Geology Of New Zealand.
Mitre Peak and Sinbad Gully, Milford Sound. Showing typical glaciated topography, with Sinbad Gully, a cirque, in the background.
MAP OF NEW ZEALAND
GEOLOGY OF NEW ZEALAND.
BY P. MARSHALL, D.Sc., M.A., F.G.s., F.R.G.S., Professor of Geology and Mineralogy, Otago University.
Illustrated.
WELLINGTON:
JOHN MACKAY, GOVERNMENT PRINTER
1912.
[All Rights Reserved.]
PREFACE.
Those who know Dr. Marshall's excellent little book on the Geography of New Zealand will herald with pleasure a companion volume on the Geology of the Dominion from the same author.
New Zealand may well be described as a geographical and geological paradise. The volcanoes and the widespread hyderthermal springs of the North Island remind one of like phenomena in Iceland and the Yellowstone National Park. The lofty snow-clad Southern Alps of the South Island, with their spacious snowfield and glaciers, rival the Swiss Alps and the Rockies in grandeur; while the West Coast Sounds almost surpass in beauty the famed Scandinavian fiords, which they resemble in mode of origin. Around our windswept coast may be seen shore-lines in every stage of formation— recently elevated, long-depressed, deeply dissected, and of mature topography.
The stratigraphical formations of New Zealand date back to remote geological antiquity, and present a wonderful range of problems to the student of palaeontology and of structural geology. Almost all possible varieties of igneous and volcanic rock are found in the Dominion—from the most basic to the most acid, and from the most alkaline to the least so.
In New Zealand's far-flung hills is found almost every known metallic mineral, while the widespread distribution of coal-bearing strata makes up for the inextensiveness of the seams in the localities where they exist.
Professor Marshall has climbed our volcanoes, explored widely in our Alps, and travelled extensively in our Southern Sounds region; he has a broad knowledge of our igneous formations and of our varied sedimentaries. Thus it seems especially fitting that he should prepare a general text-book on the country's geology.
JAMES MACKINTOSH BELL,
Director, Geological Survey of New Zealand.
The manuscript for this book was written, in 1907. During the intervening time much work has been done in New Zealand geology, especially by the geological surveys conducted by Dr. Bell. A certain amount of modification of the original matter has thus been rendered necessary, but an endeavour has been made to incorporate all new material. It has, however, not been possible to fully represent the views of Professor Park as set forth in his “Geology of New Zealand,” which has lately appeared. This, however, I have done in a work on the geology of New Zealand lately published by Carl Winter, of Heidelberg, in which the stratigraphical geology of New Zealand is described in far greater detail than is necessary in an elementary treatise on the subject.
From a physiographical standpoint it is probable that no one country of the earth —at any rate, of small size—offers greater interest to a geologist. It has been impossible to do full justice to this aspect of the subject in so small a general work. Many interesting matters in connection with volcanic action and glaciation have perforce been omitted.
The various statements that have been made from time to time in regard to the stratigraphy of New Zealand are much confused. Itis impossible to reconcile them, and it is even a matter of great difficulty to point out the extent of the divergence or of the agreement of the various opinions. While this has been attempted in the present work, prominence has necessarily been given to the author's opinions, which have the advantage of being extremely simple. Briefly stated, these are —(1) That the great series of folded stratified rocks of which the main mountain-ranges are constituted were deposited during a long continuous period, lasting almost throughout the Mesozoic age, when the present position of the country was on or near the shoreline of a great continent; (2) that another great series of rocks of Tertiary or Cainozoic age was deposited whilst a general regional depression was in progress. These views are necessarily in conflict with much that has been published before, but in the writer's opinion the facts at present known strongly support them, or even make them necessary.
I wish to acknowledge the kindness of the Mines Department in permitting the use of many photographs that have been previously used in Bulletins of the Geological Survey. I am also indebted to Mr. C. A. Cotton for the photograph on page 47.
P. MARSHALL.
Otago University, December, 1911.
AUTHOR'S PREFACE.
CORRECTIONS.
Page 23, line 25, for "18 H 2 O" read "8H2 O."
Page 29, line 8, for "629" read "729."
Page 77, line 13, for "(Fig. 44) " read " (Fig. 43)."
Page 87, line 38, for "in places and" read " and, in places."
Page 111, line 5, for "H2S + 2S O2" read " 2H 2S + S O2."
Page 124, line 17, for " and then " read " or by."
Page 124, line 18, for "parallel to the dip" read "at right angles to the strike."
Page 176, under Fig. 100, read the words " From West Wanganui" after the word " folium."
Page 179, line 37, for "East Cape" read "Cape Runaway:"
Page 203, line 7, for "eastwards" read "westwards."
CONTENTS.
PAGE
Chapter I.—Relation of New Zealand to other Physical Features of the Western Pacific
Chapter II. —Physical Structure of New Zealand .. .. .. 6
Chapter III. —Minerals of New Zealand .. .. .. 9
Chapter IV. —Rain, Rivers, and Lakes .. .. .. 25
Chapter V.—Percolating Water : Chemical Action .. .. .. 45
Chapter VI.—The Atmosphere and Physical Action .. .. .. 53
Chapter VII. —Glaciers .. .. .. 59
Chapter VIII.—Sea-coast .. .. .. 72
Chapter IX.—Volcanic Action .. .. .. 85
Chapter X.—Mountains .. .. .. 116
Chapter XI.—Rocks of New Zealand .. .. .. 136
Chapter XII. —Deposits of Economic Importance .. .. .. 141
Chapter XIII.—Geological Action of Organisms .. .. .. 154
Chapter XIV. —Metaniorphism .. .. .. 162
Chapter XV. —Geological History of New Zealand .. .. .. 170
Chapter XVI. —The Outlying Islands .. .. .. 206
Chapter XVII. —Stratigraphical Classification .. .. .. 208
ILLUSTRATIONS.
Frontispiece—-Milford Sound. Coloured geological map of New Zealand.
Fig. PAGE
1. Ocean contours in the south-west Pacific .. .. .. 2
2. Section across Pacific from Moreton Bay to Aldrich Deep .. .. 3
3. Section showing ocean-depths from Cape Howe to New Zealand .. 4
4. Heaps of rock phosphate, Clarendon .. .. .. 22
5. Erosion pillar near Henley, Otago .. .. .. 25
6. Gorge of Shotover at Arthur's Point .. .. .. 26
7. Stages of stream erosion .. .. .. 27
8. Cone of gravel, Rangitata Valley .. .. .. 28
9. Shingle-fiat of Taramakau River .. .. .. 29
10. River-system of part of west coast, North Island .. .. 31
11. Anabranch on Waihola River .. .. .. .. .. 32
12. Map of Rangitata River .. .. .. .. 34
13. Diagrams showing deposition of sediment.. .. .. .. 36
14. Stratified rocks of Oamaru age, Amuri Bluff .. .. .. 37
15. Diagrams illustrating drainage .. .. .. .. 38
16. Clutha River and upper part of Mataura River .. .. .. 39
17. Stream-system near Nugget Point .. .. .. .. 40
18. Waiau River issuing from Lake Manapouri .. .. 42
19. Tarn at foot of avalanche slope .. .. .. .. .. 43
20. Spheroidal weathering of Triassic greywacke .. .. .. 46
21. Rainfall erosion of limestone, Te Waro, Whangarei.. .. 47
22. Jurassic conglomerate, Winton, Otago .. .. .. 50
23. Diagrams showing origin of spring-waters.. .. .. .. 51
24. Diagram illustrating movement of sand-dunes .. .. ..53
25. Loess lying on volcanic scoria .. .. .. .. .. 54
26. Frost erosion on mountain-pass .. .. .. .. .. 55
26A. Frost-action on summit of Mount Aurum .. .. 56
27. Wilkinson Glacier, Westland .. .. .. .. 58
28. Upper part of Tasman Glacier .. .. .. .. 59
29. Glacial region of the Southern Alps . . .. .. 60
30. Mount Cook from Tasman Glacier .. .. .. 62
31. Striated surface of ice-worn boulder .. .. .. 62
32. Valley of North Clinton, Lake Te Anau .. .. .. 63
33. Boulder worn by glacial erosion .. .. .. 64
34. Greywacke boulder showing ice erosion .. .. .. 65
35. Upper Rangitata Valley .. .. .. .. 66
36. Roches moutonnees, Rangitata Valley .. .. .. .. 67
37. Rock-basin eroded by ice .. .. .. .. 68
ILLUSTRATIONS. VII
Fig. PAGE
38. Cirque, North Clinton Valley .. .. .. .. .. 69
39. Head of North Clinton Valley .. .. .. .. 69
40. Erratic boulder transported by ice .. .. .. .. 70
41. Cliff of horizontal Jurassic strata .. .. .. .. 73
42. Shelf of marine erosion, Brighton .. .. .. .. 74
43. Diagram of gravel-bank across inlet .. .. .. 75
44. Gravel-bank at Wakapuaka Bay, Nelson.. .. .. .. 76
45. Totara Lagoon, near Hokitika .. .. .. .. 77
46. Manukau Harbour.. .. .. .. .. .. 79
47. Microscopic section of greywacke .. .. .. 83
48, Interior of crater, Mangere, Onehunga .. .. .. .. 85
49. Volcanic bomb, Mount Eden, Auckland .. .. .. .. 87
50. Panmure basin, Auckland .. .. .. .. 88
51. Scoria cone at Waimate, Bay of Islands .. .. .. .. 88
52. Profile outline of New Zealand volcanoes .. .. .. 89
53. Section of tuff and scoria cones .. .. .. .. 89
54. Hills of scoria at base of Ngauruhoe .. .. .. .. 90
55. Interior of crater, Ngauruhoe .. .. .. .. .. 91
56. Diagrammatic section of a volcano .. .. .. .. .. 91
57. Map of volcanoes in centre of North Island .. .. .. 92
58. Ngauruhoe in eruption .. .. .. .. .. 93
59. Crater-lake of Ruapehu .. .. .. .. .. 94
60. Map of Auckland volcanoes .. .. .. .. .. 95
61. Columnar basalt, Black Head, Dunedin .. .. .. .. .. 96
62. Columnar structure, Sandymount, Dunedin .. .. .. .. .. 97
63. Granite gorge in Kakapotahi River, Westland .. .. .. .. 98
64. Microphotograph of hornblende andesite, Mount Egmont .. .. 103
65. Microphotograph of hypersthene andesite, Mount Ruapehu .. .. 103
66. Microphotograph of biotite norite, Milford Sound .. .. .. 104
67. Microphotograph of dolerite, Dunedin .. .. .. .. 104
68. Distribution of deposits from Tarawera eruption .. .. .. 107
69. Effect of Tarawera eruption on Lake Rotomahana .. .. 109
70. Growth of vegetation near Lake Rotomahana .. .. .. 110
70A. Diagram of geyser .. .. .. .. .. .. 112
71. Diagram illustrating normal faulting .. .. .. .. 117
72. Plan and section of dome and basin .. .. .. .. 118
73. Diagram showing relation of contours to outcrop .. .. .. .. 119
74. Crumpled schist, Otago .. .. .. .. .. 120
75. Contorted sandstone, Bay of Islands .. .. .. .. 121
76. Diagram of fold mountains .. .. .. .. .. .. 122
77. Diagram of residual mountains .. .. .. .. .. 122
78. Diagrams of folds .. .. .. .. .. 123
79. Fault at Landguard Bluff, Wanganui .. .. .. .. 124
80. Diagram of reversed fault .. .. .. .. .. 125
81. Section across Kaikoura Mountains .. .. .. .. .. 1 26
82. Section across north-west Nelson .. .. .. .. .. 128
Viii ILLUSTRATIONS.
Fig- PAGE
83. Section across west of Southern Alps .. .. .. .. 128
84. Section illustrating structure of Southern Alps .. .. .. 130
85. Section from Stewart Island to Waitaki River .. .. .. .. 130
86. Section from Maunganui Bluff to Three Cables .. .. .. .. 131
87. Map showing mountain-ranges of New Zealand .. .. .. .. 132
88. Diagram of block mountain .. .. .. .. .. 134
89. Section from Cape Egmont to Porangahau .. .. .. .. 135
90. Auriferous gravels at Ross, Westland .. .. .. .. .. 143
91. Gold-dredge, Central Otago .. .. .. .. .. 145
92. Martha Lode, Waihi .. .. .. .. .. .. 148
93. Diagram of lode .. .. .. .. .. .. 149
94. Parapara iron-ore .. .. .. .. .. .. 152
95. Diagram illustrating Darwin's theory of coral reefs.. .. .. .. 156
96. Diagram illustrating Murray's theory of coral reefs.. .. .. .. 156
97. Coal-seam, Nightcaps, Southland .. .. .. .. .. .. 160
98. Folded mica-schist, Otago .. .. .. .. .. .. 166
99. Mount Mackenzie, Clinton Valley .. .. .. .. .. 175
100. Graptolites .. .. .. .. .. .. .. .. 170
101. Unconformity between Baton River and Maitai systems .. .. .. .. 178
102. Triassic fossils, Nelson .. .. .. .. .. .. 183
102A. Spirifers, Nugget Point .. .. .. .. .. .. 184
103. Petrified forest, Curio Bay, Waikawa .. .. .. .. .. 186
104. Jurassic fossils .. .. .. .. .. .. .. 87
105. Gorge of Shotover River .. .. .. .. .. 189
105A. Unconformity at Waikato Heads .. .. .. .. 190
106. Section at Amuri Bluff .. .. .. .. .. .. 191
107. Oamaru strata .. .. .. .. .. .. .. 192
108. Fossils of Oamaru age (limestone) .. .. .. .. .. 194
109. Pecten athleta .. .. .. .. .. .. .. 195
110. Oamaru strata, Millburn, Otago .. .. .. .. .. 196
111. Map showing Pleistocene glacial extension in New Zealand .. .. .. 201
112. Rock-basins eroded by glacial action .. .. .. .. 202
GEOLOGY OF NEW ZEALAND.
CHAPTER I.
RELATION OF NEW ZEALAND TO OTHER PHYSICAL FEATURES OF THE WESTERN PACIFIC.
As judged by the present position of the nearest land-areas, the isolation of New Zealand appears to be complete. At first sight the isolation appears almost as complete when the contours of the floor of the surrounding ocean are examined.
On the west an ocean-basin 2,500 fathoms deep separates our Islands from Australia. To the east the ocean-floor dips down steeply to a depth of more than 2,000 fathoms. Southwards the depth is distinctly less—indeed, a great part of the South Pacific is less than 2,000 fathoms deep. Northwards the isolation is far less pronounced, for at no great depth below the surface there are submarine ridges which have the direction of the most salient surface features of the North Island. The long narrow range that extends from Cape Terawhiti to the East Cape, itself a continuation of the Kaikouras, is continued beneath the surface of the Pacific. At first it reaches depths of over 1,000 fathoms below the sea-level, but soon rises and remains at a depth of less than 830 fathoms for a long distance. On this ridge are situated first the Kermadecs and then the Tonga Islands.
Close to this submarine ridge and on its castern side there is a profound ocean deep or abyss. For a long distance from its commencement, 100 miles north-east of the East Cape, it is over 4,000
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GEOLOGY OF NEW ZEALAND.
fathoms deep, and in some places it dips below the 5,000-fathoms level. There is only one locality in the world —in the northern
Fig. 1.—Ocean Contours in the South-west Pacific.
Pacific —where soundings have been obtained deeper than the 5,155 fathoms of the Aidrich Deep.
3
GEOLOGY OF NEW ZEALAND.
Another submarine ridge, following roughly the direction of the Auckland Peninsula, extends slightly to the west of north through Norfolk Island to New Caledonia, but is separated by a narrow strait of deep water from eastern Australia. Over this ridge the water is always less than 1,000 fathoms in depth; and in places, such as the Balfour Shoal, it is distinctly less than 500 fathoms over a fairly wide area. It is noticeable that, though this submarine ridge has the same direction as the North Auckland Peninsula, it is not a direct continuation of it, but must be regarded rather as a parallel line of elevation.
Fig. 2.—Section showing the Floor of the Pacific- from Moreton Bay to Aldrich Deep— 1,700 Miles.
The strike of the folds of the New Zealand rocks has the direction of the Tongan ridge, yet there is to be found in those portions of the ridge that rise above sea-level little geological evidence that can be used to illustrate a similarity in structure between New Zealand and the material of the ridge. The Kermadecs are volcanic, but the rocks show no more than a general resemblance to those of the Ruapehu region, which within New Zealand has its axis directed towards the north-east. The rocks of the Tongan Group appear to be more basic; but little is yet known of them. In Samoa, just beyond the end of the ridge, volcanic action is still in progress, and basaltic rock is being ejected. The granite that is found in the form of boulders in the Kermadecs does not appear to have any parallel in the North Island.
The rocks of the islands situated on the western ridges appear to be much more closely related to those of New Zealand. Leaving aside Norfolk Island, which is said to be formed of basalt, the rocks of New Caledonia have a great interest from this point of view. There is a large mass of serpentine associated with deposits of nickel. This at once reminds us of the masses
4
GEOLOGY OF NEW ZEALAND.
of ultra-basic rooks in New Zealand, which at the Red Hill in the west of Otago contain nickeliferous iron. There is also a strong development of Triassic rocks containing the well-known fossil Monotis and other characteristic forms that occur abundantly in the Triassic rocks of the Wairoa Gorge (near Nelson) and in other places in New Zealand. In the New Hebrides and in Fiji andesitic rocks are found in some quantity. These characteristics are maintained in the lands that stretch farther north and west through New Guinea and the Malay Archipelago. We are naturally led to conclude that this submarine ridge marks the direction of a continuous structural fold of the earth's crust, which here forms the westerly limit of the deep basin of the Pacific Ocean. This western ridge is the Australasian festoon of Gregory, who ignores, however, the more direct ridge through the Kermadecs. Suggestive though the existence of the Tongan ridge is, our knowledge of the rocks and of the structure of the islands that rise from its crest is still too imperfect to allow us to speculate as to its exact relationship to New Zealand. Whatever its true relationship to the structure of New Zealand may be, it is certain that the remarkable oceanic abyss that lines it on the east has no southern extension, for the water between New Zealand and the Chatham Islands appears to be nowhere more than 1,300 fathoms in depth.
Fig. 3.—Section showing Ocean-depths from Cape Howe to New Zealand.
It must be remembered that Captain Hutton has stated that the flora and fauna of New Zealand present some remarkable affinities with those of Malaysia. So marked are these that he has often stated that the early Cainozoic elevation of New Zealand was sufficient to
5
GEOLOGY OF NEW ZEALAND.
unite all these western Pacific lands together, and that many of the plants and animals of New Zealand reached us from the north at that time. To the east the Chatham Islands are so closely related to New Zealand in flora and fauna that they must have remained an integral portion long after all union with the northern islands had terminated.
The southern islands are still more closely related in the forms of their plants and animals, and the structure of the land and the nature of the rocks show so close an approach to New Zealand types that there can be little doubt that they really formed part of the same land-mass. Their separation from the larger islands must be ascribed to the relatively late depression of the whole area, and of this we have abundant evidence in the features of the coast-line of New Zealand. The occurrence of granites and gabbros in these islands shows that they have been land-areas long enough to allow of the removal of thick masses of rock by the slow action of geological agents. The volcanic rocks of which they are chiefly formed do not differ in any important respects from those that are common in New Zealand. In Campbell Island there is a Cainozoic limestone that does not in any essential respect differ in structure or in fossil contents from those that are so abundant in New Zealand.
Though the plants and animals of the southern islands are so similar to those of New Zealand, there is also an important Fuegian element. In New Zealand also this can be recognized in the Acaena, Sophora, shrubby Veronicas, and many other plants. This points to the conclusion that an elevation in the later Cainozoic not only united these islands to New Zealand, but also connected the whole area with South America, or with some southerly continuation of it. If separated at all, these areas were so developed as to be divided by such a narrow and shallow water-area that the distribution of the Mollusca was effected as well as that of the land-plants.
It must be remembered that Professor Suess, in dealing with the relationships of southern lands to one another, has compared the whole distance between Western Australia and New Zealand to the structure of South America. The comparison is based upon rock structure and succession, and upon a similar succession of mountain and plain, though the difference in magnitude, especially in respect to distance, is most striking. It is not considered that the comparison is of sufficient importance to justify a statement of it in detail in this work.
6
CHAPTER II.
PHYSICAL STRUCTURE OF NEW ZEALAND.
It has been already shown that the long narrow Islands of New Zealand are bordered by the deep waters of the Pacific on the east and by the deep waters of the Tasman Sea on the west. The ocean contours show clearly that New Zealand is only the higher emergent crest of an extensive submarine ridge extending northwards almost up to the Samoan Islands and southwards as far as Macquarie Island. The ridge may extend even farther south; but in regard to this we cannot speak with certainty, as only a small number of soundings has been taken.
Of the land-mass itself, there are some general features that claim attention.
The South Island.
The great mountain-chain of the Southern Alps extends throughout the length of the South Island from south-west to north-east. On the eastern side spurs and lower ranges extend thirty miles from the main divide. In Otago there are many ranges running nearly north-and-south, the most easterly being near the east coast. A similar feature is found in Marlborough, where, however, the eastern ranges are parallel to the main divide, and, in the case of the Kaikouras, surpass it in altitude. South of the Otago mountains there are smaller hills—the Hokonuis —running from W.N.W. to E.S.E. Between the separate ranges of the Otago mountains and south of them there are extensive gravel plains. There are smaller gravel plains between the mountain-spurs in Canterbury, while to the east of the mountains is a great expanse of gravel two hundred miles long —the Canterbury Plain.
To the west of the main divide there are no parallel ranges, except in Nelson, where they form high mountainous land to the west of Tasman Bay and the Motueka River. On the western side the spurs are steep, and the declivity of the main range is abrupt. From its
GEOLOGY OF NEW ZEALAND.
7
GEOLOGY OF NEW ZEALAND.
foot morainic masses and gravel plains extend westward for five or ten miles and reach the coast, but in the south the mountains rise with precipitous sides from the shore.
Almost throughout the South Island there is a fringe of undulating country between the mountain-chain and the gravel plains. In a few instances the undulating country extends into the mountains themselves, and forms the floor of the larger basins that are found between them.
This topographical division of the country into (a) mountains, (b) undulating country or downs, and (c) plains usually corresponds with the main features of their geological structure. The rocks of the mountains are much folded, and, though usually consisting of altered shales and sandstones, are often composed of metamorphic rocks and sometimes of plutonic rocks. These rocks are never more recent than the Jurassic period. The undulating country, or downs, is formed of poorly coherent sandstones and limestones, for the most part of Middle Cainozoic age. These rocks are often horizontal, and when they are folded the action has been merely local. The gravel plains are recent accumulations deposited in the moun-tain-valleys, or they constitute a belt of gently sloping land between the downs and the sea. The material of which the gravel is composed shows clearly that the plains have been formed as a result of the action of rivers and rainfall on the mountain-ranges.
The North Island.
The structure of the North Island is in general similar to that of the South Island, though there is much modification.
The mountain-ranges of old rock are far more restricted in their extent. They contain no schists, and plutonic rocks are found in the extreme north only. There is a long continuous ridge stretching from Cape Terawhiti to near the East Cape, and remnants of other ranges are found between Kawhia and Waiheke Island, in the Cape Colville Peninsula, and in the country to the north of Auckland. On the eastern side of the main range the downs country, of Cainozoic ag e—in structure a coastal plain—covers a large area, and in places is mountainous, while the gravel plains of the Wairarapa and Hawke's Bay are relatively small. On the west of the range the undulating country is still more developed. The Wanganui and Taranaki districts are comprised within it, though its character is there somewhat
2—Geology.
8
GEOLOGY OF NEW ZEALAND.
changed by the deep and precipitous gorges that the rivers have cut through it. In the Raglan district, and from the Waikato to the far north, land of this kind binds together the isolated fragments of the old mountain-ranges of ancient rock. Gravel plans, again, are few on the west, but from Feilding to Otaki they are of importance.
Volcanic Country. —There is, however, a type of country which has a local development only in the South Island, but is almost dominant in the composition of the North Island. This type is the volcanic country. Banks Peninsula, Otago Peninsula, and a few other localities represent it in the South Island, but in the North Island the volcanic plateau is perhaps the dominant topographic feature. Commencing at the south of Lake Taupo, it stretches over the whole country between the mountain-range and the high country from Kawhia through Ngaruawahia, and extends as far as the Bay of Plenty. Relatively flat, and for the most part barren, this volcanic country rises here and there into striking and majestic cones, or even into serrated ranges; more frequently, however, flat-topped hills are the characteristic elevations. Almost everywhere the surface is covered with pumice. The lower Waikato country is mainly covered by lava-flows of basaltic rock similar to that forming the sixty small cones at Auckland, as well as many others near the Bay of Islands. The volcanic rocks of the Cape Colville Peninsula are found again in the Barrier Islands and in the many fantastic hills of Waitakerei, Whangarei, Whangaroa, and other localities.
Dunes. —There is yet another feature to which attention should be drawn. In the far north there is an immense stretch of sand which, with its rolling clunes, binds together several isolated areas of high land. The North Cape area, Hohoura, and Mount Camel would be as isolated as the Three Kings but for the wave-deposited sand which has united them to the mainland. A similar but less extensive sand-deposit is found at Cape Farewell. From Patea to Waikanae, on the west coast of the North Island, the dunes extend inland for six miles or more. In many other localities on the coast the dunes form conspicuous but less-important features.
9
CHAPTER III.
MINERALS OF NEW ZEALAND.
In the descriptions of the minerals in the following pages frequent reference will be made to specific gravity (G), hardness (H), and crystalline form. As it is quite impossible to give even a brief summary of crystallography here, if the meaning of the various crystallographic terms is not known reference must be made to a suitable textbook, such as Dana's "Mineralogy."
The specific gravity means the weight of the mineral, bulk for bulk, as compared with water.
The hardness is based on a scale for which the degree of hardness of talc is 1, and that of the diamond—the hardest mineral known —is 10. In making use of this scale it is convenient to remember that the hardness of the finger-nail is about 2.5, and that of a steel knife is about 6.
The term cleavage refers to the tendency that minerals possess to split easily in certain directions, owing to a relatively small value of molecular cohesion in a direction at right angles to the plane of cleavage.
The streak is the colour of the fine powder of the mineral; usually some of this is obtained by scratching the surface of the mineral with a knife.
Graphite.—Form, hexagonal plates. Cleavage, perfect, parallel to the base. H, 1.5. G, 2. Colour, grey. Lustre, metallic. Streak, lead-grey.
Occurs rather sparingly. Has not yet been found in sufficient quantity to justify mining operations. Has been found in gneiss at Dusky Sound. Boulders have been found in the beds of streams that flow from Mount Egmont. In this case the graphite was probably formed by the action of volcanic heat upon coal-seams through which the lava passed on its way to the surface. Occurs also at Pakawau.
GEOLOGY OF NEW ZEALAND.
10
GEOLOGY OF NEW ZEALAND.
Sulphur.—Orthorhombio pyramids with basal plane. No cleavage. H, 2. G, 2. Colour, yellow. Lustre, vitreous. Streak, grey.
Occurs abundantly in the thermal district of the North Island. Usually amorphous, as is the case at Tikitere and Waiotapu. Slender needle-shaped crystals are found near fumaroles, and well-formed crystals occur at Rotokawa, fifteen miles north of Lake Taupo.
Arsenic in reniform masses is found in the quartz veins near Coromandel.
Bismuth has been found at the Owen River, in the west of Nelson.
Gold. —Isometric. No cleavage. Usual form, the rhombic dodecahedron. H,3. G,19. Colour, yellow. Lustre, metallic. Streak, gold-yellow.
Occurs widely distributed in New Zealand: -
(a.) In beach-sands at Orepuki and from Milford Sound to Westport. At one time rich in places; now workable at a few places only, and after heavy weather.
(b.) In river-gravels and placers. Found throughout the micaschists of Otago, over nearly the whole extent of Westland, and in many parts of the Nelson and Marlborough Provinces. In Otago nuggets of large dimensions have been found. All the richer parts of these deposits have now been worked, and but few payable gravels remain.
(c.) In quartz veins traversing (1) mica-schist in Otago, (2) greywacke and shales at Reefton and the Wilberforce, and (3) andesite at the Thames and Coromandel.
The gold in the quartz occurs with pyrite. In most cases it is probably combined with sulphur in the form of a sulphide. In Otago there is but little silver with it, but on the Thames Goldfield there is three times as much silver as gold. Occasionally the gold occurs in large bonanzas, as in the Caledonia Mine at the Thames, where gold worth £550,000 was obtained within twelve months. Smaller bonanzas have been found in the Hauraki and Waiotahi Mines.
The gold that is found in gravels and beaches is certainly derived from quartz veins that have been gradually worn away. The lighter and more brittle minerals broke up and floated away, while the tougher and heavier minerals, such as gold, were concentrated by the water. The gold of the west coast of the South Island is often flaky. This has been ascribed to pressure since its liberation from the quartz; but the grains may have been originally flat before their removal from the veins.
11
GEOLOGY OF NEW ZEALAND.
The formation of large nuggets has been a subject of frequent discussion. It is evident that they are either masses of gold derived directly from veins, or that they have been increased in size, or have been completely formed in the gravels in which they are found. The first theory is supported by —(1) the knowledge that masses of gold as large as nuggets do occur in mines, (2) the occurrence of quartz with the gold of nuggets, (3) the rolled nature of the material in which they occur, and (4) the actual shape of the nuggets. The "growth in place" theory is supported by —(1) the experimental precipitation of gold from chloride-of-gold solution on a surface of gold or pyrite, (2) the purity of the gold of nuggets, (3) the frequent absence of quartz, and (4) the fact that gold dissolves slightly in natural waters.
Platinum.— Isometric. H,4. G,17. Colour, silver-white. ustre, metallic.
Occurs with gold to a slight extent at Orepuki and Collingwood, and is found in small quantities at Waikaia and elsewhere. A little iridium, osmium, and other allied metals occurs with the platinum.
Native Iron.—A few meteorites have been found. One discovered in the Wairarapa was of relatively large size.
Awaruite. —An alloy of iron and nickel, containing two parts of nickel to one part of iron, has been found in some quantity in the sands of the Gorge River and some other streams that flow from the olivine country of the Red Hill district, in west Otago. The alloy is apparently an impregnation in serpentine, for a grain of it has been found embedded in this rock.
Stibnite. —Orthorhombic. Cleavage, brachypinacoid. H,2. G, 4-55. Colour and streak, lead-grey or whitish. Composition, sulphide of antimony (Sb2S 3 ). Fuses easily.
Occurs as fine crystals in druses at the Thames; found in more compact forms at Alexandra (in Otago), Endeavour Inlet, and the Bay of Islands.
Molybdenite.—Hexagonal. Tabular form. Cleavage, basal. H,1. G,2.7. Colour, lead-grey. Lustre, metallic. Leaves a bluishgrey mark on paper. Composition, sulphide of molybdenum (MoS 2 ).
Occurs at Dusky Sound, Mount Radiant, and the Thames.
Galena. —Isometric. Cubes and octahedrons. Cleavage, highly distinctive, cubic. H, 2.6. G, 7.5. Colour, lead-grey with a bluish tint. Lustre, metallic. Fuses easily. Composition, sulphide of lead (PbS).
12
GEOLOGY OF NEW ZEALAND.
Occurs somewhat sparingly in New Zealand, at Te Aroha and the Thames.
Argentite.—lsometric. H, 2. G, 7.2. Colour, lead-grey with a blackish tint. Composition, sulphide of silver (Ag 2 S).
Occurs in the ore of the Waihi Mine.
Chalcocite. —Orthorhombic. H, 2.5. G, 5.6. Colour, lead-grey. Lustre, metallic. Usually massive. Composition, sulphide of copper (CuS).
Occurs in the Dun Mountain area, near Nelson.
Sphalerite.—lsometric. Tetrahedral. Cleavage, rhombic dodecahedron. H, 3.5. G, 4. Colour, black to yellow. Streak, brown to white. Lustre, metallic or adamantine. Composition, sulphide of zinc (ZnS).
Occurs sparingly in New Zealand, at Te Aroha, the Thames, and the Great Barrier Island.
Cinnabar. —Hexagonal. Usually granular; isolated crystals rare. Cleavage, prismatic. H, 2. G, 8. Colour, red. Streak, scarlet. Composition, sulphide of mercury (HgS).
Occurs at Waitahuna (near Dunedin) and the Thames.
Pyrrhotite. Hexagonal. Usually tabular. H, 4. G, 4.5. Colour, bronze-yellow. Streak, black. Lustre, metallic. Slightly magnetic. Composition, sulphide of iron (Fe11S12).
Occurs at Dusky Sound, and at the Dun Mountain, Nelson.
Chalcopyrite.—Copper-pyrites. Tetragonal. Form closely resembles tetrahedrons. H, 3.5. G, 4.2. Colour, brass-yellow, but often tarnished. Streak, greenish-black. Lustre, metallic. Composition, CuFeS 2 .
Occurs near Nelson, in the Dun Mountain area; at Mount Radiant, near the Karamea; Kawau Island; the Great Barrier; and Moke Creek, near Lake Wakatipu.
Pyrite.—Mundic. Isometric cubes, octahedrons, and other forms. H, 6. G, 5. Colour, pale brass-yellow. Streak, brownish-black. Lustre, metallic. Composition, FeS 2 .
Occurs as minute crystals in many rocks. Frequent in quartz veins, often associated with gold. Good cubes occur at Mount Aurum,
in the Shotover district. Good octahedrons are found at Parapara, near Collingwood.
Marcasite. —Orthorhombic, distinct crystals unusual; more often massive, or in rounded forms with pyrite. H, 6. G, 4.8, Colour,
13
GEOLOGY OF NEW ZEALAND.
pale bronze-yellow. Streak, brownish-black. Lustre, metallic. Composition, FeS 2 , the same as pyrite. Decomposes readily.
Occurs commonly in concretions and in coal.
Arsenopyrite.—Orthorhombic. Cleavage, indistinct. H, 6. G, 6. Colour, silver-white to grey. Lustre, metallic. Streak, dark grey. Composition, FeAsS.
Occurs abundantly in auriferous lodes at Coromandel.
Pyrargyrite.—Hexagonal. H, 2.5. G, 5.8. Colour, black to deep red. Streak, red. Lustre, metallic. Composition, Ag 3 SbS 3 .
Occurs at Puhipuhi, near Whangarei.
Richmondite.—An ore containing copper, silver, and several lessvaluable metals. Has a massive structure and a black colour.
It occurs at the Richmond Hill, near Collingwood.
Fluorite.—Isometric. Colour, green or purple to white. Cleavage, octahedral. H, 4. G, 3. Composition, CaF 2 .
Occurs near the junction of the Baton branch with the Motueka.
Quartz. —Hexagonal. No cleavage. H, 7. G, 2.6. Colourless when pure, but yellow, brown, or red when it contains impurities. Streak, white. Lustre, vitreous. Composition, Si O2.
Occurs abundantly as grains in nearly all rocks. Good crystals are found in cavities. Masses of nearly pure quartz form metalliferous veins. Various coloured varieties occur in the Clent Hills, near Ashburton. Yellow quartz crystals arranged in star-like groups are found near Oamaru.
Chalcedony.—A form of silica in which perfectly amorphous bands alternate with finely crystalline bands. Massive and very fine-grained in the Clent Hills. Agate is a variegated chalcedony.
Flint, a dense dark mineral with a curved fracture, occurs at Amuri Bluff, Oxford, and Campbell Island.
Plasma and piase, green minerals allied to chalcedony and flint, are found in the Clent Hills.
Jasper, a dense red variety, is widely distributed.
Opal differs from chalcedony in containing some water combined with the silica. It occurs at Mount Somers. Jasp-opal is found at Cape Saunders. A little precious opal-a variety with a play of colours —has been found at Cabbage Bay, in the Coromandel Peninsula.
Hyalite, a perfectly clear glassy variety, is found at Dunedin.
Geyserite, a deposit from the water of hot springs, is abundant in the Taupo and Rotorua districts.
14
GEOLOGY OF NEW ZEALAND.
Cuprite.—lsometric. H, 3.5. G, 6. Colour, black to red. Streak, red. Lustre, submetallic. Composition, Cu 2 O.
Occurs in the Dun Mountain area.
Corundum.—Hexagonal. H, 9. G, 4. Colour, very variable. Lustre, adamantine. Composition, AI 2O3 .
A dark-red variety (ruby) occurs with a green mica in the rock called goodletite, which occurs as boulders in the sluicing-gravels of Rimu, near Hokitika. The mineral has not got such a colour as to render it of any commercial value.
Haematite.—Hexagonal. H, 6. G, 5. Colour, iron-black or bright red. Streak, red. Lustre, metallic. Composition, F e2 O3 .
Does not occur frequently in New Zealand. A micaceous variety occurs near Queenstown, and in many localities shales are coloured red with this mineral.
Ilmenite.—Hexagonal. H, 5.5. G, 5. Colour, black. Streak, black. Lustre, metallic. Composition, FeTi O2.
Occurs abundantly in igneous rocks as minute grains, and also forms a constituent of the black sands on the western coast of the North Island.
Magnetite.—lsometric, octahedrons. H, 6. G, 5.2. Colour, black. Streak, black. Lustre, metallic. Composition, F e3 O4 Strongly magnetic.
Occurs as minute crystals in many igneous rocks, to which it imparts a black colour. Distinct crystals are found in chloriteschists in Otago and Westland. An important constituent of black sand.
Chromite.—lsometric, octahedrons. H, 5.5. G, 4.4. Colour, black. Streak, brown. Lustre, metallic. Composition, FeCr 2O4 .
Occurs as small grains in the olivine rocks of the Dun Mountain and of the Red Hills in Otago and Nelson. Sometimes in large masses in these rocks.
Cassiterite.—Tetragonal; knee-shaped twins common. H, 6-5. G, 7. Colour, black to yellow. Streak, white. Lustre, adamantine. Composition, Sn O2.
Occurs in the form called "stream tin" at Port Pegasus in Stewart Island, and to a slight extent in some sands in Westland.
Rutile. —Tetragonal; knee-shaped twins common. H, 6. G, 4.2. Colour, reddish-brown. Streak, pale brown. Lustre, adamantine to metallic. Composition, Ti0 2 .
15
GEOLOGY OF NEW ZEALAND.
Occurs as small embedded crystals in the gneisses of the West Coast Sounds and in some schists.
Limonite. —Massive. H, 5. G, 4. Colour, brown to black; often with a bright polished exterior. Streak, yellow. Composition, 2F e2 O3 H 2 O.
Occurs in large masses forming important iron-ores at Parapara, near Collingwood; also as spherical hollow bodies in many auriferous gravels. Abundant as nodules in the clay soils in the north of Auckland. An important colouring-substance in clay soils, though the actual percentage of limonite in these clays is quite small.
Psilomelane.—Massive. H, 5.5. G, 4. Colour, black. Streak, black. Lustre, dull. Composition, H4Mn O 5.
Occurs in association with other manganese minerals in red shales at the Bay of Islands, Waiheke, Wellington, Taieri Mouth, and elsewhere.
Calcite. —Rhombohedral. Cleavage, rhombohedral. H, 3. G, 2.7. Colour, usually white. Streak, white. Lustre, vitreous. Double refraction strong, and conspicuous in cleavage pieces. Composition, CaC O3.
Occurs as distinct crystals in cavities in many rocks. In the earthy form it constitutes nearly all the material of limestone rocks. In the crystalline granular form it constitutes marble.
In New Zealand large cleavage pieces are found in the Shotover gravels, near Lake Wakatipu. Crystals are found at the Thames, and at Limestone Island, Whangarei. There are large deposits of marble at Parapara and Takaka, and in many other places in the Nelson Province. It also occurs at Caswell Sound, in Otago.
Aragonite.—Orthorhombic. H, 3.5. G, 3. Colour, white. Streak, white. Lustre, vitreous. Twinning frequent, parallel to the face of the prism; this may produce apparently hexagonal prisms. Composition, CaC O3.
Occurs in cavities in basaltic rocks at Dunedin, Oamaru, and many other places.
Cerussite. —Orthorhombic. H, 3.3. G, 6.5. Colour, white. Streak, white. Lustre, vitreous. Cleavage, prismatic. Composition, PbC O3.
Occurs at Te Aroha, in the upper part of the veins that contain galena in their lower portions.
16
GEOLOGY OF NEW ZEALAND.
Malachite.—Manachite; but crystals are not distinct. H, 4. G, 3-9. Colour, green. Streak, pale green. Lustre, vitreous. Composition, CuC O3CuH 2O2 .
Occurs at Moke Creek, near Lake Wakatipu; Dun Mountain; Kawau, near Auckland; and the Great Barrier.
Another copper-carbonate, azurite, often occurs with malachite. It is usually crystalline, and has a bright-blue colour.
Orthoclase. —Monoclinic; twinning general. (1.) Carlsbad law: The clinopinacoid is the composition plane, and the orthopinacoid is the twinning plane. (2.) Baveno law: The clinodome is the composition and twinning plane. (3.) Manebacher law: The plane of twinning and composition is the basal plane. H, 6. G, 2.5. Colour, white or pink. Streak, white. Lustre, vitreous. Composition, KAIS i3O8. Cleavage, parallel to the base and clinopinacoid. Often called feldspar.
Occurs in all the acid igneous rocks. Large crystals with welldeveloped base and clinopinacoid are common in the granite of Separation Point, near Nelson. These crystals weather out readily, and form the pebbles on the beaches.
Microcline. —Not distinguishable from orthoclase in ordinary hand-specimens, though examples from foreign localities often have a characteristic green colour. Composition, the same as orthoclase.
Occurs at Port Pegasus and Golden Bay, in Stewart Island, and also in granite at Separation Point.
Albite Oligoclase Andesine Labradorite Bytownite Anorthite
These constitute a group of feldspar minerals that are not usually distinguishable from one another in hand-specimens. All of them are triclinic. Cleavage, parallel to the base and the brachypinacoid. H, 6. G, varies from 2-6 in albite I to 2.75 in anorthite. The twinning in nearly
every crystal is polysynthetic—that is, indefinitely repeated—with the brachypinacoid the composition and twinning plane. The crystals are also twinned in many instances after the same laws as orthoclase. Colour, white. Streak, white. Lustre, vitreous. Composition —albite, Na 2 OAl 2 O36S iO2 ; anorthite, CaOAl 2O3 2Si O2 . If these formulae are represented by Ab and An respectively, the minerals may be represented in the following way: Oligoclase—Ab, 3; An, 1. Andesine—Ab, I; An, 1. Labradorite—Ab, 1; An, 3. BytowniteAb, 1; An, 6.
17
GEOLOGY OF NEW ZEALAND.
One, if not more, of these minerals occurs in nearly every kind of igneous rock, especially in those of a basic type. In New Zealand there are no recorded instances in which these minerals weather out from the rocks in which they are embedded. Collectively they are called triclinic feldspars or plagioclases. Microscopic methods have to be used to identify them. These feldspars form the white portion of the gabbros at the Bluff and the West Coast Sounds, and also the glassy crystals in the rocks of Banks Peninsula and of Mount Egmont and Mount Ruapehu.
Enstatite. —Orthorhombic. Cleavage, prism and pinacoids. H, 5.5. G, 3. Colour, grey. Streak, white. Lustre, vitreous. Composition, MgSi O3.
Occurs in large plates in some olivine rocks at Milford Sound. It is also abundant in some of the olivine rocks near Nelson.
Hypersthene is very similar to enstatite, but its colour is black, its lustre metallic, and it contains some iron in place of magnesium.
It occurs widely in New Zealand, especially at the Bluff, and at the Darran Mountains, near Milford Sound. It is an important constituent of the andesites of the Thames goldfields, and of those of Ruapehu and other volcanoes of the central region of the North Island. In these rocks, however, the crystals are small. Since in many of the more vesicular rocks the crystals of hypersthene are the hardest and most resistant to rock-destruction, they now form an important ingredient of the black sand on some of the beaches of Lake Taupo.
Augite.—Monoclinic. Cleavage, prismatic; the cleavage surfaces are inclined at angles of 87° and 93°. Twinning, common, with orthopinacoid the twinning and composition plane. H, 5.5. G, 3.3. Colour, black. Streak, grey. Lustre, vitreous. Composition, variable; a silicate of lime and magnesia, with some iron and alumina.
Abundant in all the more basic igneous rocks. The crystals are firmly embedded in the rock, but may often be distinguished by their colour and cleavage. Well-formed crystals can be obtained from weathered dolerite at Lyttelton and Dunedin.
Aegerine is a green variety of augite, though in large crystals it appears black. Embedded in colourless and transparent minerals it gives a pale-green colour to some Dunedin rocks.
Diallage differs from augite in possessing a very distinct pinacoidal cleavage; its lustre, too, is rather metallic.
18
GEOLOGY OF NEW ZEALAND.
Occurs as a rock-constituent at the Darran Mountains, near Milford Sound, and is almost the sole constituent of some rocks in the Dun Mountain.
Hornblende. —Monoclinie. Cleavage, prismatic; the cleavage surfaces are inclined at angles of 56° and 124°. Twinning, common as in augite. H, 5.5. G, 3.2. Colour, green to black. Streak, white to brown. Lustre, vitreous. Composition, generally similar to that of augite.
Occurs as embedded crystals in many intermediate igneous rocks. The diorites of the Sounds of Otago, tie pionolites of Dunedin, and the andesites of Mount Egmont are examples.
Actinolite is a light-green variety of hornblende that occurs in many metamorphic rocks in Westland. Nephrite, or greenstone, is a mineral composed of minute densely matted fibres of actinolite. It occurs as boulders in the gravels of the Taramakau and neighbouring rivers. It occurs in situ in the adjacent mountains.
Cossyrite is a triclinic variety containing much soda. It occurs in grains of microscopic size in some Dunedin rocks.
Nepheline.—Hexagonal. H, 6. G, 2.6. Colour, white, or transparent and colourless. Streak, white. Lustre, greasy. Composition, 3Na 2 OK 2 O4Al2 O39SiO2. It gelatinizes when the powder is treated with dilute HCI.
It occurs in many volcanic rocks at Dunedin and in boulders near Brunner; more sparingly at Auckland. A rock of Rarotonga, Cook Islands, is particularly rich in this mineral.
Garnet. —Isometric; generally dodecahedrons. Cleavage, dodecahedral. H, 7. G, 3.8. Colour, variable; generally pink or red, but sometimes green or black. Streak, white. Lustre, vitreous. Composition, variable; the commonest kind is 3CaOFe 2 O3SiO2.
Occurs in many rocks of the Sounds district in Otago, and in schists in Westland and Nelson. It is the mineral that gives "ruby" sand its red colour. A green chromium-bearing garnet (ouvarovite) occurs in Dusky Sound. A white variety (grossularite), which contains aluminium in place of iron, occurs widely at the Dan Mountain.
Olivine. —Orthorliombic. H, 5. G, 3.3. Colour, greenish-yellow. Streak, white. Lustre, vitreous. Composition, (MgFe) 2 Si O4. Weathers readily from oxidation of the iron that it contains.
GEOLOGY OF NEW ZEALAND.
19
Occurs in basalts throughout New Zealand, and also forms the mass of ultra-basic rocks of the Dun Mountain, Nelson; of Anita Bay, Milford Sound; and of the Red Hills, in the west of Otago.
Zircon.-Tetragonal. H, 7.5. G, 4.7. Colour, reddish-brown. Streak, white. Lustre, adamantine. Composition, ZrSi O4.
Occurs rather rarely in New Zealand, chiefly in the sands of Westland.
An igneous rock (porphyry) at Campbell Island contains numerous crystals of deep colour.
Beryl.— Hexagonal prisms. H, 8. G, 2.7. Colour, yellowisbgreen.
Occurs at Dusky Sound and Stewart Island.
Epidote.—Monoclmie. Cleavage, basal. The crystals are elongated in the direction of the ortho-axis. H, 6.5. G, 3.4. Colour, pale green. Streak, white. Lustre, vitreous. Composition, HC a2 (AlFe)3 Si 2O12 .
Common in metamorphie rocks, to which it imparts a pale-green colour. Especially common in Westland, but no large or isolated crystals have been found.
Tourmaline. —Hexagonal, generally long prisms. H, 7. G, 3. Colour, usually black. Streak, white. Lustre, vitreous. Composition, variable; a silicate of aluminium with some alkaline silicate and water, and about 10 per cent, of boric acid.
Occurs in some granites at Stewart Island and Westland. Fine crystals are obtained at Richmond Hill, near Collingwood.
Chabazite. —Rhombohedral. H, 4. G, 2. Colour, white. Streak, white. Lustre, vitreous. Composition, (CaNa 2 )Al2Si4O126H 2 O.
Occurs at Dunedin as small crystals in cavities in volcanic rocks. This and the two following minerals are included in the group of minerals called zeolites, because they give off so much water when the powder is heated with the blowpipe that they appear to boil.
Analcite. —Isometric. H, 5. G, 2.3. Colour, white. Streak, white. Lustre, vitreous. Composition, NaAlSi 2 O6H 2 O.
Occurs in fairly large crystals in alkaline rocks near Dunedin. Natrolite.—Orthorhombie.
It is found in the form of needle-shaped crystals in the basalts at Caversham.
Muscovite. —Monoclinic; generally hexagonal plates with welldeveloped basal plane. Cleavage, basal, highly perfect. H, 2.5.
20
GEOLOGY OF NEW ZEALAND.
G, 2.8. Colour, grey. Streak, white. Lustre, pearly. Composition (HK)AISiO4.
Occurs widely in schists and granite. Plates 1 inch thick and 10 inches in diameter have been obtained from granite near the head of George Sound. Often called potash-mica.
Biotite.—Monoclinie; has the same form as muscovite. H, 2.5. G, 3. Colour, black. Streak, white. Lustre, submetallic. Composition, variable; a silicate of aluminium and iron, with some potash and water.
Occurs commonly in granites and in many schists. Some fine specimens have been obtained from Port Pegasus, Stewart Island. Often called magnesia-mica.
Chlorite. —A group of micaceous minerals that give a green colour to many schists and some decomposed igneous rocks. Clinochlore is the commonest variety in New Zealand. Form and cleavage as in muscovite. H, 2.5. G, 2.7. Colour, green. Streak, greenish-white. Lustre, pearly. Composition, variable, but typically H5Mg 5 Al 2 Si O18.
Occurs abundantly in schists in Otago.
Fuchsite is a bright-green mica containing chromium.
It has been found at Rimu (near Hokitika) and in Central Otago. Serpentine. Monoclinic, but usually massive. H, 3. G, 2.5.
Colour, pale green to black. Streak, white. Lustre, greasy. Composition, H 4 Mg 2 S i2 O9 .
Occurs abundantly, forming rock-masses in the Dun Mountain, at the Red Hills in Nelson and Otago, near Browning's Pass, and elsewhere. It is also common as an alteration-product of olivine in basaltic rocks.
Bowenite is a transparent variety with an apple-green colour. It has a lamellar structure. Its hardness reaches 5.5. This is the tangiwai of the Maori; it is often confused with nephrite. The mineral occurs in veins in talc at Milford Sound.
Chrysotile, often called asbestos, is a fine fibrous form of serpentine. It occurs at Mount Arthur (in the north-west of Nelson) and in other masses of serpentine rocks.
Talc.—Orthorhombic, but usually massive or in tabular forms. H, 1. G, 2.7. Colour, apple-green to white. Streak, white. Lustre, pearly. Composition, H 2 Mg 3 Si 4O12 .
Occurs at the Caples River, Lake Wakatipu, and many places in Westland.
21
GEOLOGY OF NEW ZEALAND.
Glauconite. —Amorphous. H, 2. G, 2.3. Colour, greyishgreen to blackish-green. Composition, a hydrous silicate of iron and potassium.
Occurs as green grains in marine sandstones. In New Zealand is very abundant in the sandstones of the Oamaru system, beneath the limestone.
Kaolin.—Monoclinic, but usually without any appearance of crystalline structure. Colour, white to yellow. H, 2. G, 2.6. Composition, H4Al 2 Si 2 O9.
Occurs abundantly as a decomposition-product of many kinds of feldspar, but principally of orthoclase. With sericite (finely divided muscovite), which is also a result of the decomposition of feldspars, it forms nearly the whole of ordinary clays. Pure kaolin is called pipeclay.
Halloysite is a similar substance with more alumina and less silica. It occurs as a decomposition-product of alkaline rocks near Dunedin.
Sphene, or Titanite. —Monoclinic. H, 5.5. G, 3.4. Colour, white to brown. Streak, white. Lustre, adamantine. Composition, CaTiSi O5.
Occurs as minute embedded crystals in granites and other acid rocks. Is abundant in the granites of Separation Point.
Apatite.—Hexagonal. H, 5. G, 3.2. Colour, white to pale green. Lustre, vitreous. Composition, Ca 3 P 2 O8.
Occurs as minute needles in nearly all igneous rocks. An earthy form, called phosphate rock, or phosphorite, occurs to an important extent at Clarendon, near Milton, in association with Cainozoic limestones.
Vivianite. —Monoclinie, but often earthy. H, 2. G, 2.6, Colour, blue. Streak, bluish-white. Lustre, vitreous. Composition. F e3P 2 O88H2O.
Occurs in the earthy form in many swampy areas throughout the Dominion. Crystals have been found in moa-bones near Dunedin.
Barite. —Orthorhombic. Cleavage, basal and prismatic. H, 3. G, 4.5. Colour, white. Streak, white. Lustre, vitreous. Composition, BaS O4.
Does not occur widely in New Zealand. Simple tabular crystals are found in some of the mines at the Thames. In other places crystals are found occasionally in geodes or cavities.
22
GEOLOGY OF NEW ZEALAND.
Fig. 4.—Heaps of Rock Phosphate mined at Clarendon, Otago.
Lent by Department of Mines, New Zealand.
23
GEOLOGY OF NEW ZEALAND.
Gypsum.—Monoclinic. Cleavage, clinopinacoid. H, 1.5. G, 2.3. Colour, white. Streak, white. Lustre, pearly. Composition, CaS O42H 2 O. Crystals are often twinned with the clinopinacoid as the twinning and composition plane.
Occurs at the Thames as good transparent crystals (selenite), and in many localities as encrustations on rock-surfaces. Masses of compact gypsum are unknown in New Zealand.
Melanterite. —Monoclinic. H, 2. G, 1-9. Colour, pale green. Streak, white. Composition, FeS O47H 2 O. Taste, astringent.
Occurs in large masses in many of the mines of the Thames region. It is deposited rapidly from the mine-waters.
Epsomite.—Delicate white fine needles. H, 2. G, 1.7. Soluble. Taste, bitter. Composition, MgS O47H 2 O.
Occurs in many of the Thames mines.
Alum. —Isometric.
A massive white substance that occurs in many hot-spring districts is a true alum, or a closely allied substance. It is particularly abundant at Waiotapu and Orakei Korako. Occurs also in association with pyritous shales in various localities.
Alunite. —Rhombohedral. H, 3.5. G, 2.6. A fibrous massive or earthy substance. Colour, usually white. Composition, K 2 O3A l2O34SO36H 2 O.
Occurs at Rotorua.
Alunogen. — A colourless soluble substance. Composition, A l2S 3O12 I 8 H 2 O.
Occurs with many brown coals.
Scheelite. —Tetragonal. H, 4.5. G, 6. Colour, white. Streak, white. Lustre, vitreous. Composition, CaW O4.
Occurs in several localities in quartz veins that penetrate schist rocks. Found in Otago at Glenorchy and Alexandra; also in Marlborough.
Ambrite,—A substance probably referable to this mineral occurs rather frequently in the lignites of Otago, and generally in the coals and lignites throughout the Dominion. It is reddish-yellow in colour. H, 2. G, 1. It is clouded and fissured.
Kauri-gum.—This is widely distributed in the clays and soils of the country extending from the Thames to the North Cape. Often clear and transparent, with a pale-yellow colour. If impure, it is whitish or blackish in places.
3—Geology.
24
GEOLOGY OF NEW ZEALAND.
Petroleum. —In its natural occurrence this substance is a mixture of many hydrocarbons that differ widely in volatility. The substance is viscous, and usually of a dark-brown colour. It generally occurs in beds of sandstone, but appears to be formed by the destructive distillation of shales that contain remains of fish and other marine animals.
In New Zealand petroleum occurs at New Plymouth, near Greymouth, and in the Poverty Bay district.
The following minerals occur in New Zealand, but are not of sufficient importance to merit special description:—
Yesuvianite. Dolomite. Rhodonite.
Bornite. Elaterite. Spinel.
Calamine. Fuller's earth. Topaz.
Cervantite. Idrialite. Wavellite.
Chiastolite. Iridosmine. Witherite.
Chalcanthite. Kyanite. Wolfram.
Covellite. Native lead. Wollastonite.
Delessite. Native silver. Wulfenite.
Dioptase.
25
GEOLOGY OF NEW ZEALAND.
CHAPTER IV.
RAIN, RIVERS, AND LAKES.
The surface features of New Zealand owe their forms in a large measure to the action of the water that falls oil the surface of the land as rain. Each drop of rain when it falls disturbs particles of the soil. Some of these are suspended, and are carried away with that portion of the drop that runs off the surface. If the rainfall is heavy a trickle is soon developed, and a streamlet is formed. By the union of streamlets, streams and rivers are formed. By means of these the raindrops reach the sea. The water still retains much of the soil that was suspended in it, and upon this material the wearing action of the stream depends. Hard stones lying on loose matter protect it from the action of rain, while surrounding unprotected matter is removed. This protective action finally results in the stones crowning an earth pillar. A few of such pillars occur near Henley (in Otago) and in other parts of the South Island.
Fig. 5.—Erosion Pillar near Henley, Otago.
A cemented stratum at the top of the pillar has protected the material below it from the action of rain.
26
GEOLOGY OF NEW ZEALAND.
River Corrosion. —Sooner or later each particle of soil knocks against the banks of the stream or against the rocks of its bed. Every impact breaks off a microscopic fragment of rock, and thus the bed of the stream is gradually worn down, as by a file. It is this action —continuous day after day, year after year, but especially rapid in times of flood—that has worn down the gullies of streams, the gorges of torrents, and the valleys of rivers. Here we are brought face to face with the main demand of geology—a great lapse of time. During a human lifetime little perceptible change, if any, takes place in any of the stream-beds; yet if we look at the rounded surfaces of the
Fig. 6. —Gorge of Shotover at Arthur's Point.
Typical form of gorge in schist rocks.
rocks past which the stream flows, they are found to be smooth and polished; if we listen by the side of a flooded torrent, above the dull roar of rushing waters we can from time to time hear the sharp knock of stone against stone. The rounded surfaces are evidence of wear-and-tear; the knock of the stones tells a tale of broken fragments, of the supply of new material to the water with which the wearing may be continued. Active work is in progress —time alone is needed for results of enormous magnitude. Everywhere the face of nature
27
GEOLOGY OF NEW ZEALAND.
bears the marks of the hand of time, and the demands of geology cannot be refused.
The action of the stream is extremely rapid on the surface of the rocks over which it flows: layer after layer is worn away, and in time a steep-sided gorge is formed (Fig. 6). It is by this simple action that the profound gorges of Wanganui, Taieri, Manawatu, Otira, and hundreds of others throughout the Dominion have been shaped. This corrosive action cannot continue for ever. In time a level is reached which gives so slight a slope to the sea that the action ceases, or, rather, any further wear is at once repaired by the deposition of other material.
The attainment of the base-level of erosion, as this gentle slope from the source to the sea is called, marks the end of the first, or youthful, or gorge stage of the life - history of a stream (Fig.7(1)). A marked change then takes place. As the wearing action of the stream on its banks now always acts on the same level, the sides of the gorge are undermined, landslips fall, the steepness of the gorge decreases, and the valley changes gradually from a gorge to an open vale with a flat bed over which the stream winds and swings backwards and forwards. This is the mature stage of the valley (Fig.7 (3)). Still there is no rest, for the destructive action will
Fig. 7.—Stages of Stream Erosion.
1. Gorge or youthful stage.
2. Valley in more mature stage.
3. River-valley in mature stage of development.
28
GEOLOGY OF NEW ZEALAND.
continue until the dividing ridges between the streams are worn down, and the whole country is reduced to the level of the base-level of erosion. The plain extends impartially across hard and soft rocks, truncating those that are inclined and coinciding with those that are horizontal. Such a plain is called a peneplain. It is not often of large extent, nor is its surface very even. In New Zealand a small peneplain extends northward from Clinton towards the Blue Mountains. In order that a peneplain should be formed the land must remain at a definite level for an extremely long period. The
Fig. 8. —Upper Rangitata Valley, Canterbury.
In the middle distance is a profile of a cone of gravel carried down by the torrent which flows in Tank Gully.
period that elapses while rivers reduce the surface of a country to a peneplain is called a cycle of erosion.
It has been previously stated that streams carry the particles of soil in suspension until they reach the sea. This is not always true. There is a definite law that expresses the relation between velocity of a stream and the size of the particles of rock that it can carry in suspension. The law states that the size of the particles varies as the sixth power of the velocity of the stream—that is, if the
29
GEOLOGY OF NEW ZEALAND.
velocity of a stream is doubled, its suspending-power is increased 64 times; if the velocity is trebled, a stream can carry particles 629 times as large as it could before. It follows from this that a very slight check to the velocity of a stream causes it to deposit the material that it carried before the check. This happens where a rapid stream joins a slow-going river. The gravel that is brought
down by the stream is not carried on by the river (Fig. 8). It is deposited and accumulates, confining the waters of the river to a narrower channel, and forming a rapid. It is in this way that the rapids in the Wanganui River have been formed, and the fans at the bottom of mountain gorges have a similar origin. At the mouths of rivers the current is stopped. Here all the load is deposited. The amount brought clown annually by a large river is enormous. The Mississippi deposits
Fig. 9.—River-flat of Taramakau from Summit of Turiwhate, 4,000 Feet above Sea-level.
The detritus of the tributaries, Wainihinihi in foreground and Taipo in background, have much constricted the flat of the main stream.
63,000,000 tons annually, the Thames 500,000 tons. The New Zealand rivers have not been properly gauged as yet, but the author has calculated that a small stream—the Water of Leith, at Dunedin —carried down 65,000 tons during a six days flood in 1905.
Effects of Land Elevation or Depression.-It has hitherto been tacitly supposed that the level of the land remains stationary. It is
30
GEOLOGY OF NEW ZEALAND.
evident that if the land were slowly elevated the gorge stage of the streams would last longer; while if the land were slowly depressed deposition would take the place of erosion, or, in other words, aggradation would supersede degradation. A river-valley that had previously been worn down would be filled up, and a gravel plain would occupy the valley (Fig. 9). The river-plains of Southland have this origin, especially the Waimea, Oreti, and Mataura Plains; the upper portion of the Canterbury Plain, the Waimea Plains in Nelson, and the Wairarapa Plain in the North Island are other examples. If elevation succeeds such a depression, the rivers will recommence their work on the gravels that have been laid down. The gravels are easily removed, and as the river impinges first on one bank and then on the other wide flats are formed. The river may swing to one side and not return to a flat that had been formed on the other until its bed is distinctly lower. When the river swings back the old flat may not be entirely removed, and its remains will exist as a terrace. Large numbers of terraces may be seen in many of the valleys of New Zealand, especially where the Canterbury rivers (e.g., the Waimakariri) issue from their mountain-gorges. A movement of elevation always inaugurates a new cycle of erosion.
Other Features of River-valleys.—Many features besides those that have been mentioned are to be found in river - valleys. The course varies in directness in different portions ; a river - valley contracts in some places and widens out into lake-like expanses in others ; reaches of still water alternate with rapids ; here and there is a plunge over a waterfall; and in the bed of a stream the rock may be worn into fantastic shapes, and circular potholes may be found. All these features can in most cases be easily explained, but it is often the case that an explanation that is satisfactory in one instance fails absolutely in others. Bach feature has to be considered in the light afforded by the surroundings, and intelligent observation must be brought to bear on each individual case. At the same time it is advisable to point out the most usual causes for the features that have been mentioned above.
The directness of the course of a river is in the main dependent upon the original slope of the country over which the stream flows. If the slope is steep, as in the case of the volcanic cones of Egmont and Ruapehu, the sides of which have a slope of 30°, the streams rush down with impetuous directness (Fig. 10). Even the slope of the
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GEOLOGY OF NEW ZEALAND.
Canterbury Plain and of the Southland plains is sufficient to make the rivers that flow over them take a straight course. On the other hand, in those districts where the slope of the country is slight the
Fig. 10. —River-system of Part of West Coast, North Island.
Compare the straight courses of the streams that flow down the steep sides of the mountain with the complicated meanders of the streams that flow across the old and elevated marine plain of deposition.
stream has a tortuous course, and flows along a series of recurring meanders. This is particularly noticeable in the Wairoa .River
32
GEOLOGY OF NEW ZEALAND.
(north of Auckland), the Piako, the Thames, the Ruamahunga, and in the Aparima River of Otago. When once a direct or a meandering course has been taken it is maintained whilst a movement of elevation is in progress; but if there is depression sufficient to cause the valley to be filled the stream may adopt a more meandering course over the gravels it has deposited.
The Wanganui River and other rivers of the west of the North Island have complicated meandering courses, though everywhere their valleys are narrow and are bounded by steep cliffs. The country over which they flow is an old sea-bottom that had a surface nearly
Fig. 11. —Anabranch on Waihola River.
Waihola River on left, with anabranch extending to right in middle distance.
level. As it was gently raised the streams meandered across it, and their meandering channels have been incised deeply into the old seabottom. The contrast is well marked in the Patea River, which has a straight course where it flows down the steep slopes of Mount Egmont, and a complicated meandering course where it flows across the raised sea-bottom. A meandering stream is always changing its course; from time to time it wears away the neck of a meander, and its old course round the meander constitutes an anabranch, as in the case of the Waihola River (Fig. 11).
33
GEOLOGY OF NEW ZEALAND.
The alternate contraction and expansion of a stream-valley is often due to the differences in the hardness or resisting-power of the rooks through which the stream passes. In a region of hard rock, where corrosion is slow, the youthful stage of a river may be long maintained, while above or below, where rocks are soft or less resistant, the mature form may have been reached. The Grey and lower Waimakariri gorges are examples. A stream may also be partially blocked by the materials brought by a tributary. The energy of the stream is exhausted by moving this material along, and corrosion proceeds slowly. This is most marked in the smaller mountain-streams.
There may be unequal elevation of the land along a streamvalley. This may cause a stream that has throughout attained the mature stage to return to the youthful stage where the elevation is most rapid. The alternating gorges and plains in the river-valleys of Otago may in part be thus explained. Portions of stream-valleys may have been overdeepened by glacial erosion. The streams that succeed the glaciers fill up the overdeepened portions with gravel, while the water that issues from the temporary lake has no detritus to do its filing-work. Thus a plain is formed in the upper portion, while the gorge below is unchanged. This accounts for the wide valleys between the mountain-ranges, and the narrow gorges at the plains, which characterize the rivers of Canterbury (Fig.12). The most important cause of this contraction and expansion is the formation of younger and of subsequent valleys, to be afterwards described.
For the alternation of reaches of still water with rapids some of the causes outlined above offer sufficient explanation. A change from hard to soft rock in the river-bed, obstruction by detritus of a tributary flowing down a steeper slope, and the work of ice all afford explanations in different localities.
Waterfalls are generally caused by the presence of a hard layer of rock resting on a soft stratum. The corrosion of the soft rock is most speedy. A rapid over the hard stratum results. The hard stratum becomes undermined, and from time to time masses break off. These are soon worn away by the mad swirl of the water below the rocky edge. The Waiangi Falls, at the historic locality in the Bay of Islands, occur where hard basalt rests on softer clays. The Waiwera and Whangarei Falls are similarly explained. The
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GEOLOGY OF NEW ZEALAND.
Huka Falls, on the Waikato, occur where a hard mass of siliceous rock rests oil softer scoria. The Mataura Falls rush over a hard stratum of resistant sandstone with softer shales below. The picturesque miniature falls that one sees on the cliff banks of the Wanganui are a feature of a stream-valley that is still in its youthful stage.
Fig. 12. —Map of Rangitata River.
Compare the broad valley in the upper part, due to glacial overdeepening, with the narrow gorge are Moraines cover the area between the dotted lines.
Here the main stream, with its large body of water, corroded its bed more rapidly than its tributaries with their smaller volumes. The valleys of the latter are left behind in the filing process, and
35
GEOLOGY OF NEW ZEALAND.
remain hanging on the cliffs. Similar features are very noticeable in the Taieri Gorge. This is only one class of hanging valley, but not the main class, to which the lofty falls of the West Coast Sounds belong.
In rapids which pass over a virgin bed of rock, fantastic shapes of rocky hollows and projections are formed. The swirl of the waters as they rush hither and thither causes a wearing action in everchanging directions, and prevents an even surface from being formed.
Most remarkable are the potholes. These are hemispherical hollows with loose stones in them. Usually at the base of some little fall in a rapid an eddy is formed which whirls stones round and round. Their movement wears out a basin, the size of which depends upon the time during which the position of the small fall remains unchanged. Splendid examples are found at the Mataura Falls; and the "Bell Rock," on the Te Anau-Milford track, takes its name from a pothole excavated in an enormous boulder that has since been overturned.
Deposition.
Under ordinary conditions all the material carried in suspension by rivers is deposited at the river-mouth. The current is here completely stopped, and the power of suspension is lost. The gravel is deposited directly the strength of the current is reduced, but the finer material may be carried out some distance from the shore. The larger grains of mineral particles that constitute the sand are laid down outside the gravel, and then comes the mud, which all sinks to the bottom within a few miles of the beach.
If the coast-line remains at the same level it is gradually built outwards, and the gravel extends over the area on which sand was previously deposited, the sand invades the area of mud, and the limits of the deposit of mud extend further seawards. Thus it is evident that there will be a normal order of deposit of sediment, the mud being the lower, on that the sand, and on the top the gravel (Fig. 13). Movements of the coast-line up or down will cause these deposits to be laid down in quite a different order. If the land is being depressed at the same rate as the gravel is being deposited, a bed of gravel hundreds of feet in thickness will be formed, and the sand and mud will remain quite separate from one another. If the movement of depression is more rapid than the deposition of the gravel, the materials will be laid down in the reverse of the normal order —that is to say,
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GEOLOGY OF NEW ZEALAND.
the mud will lie on a deposit of sand, and the sand will in its turn rest on gravel. On the other hand, rapid elevation will cause deposition to take place in the normal order, but the amount of any kind of sediment at a particular spot will be relatively small. Thus, from the observation of the order in which different kinds of sediment succeed one another, and of their relative thickness, valuable conclusions may be drawn as to the land-movements that were in progress at the time that the sediments were being deposited.
Fig. 13.. —Showing Deposition of Sediment.
All sediments, and particularly the finer ones, are laid down in definite layers or strata. These are distinct because the rate of deposition is constantly varying, and the nature of the material supplied by the rivers changes from time to time. Its tributaries will in many cases derive their waters from districts over which the rocks are different. They will therefore be supplied with different kinds
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GEOLOGY OF NEW ZEALAND.
of sediment, and if they are not all in flood at the same time the average nature of the sediment carried by the river will vary somewhat.
These layers remain conspicuous in rock-masses, and are then called strata. When laid down they are horizontal, or nearly so. The inclination that they often have after the rocks have been elevated is one of the chief facts employed in determining the nature of the rock-movements by which the earth's crust has been disturbed.
As before mentioned, gravels are deposited before the current of a river has been completely checked. The variable nature of the
Fig. 14.—Stratified Rocks of Oamaru Age, Amuri Bluff.
current by which they are laid down causes it to scour out hollows in material already deposited, and these will be afterwards filled with more gravels, often arranged in inclined layers. This is the origin of current or false bedding, which is seen in many of the gravels of Pliocene age at Wanganui.
Beheaded Rivers.
It- has been previously stated that the direction of the flow of streams is the result of the direction of the steepest initial slope in the country over which they flow. This is well seen in the streams that course radially outwards down volcanoes {e.g., Mount Egmont), and in the
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GEOLOGY OF NEW ZEALAND.
rivers that flow across the Wanganui marine plain. There are, however, numerous instances in which a river seems to flow in opposition to the general slope of the country, or, rather, instances in which rivers leave level country or mature valleys and enter tortuous gorges through mountain-ranges. The Manawatu River leaves the flat country at Woodville to enter the deep gorge between the Tararua and Ruahine Mountains. The Waikato leaves the plains that slope gently to the Firth of Thames and strikes straight through the Taupiri Range. The Mataura forsakes the mature valley that forms the continuation of the Wakatipu basin, and passes through the twists and turns of the gorge through the Dome Mountains. In each of these cases the gorge portion of the course must be regarded as a new valley that has been cut by the activity of some stream after the main valleys had been formed. The river flowing through the new valley had a shorter
Fig. 15.—Diagrams illustrating Drainage.
1. Diagram to illustrate parallel consequent streams on a sloping surface.
2. Diagram to illustrate readjusted drainage. A, B, strata of hard rock inclined to surface of country; D, C, escarpment formed by outcrop of B; a, b, c, consequent streams; d, e, subsequent streams; f, obsequent stream; g, water-gap; h, h, air-gaps.
course to the sea than the older rivers. This shorter course to the sea gave it greater velocity, and hence a greater erosive power. It was therefore able to deepen its valley and extend its head-waters backwards, thus beheading the valleys of the main streams. A tributary of the Nokomai took the waters of the Mataura. A tributary of the Pohangina committed piracy on the waters of the Manawatu, which must have flowed out to the east coast previously, but in this case elevation of the Puketoi Hills to the east possibly slackened the flow of the Manawatu in its upper reaches.
Other conditions, such as the occurrence of soft belts of rock or the variation of rainfall-supply to different streams, may favour rapid erosion and piracy (Fig. 15). A slight inclination of stratified rocks of
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GEOLOGY OF NEW ZEALAND.
Fig. 16. —Clutha River and Upper Part of Mataura River.
The upper part of the Mataura from 1 to 2 lies in a continuation of the Lake Wakatipu Valley; from 2 to 3, the Dome Gorge; from 3 to 4, the Nokomai tributary.
4—Geology.
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GEOLOGY OF NEW ZEALAND.
different hardness is specially favourable if the original streams flowed across the strike. The tributaries that join a stream in soft rock above a hard belt perform more rapid corrosion than the main streams that cut through the hard rocks. They will therefore behead all but the strongest of the main streams, and many of the old gorges through the hard rock will become deserted. Such deserted gorges are airgaps, and those that are still occupied by rivers are water-gaps. For such active streams as those tributaries described the term subsequent is employed, while the original streams are termed consequent. As the beds of the subsequents are more and more
Fig. 17.—The System of Streams in the Hilly Country near Nugget Point, Otago.
The original consequent streams probably flowed radially outward. Only the larger streams have maintained this course; the minor streams are mainly subsequent, and run along the strike of the softer strata, which are highly inclined.
owered, small streams will flow from the air-gaps to them. To streams such as these the name obsequent has been applied.
The Mataura appears to afford an example of what has been just described. The strata dip steeply E.S.E. The old consequent valley, through which the railway-line now passes, runs S.S.W. across the
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GEOLOGY OF NEW ZEALAND.
strike.' The gorge through the Dome Pass is a subsequent valley, and is parallel to the strike. Down the old air-gap the obsequent Pass Creek runs to join the Mataura before it enters the Dome Pass (Fig. 16). In a district where this structure is pronounced an adjusted system of drainage results, and, except for a few major rivers that have been able to maintain their courses across the strike, the streams have courses parallel to one another and in the direction of the strike. A system of partially adjusted drainage is most noticeable in the Nuggets district. In every case the side of the valley on the rise side of the strata has a slope parallel to the dip. On the other side hard strata of rock crown the crests with a bold escarpment. From the map on page 40 it will be seen that nearly all the smaller streams in the Nuggets region run in parallel courses E.S.E. and W.N.w., while a few of the larger ones run at nearly right angles to this direction. The strike of the rocks is in the same direction as the smaller streams, which in reality run along the strike of the softer strata (Fig. 17). When the land-surface first emerged the streams ran outwards from it, but, as time passed, the influence of the relative hardness of the strata asserted itself, and all those steamlets that ran along the strike of the softer strata maintained their courses, and from time to time captured the waters of the streamlets that ran counter to the strike. Only the larger streams, such as the Glenomaru and Romahapa, have been able, in virtue of their rapid corrosion, to maintain their directions across the strike.
Lakes.
In many river-valleys there are found expansions, often of so large a size that no current is perceptible; these expansions are lakes. Their origin is often obscure, but it is generally true that they are not due to the activity of the stream itself. A lake must be regarded as a transient feature of a landscape, for the streams that enter it all contribute their load of sediment, which gradually fills it up and converts it into a flat alluvial plain. The chief causes of lakes are as follows: —
1. Many of our small lakes result from the damming action of some obstruction in the course of a river.
(a.) Rapid deposition by a small tributary. Lakes Pearson and Grassmere, in Canterbury.
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GEOLOGY OF NEW ZEALAND.
(b.) De-position by the main stream across the mouth of a tributary. Lake Whangape, in the Waikato.
(c.) Deposition of glacial moraine in a drainage-channel. Lakes Pukaki, Tekapo, Ohau, and (partly) Manapouri and Te Anau.
(d.) Landslip blocking a river. Lake Ada.
(e.) Wind-blown sand blocking a stream. Lake Virginia, and Lake Westmere, Wanganui.
Fig.18.—Waiau River, flowing out of Lake Manapouri.
(f.) Sea-drifted sand forming a bar across a stream. These are usually salt-water lagoons. Lake Ellesmere, Totara Lagoon in Westland, and Lake Mahinapua.
(g.) Lava-flows extending across stream-valleys. Lake St. John in Auckland, Lake Rotoaira, and Lake Omapere.
2. Lakes are frequent in volcanic districts, where they may occupy craters or more irregular explosion cavities.
(a.) Craters. The Blue Lake on Tongariro, and Lake Takapuna.
(b.) Explosion cavities. Lake Rotomahana, probably Lake Rotorua, and other lakes of the thermal district.
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GEOLOGY OF NEW ZEALAND.
3. Lakes may occupy infallen areas of the earth's crust.
(a.) Lake Tawpo has been supposed to have this origin, for the immense eruptions in its neighbourhood may have disturbed the equilibrium of the earth's crust locally, though the recent formation of Lake Rotomahana by a volcanic explosion suggests this origin for Lake Taupo also.
Fig. 19. —Tarn at Foot of Avalanche Slope, North Branch of Clinton River.
(b.) Soluble rocks may be dissolved by percolating water, and huge cavities may be left into which the overlying rock may afterwards fall. Such is the origin of the small lagoons in the Oamaru district, and perhaps of Lake Waikaremoana.
4. Glaciers may erode basins in rock. Lake Harris is an example on a small scale, and Lakes Wakatipu, Te Anau, and Manapouri
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GEOLOGY OF NEW ZEALAND.
examples on a larger scale. In the last two examples the size of the lake has been increased by the deposition of moraine across the outlet (Fig. 18). It must be remembered that many authorities ascribe the origin of these lakes to the subsidence of blocks of the earth's crust, and to the formation of graben, or trough faults. The deposition of moraine is often very irregular, and in the basins formed by such irregularities lakes of some size may be found. Small lagoons occur in the Tasman Valley. Lakes Mapourika and lanthe, in Westland, are examples on a larger scale. Tarns are formed at the foot of avalanche slopes in many valleys of south and west Otago. Though the early avalanches of spring bring down no rock debris, as the summer advances much of this material accumulates round the margins of the first avalanches. When the ice melts a ring of debris appears, and within this a tarn is formed (Fig. 19).
5. Occasionally the meanders of a river-valley are cut off by the erosive action of the river, and the entrance and exit of the anabranch thus formed may become silted up and a semicircular lake formed. In New Zealand there are no examples on a large scale, though small ones are not infrequent.
The erosion performed by lake-waters is of a nature similar to that of the sea, but is oil a much smaller scale because of the comparatively insignificant size of the waves and the absence of tidal and ocean currents. Shelves and terraces are very common on lakeshores, for the level of the outlet is constantly being lowered by the corrosion of the outlet, and the level of the lake is thereby reduced.
Deposition in lake-basins is similar to that in marine areas, though, the area of water is so limited that terrigenous sediments only and principally those of a relatively coarse nature, are deposited.
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GEOLOGY OF NEW ZEALAND.
CHAPTER V.
PERCOLATING WATER: CHEMICAL ACTION.
About a quarter of the rainfall flows off the surface of the land. The larger portion of the other three-quarters sinks into the ground. It carries with it oxygen from the atmosphere as well as carbonic-acid gas, and, armed with these chemical reagents, it produces the most startling and important effects. The action is slow, and, even where it is most rapid, little, if any, effect can be observed during a lifetime. The results, therefore, are not known from actual observation of the whole process in any single case. Rocks in various states of decay, however, are to be seen, and the chemical action of water, oxygen, and carbonic-acid gas on the chemical compounds of which rocks are composed is well known. It is on observation and knowledge such as this that the following statements and reasoning are based.
Feldspars are readily attacked; alkaline carbonates are formed and pass away in solution; aluminium-silicate combines with water and forms kaolin; silica is liberated, but as it is fairly soluble in solutions of alkaline carbonates most of it goes away in the water that soaks gradually through the rocks. Finally, in place of the feldspar there remains only kaolin, or clay, and perhaps a little silica. Augite, hornblende, and olivine are also readily attacked, especially the last. The iron-silicate that they contain is first changed into carbonate, and then into oxide, or iron-rust. The calcium-silicate is changed into carbonate, and is removed in solution. The mag-nesium-silicate combines with water and forms serpentine. Any aluminium-silicate that there may be in the mineral combines with water and forms kaolin. Many minerals are almost, if not quite, unaffected by chemical action. Of the common minerals quartz and muscovite remain practically unchanged.
Where chemical action does take place it is evidently most rapid where the percolation of water, with its load of oxygen and carbonicacid gas, is most abundant —that is, near the surface and on the margin of those crevices that penetrate all rook-masses. The surface
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GEOLOGY OF NEW ZEALAND.
of a rock, therefore, weathers down into clay, which is coloured red when the rock contains much augite or other iron minerals, but is of a lighter colour where the rock is relatively poor in such minerals. The red clays of the Waikato and the North of Auckland are formed from basalts. The lighter-coloured clays of Dunedin represent original phonolite rock, and in regions where granite has weathered away a pure white pipeclay may result. A little distance from the surface the rock is not completely changed into clay, but hard cores, generally noticeably spherical, are found embedded in the clay (Fig. 20).
As was stated before, the water, percolating along the natural crevices of rock and along the margins of the crevices, changes the rock into clay. At the corners of the blocks into which the rock is divided three surfaces meet, and the action is therefore three times as rapid there as on the side of the block. This causes the corners to be gradually rounded off, and in time the block becomes nearly round.
Fig. 20.—Spheroidal Weathering of Triassic Greywacke, Railway Cutting, Dipton, Otago.
If a section that shows a considerable depth of rock be found, it will be seen that under the surface clay the solid rounded cores become larger and more and more numerous the greater the distance from the surface. At the same time the rounding is found to be less complete, until finally the unaltered rock, traversed only by natural crevices, is reached at the bottom of the section. The very general
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GEOLOGY OF NEW ZEALAND.
spherical form of the weathered surfaces of the cores has given rise to the term spheroidal weathering to denote the action described. The percolating water is often robbed of its oxygen before it has penetrated far into the rock. When it has lost its oxygen it ceases to produce a red iron-oxide, but the formation of serpentine and of clay may continue to a greater depth. If the rock contains much magnesia it is changed to a green colour, and the clay also may have a green
Fig. 21.—Miocene Limestone, Te Waro, Whangarei.
The effect of rain-water and its dissolved carbonic acid is seen in the fantastic shapes into which the limestone has been worn.
colour. Spheroidal weathering is very pronounced in many volcanic rocks throughout the Dominion, and even in the greywackes and sandstones of the Mesozoic age, especially in Southland.
Limestone is even more readily attacked than rocks that contain feldspar. The carbonate of lime of which, when pure, it is wholly formed is changed into bicarbonate, which is a soluble compound, and therefore passes away with the water. The little narrow
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GEOLOGY OF NEW ZEALAND.
crevices through which the water percolates are enlarged, and in time huge passages, perhaps a mile or so in length, are dissolved out. This is the origin of the caves that are so frequent in limestone country.
The surface of the limestone is gradually dissolved also, but irregularly, for there is always a depression where a crevice reaches the surface. This action is called chemical erosion (Fig. 21). When the limestone is impure, the grains of sand are left behind and form a deposit that sinks into the irregularities of the surface subject to solution. The dissolving action produces so great an effect that in some districts streams flow into crevices and issue again as huge springs some miles away. Such springs occur at Bubo, near Takaka.
It has just been stated that the flow of water along crevices in limestone gradually enlarges them, and subterranean tunnels, often of great width and height, are formed. To the appearance of these caves the formation of stalactites and stalagmites often gives a great beauty and variety. Stalactites are long pendulous masses that hang downwards from the roof of a cave; stalagmites are columns that rise up from the floor of the cave. Both of these objects owe their formation to the precipitation of calcium-carbonate from the bicarbonate that the water holds in solution. The rain-water that sinks into the limestone rock contains in solution some carbonic-acid gas that it has dissolved out of the atmosphere; charged with this it changes a little calcium-carbonate into bicarbonate in accordance with the equation H2O + CO2 + CaCO3 = CaH2(CO2) 2 (calciumbicarbonate). This is retained in solution until some of the percolating water issues as a drop from the roof of the cave. In this situation some calcium-bicarbonate undergoes decomposition. Carbonic-acid gas is given off, and the compound is changed back again to its original components. The calcium-carbonate, being insoluble, is precipitated, and forms a small projection on the roof of the cave. Drop follows drop, and the projection is increased in size. This action is extremely slow, but in time a long pendent stalactite is formed.
When a drop of water falls to the floor of the cave, a little more carbonic-acid gas is given off and a little more calcium-carbonate is precipitated. This gradually builds up an erect stalagmite, and in time stalactite and stalagmite may unite and form a complete column.
The rate of growth of these objects is extremely slow, but at present
49
GEOLOGY OF NEW ZEALAND.
only rough estimates have been made of the actual rate. The sides of the cave are always covered with a crust of material formed in the same way as the stalactites. The Waitomo caves, in limestone of Cainozoic age, and others near the Mokau River are well known. Less-perfect caves are found in many other districts where the limestone is thick and massive. In the Pikikiruna Range, near Motueka, there are extensive caves of a similar nature in marble rock of Ordovician age.
The action of percolating water is not entirely of a destructive nature. There are many conditions under which deposition instead of solution may take place. This is most frequent in beds of limestone, especially round fossil remains. When the solution of biearbonate reaches the organic matter a little carbonate is deposited. When once deposition has commenced it may continue for a long period, and coat after coat of carbonate be deposited. The concretion, as such a mass is called, gradually grows and maintains a spherical shape if the rock has a structure that allows water to percolate with equal ease in all directions towards it. If, however, water percolates more readily along one plane than along others, the concretions grow more rapidly in that direction than in others. Owing to various changes in the circulation of water the concretions may have very different forms —in many cases so peculiar as to baffle description.
The well-known Moeraki boulders are concretions. The papa cliffs on the banks of the Wanganui River contain many concretions which are worn out of the banks by the running water. Since they are harder than the marl in which they are embedded, they constitute the boulders in the tributary streams. They are carried down in large numbers by them, and in many places partially block the channel of the main stream.
Sometimes concretions appear to undergo contraction, and curved radial crevices arise which increase in diameter towards the centre of the concretion, and thus divide it into a number of sections. Such concretions are called septarian nodules. The boulders at Moeraki are examples of these.
Carbonate of calcium is not the only material of which concretions are formed. In different rocks there are many substances that form them. Pyrite occurs as concretions in the leaf-beds of Fraser's Gully, Dunedin. Flint concretions are common in limestones of Campbell
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GEOLOGY OF NEW ZEALAND.
Island, the silica of which they are formed being supplied by the spicules of the sponges that occur in some numbers in the limestones. Limonite concretions are common in the auriferous drifts of Otago.
The cementation that consolidates many rocks of a fragmentary nature is also due to the action of percolating water. The action is dependent upon the solution and subsequent precipitation of some of the materials of the rock. Silica, calcite, and limonite are the commonest natural cements (Fig. 22). The rusty - coloured quartz grits that rest on the coal are siliceous gravels cemented by limonite. The quartzites of Central Otago and Landslip Hill have a siliceous cement, and the limestones that occur throughout the country have a cement of calcite. It must be remembered that the hard greywackes of which the mountain-ranges are mainly composed are not cemented, but welded.
Fig. 22.-Jurassic Conglomerate, Winton, Otago.
Springs.
The water that soaks into the rocks percolates gradually through them-it may be, to great depths. Crevices often join together, and an underground stream of some size may be formed. If any crevice extends to the surface the stream may flow through it and, issue at the surface of the ground as a spring. In all cases, however,
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GEOLOGY OF NEW ZEALAND.
the orifice from which the water escapes must be at a level lower than the area over which soakage takes place. The simplest case is that in which the soakage-area is the surface of a hill that has a layer of clay or impervious rock sloping outwards near its base. If the upper rock is pervious, the rain soaks through it until the clay is reached; it then runs along the surface of the clay, and issues as a spring where the clay reaches the surface of the ground. Where the country has such a structure a landslip is particularly likely to take place, for the passage of the water along the surface of the impervious clay or rock lubricates it, and all the overlying material may slip along the sloping surface. Springs are often found in districts where the rock-structure is much less definite. Even if the water has
Fig. 23. —Diagrams showing Origin of Spring-waters.
1. Diagram of spring: C, area of soakage; B, pervious rock; A, impervious rock; S, spring.
2. Diagram of hot spring in mountainous country: C, soakage-area; A, subterranean channel; HS, hot spring.
3. Diagram of artesian spring: A, pervious layers; B, impervious layers; S, spring.
soaked to a great depth, it may return to the surface through a natural channel that has an outlet at a lower level than the soakage-area. If the water has soaked to a deep level it will have acquired the temperature of the rock at that level, and will issue as a hot spring. It is thus that the hot springs in the Copland, Waiho, and Taipo Valleys, in Westland, have been formed.
Artesian springs have a very similar origin. The country in which they occur is composed of gently sloping layers of pervious
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GEOLOGY OF NEW ZEALAND.
and impervious material inclined in the same direction, but rather more steeply than the surface of the country itself. The rain soaks into the pervious layers and flows down them towards the sea. The outlet below the sea-floor may be partly blocked by a deposition of mud, and in such case the water accumulates in the porous layer of rock. If there is a natural outlet to the surface at a level lower than that of the soakage-area, or if an artificial outlet is provided by driving down a pipe, the water will issue from the outlet, and an artesian spring is the result. The important artesian districts in New Zealand are the Canterbury Plain near Christchurch and the Hawke Bay Plains near Hastings, the Hutt Valley, and Wanganui.
It must not be supposed that the landslips on freshly cleared hills are due to the cause previously described. They occur during heavy rain, which soaks into the soil and surface clay and renders them almost liquid. The decay of the roots that previously held the soil together deprives it of its former tenacity, and the soil and clay flow down the hillsides in a stream or slip. Such slips are extremely conspicuous on the sides of the foothills of the Tararua Mountains.
GEOLOGY OF NEW ZEALAND.
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CHAPTER VI.
THE ATMOSPHERE AND PHYSICAL ACTION.
Great though the force of the wind may be in exposed localities, it is, so far as direct action is concerned, of relatively little importance. The hardness of rock-masses and their high specific gravity make them stand firm and unmoved by the most violent tempests. Exceptions are, however, found where rocks are exposed to tumultuous gales, and are of a flaky nature, as on the west coast of the Campbell Islands. There the ground for 100 yards or more from the face of the cliffs is strewn with flakes, sometimes weighing over a pound, which have been torn by the wind from the solid rock, and have been scattered
Fig. 24.—Illustrating the Movement of Sand-dunes under the Influence of a Prevailing Wind.
over the land to the leeward. Indirectly, however, the wind is one of the most important of all geological agents. It controls the rainfall, and disturbs the placid surface of the sea, raising the great waves that do so much work on the coast-line, where mountains are washed away and their remains scattered broadcast over the ocean-floor. The effects of the action of rain and waves will be considered in some detail later.
Here, however, it is necessary to notice the movement of the sand, which is suspended by the wind as it passes over the coast-line or over dry and sandy desert areas. But little of the sand is actually suspended, though much is rolled along the surface until it reaches a sheltered place. In the course of time hills are built up from the sand that the wind takes from the edge of the sea at low tide. These sand hills or dunes have a gentle slope on the side that faces the direction of the wind, but an abrupt slope on the other side, for the sand simply rolls over the crest of the dune, and lies at as steep an angle
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GEOLOGY OF NEW ZEALAND.
as the size and shape of the grains will allow—that is, at the angle of rest for the sand.
Sand-dunes are in constant motion, though, their progress is slow. Whenever the windward face is dry the sand on its surface is carried or rolled along, and is deposited on the leeward side of the dune. This slow and measured transportation of sand enables the dunes to advance almost irresistibly inland. Fences and buildings are buried, and fertile fields become barren wastes. In its progress the sand may be raised to great heights. At the Taiaroa Heads, Dunedin, the sand has within forty years been blown to the top of a hill 500 feet high. At Sandymount it passes over a rock saddle 1,000 feet above sea-level, When these grains of sand driven before the wind come into contact with any solid object they soon demonstrate their power. Each grain knocks off a solid particle; but so minute are these particles that the surface of the rock is polished by the sand-blast. Stones lying on the sandhills in localities where the winds blow strongly and alternately from opposite directions acquire peculiar and characteristic forms. The two surfaces facing the direction of the winds are worn down, and the polished faces meet in a sharp ridge along the summit of the stone. Excellent examples of such aeolian stones are found at Lyall Bay (Wellington), at St. Clair (Dunedin), and elsewhere.
Fig. 25.—Loess lying on Volcanic Scoria, Governor's Bay, Lyttelton Harbour.
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GEOLOGY OF NEW ZEALAND.
Even hail and snow driven by the wind are able to effect similar results. On several of our mountain-passes—Browning's Pass, for instance—the western faces of the rocks are worn smooth by the frequent assaults of stinging hail to which they are subjected.
It is well known that wind is capable of lifting and carrying a large quantity of fine dust. The flooded Canterbury rivers deposit large quantities of mud on the wide beds over which they flow. When the floods subside, the dust is transported over the neighbouring lands. It is caught by the grass and vegetation, and has thus formed the rich tracts of land of the Canterbury Plain. The deposit of dust is thick — often 10 feet or more (Fig. 25) —and it is evident that a great lapse of time would be necessary for the deposition of such a thickness. There are, however, reasons for thinking that the mountains along the west of New Zealand were once higher than at present. The rivers would then be subject to more frequent and more destructive floods; the gales would be more violent, and all conditions would conspire to bring about a more rapid deposition of dust. The beds of dust formed in this way are called loess. In some parts of the world they are of great thickness. In Mongolia they are at least 300 feet thick. In New Zealand, loess is confined almost entirely to Canterbury.
Fig. 26.—Frost Erosion on Mountain-pass near Lake Te Anau.
5 —Geology.
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GEOLOGY OF NEW ZEALAND.
Fig. 26A.—Summit of Mount Aurum, Shotover, showing Result of Shattering Action of Frost.
Lent by Department of Mines, New Zealand.
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GEOLOGY OF NEW ZEALAND.
The changes of temperature to which rocks are subject have great influence upon the rate at which they decay. All rocks expand when heated, and contract when cooled. This constant change in volume causes strain within the rock. This brings about the formation of small crevices, which gradually enlarge. In time the rock breaks into fragments. This is most noticeable on mountain-sides where no vegetation covers the surface of the rock and there is no protecting mantle of soil. In such localities, too, the changes of temperature are great and very rapid. The heating effect of the sun's rays is more intense than on the flat lands, and during the night the radiation is rapid. This action accounts for much of the broken material of the scree slopes which clothe the sides of many of our mountains, but it is greatly intensified by the action of frost.
Frost. —Water soaks into the crevices of the rocks by day, and freezes when the cold of night comes on. Ice takes up more space than water, and during the change enormous force is exerted on any object that confines it (Fig. 26). The freezing water thus acts like a wedge, and forces the rocks apart. The importance of this action is often seen in mountainous regions, for in the morning, after the sun strikes the frozen slopes, avalanches of rocks and stones soon come hurtling down, and add to the huge stacks of debris that are found on the lower slopes of the mountain.
Lightning is not of the importance that its dazzling appearance and the alarming roll of accompanying thunder suggest. When a flash strikes sand a small tube of fused glassy material is formed, usually only a few inches long. When a rock is struck by lightning only a small hole is drilled in its surface; seldom is there any disruptive effect.
Chemically the atmosphere is of little importance. It is only in the presence of water that chemical action of my consequence takes place.
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GEOLOGY OF NEW ZEALAND.
Showing neve icefall and ninnacled moraine-covered surface of lower nortion
Fig. 27.—Wilkinson Glacier, Westland.
Lent by Department of Mines, New Zealand.
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GEOLOGY OF NEW ZEALAND.
CHAPTER VII.
GLACIERS.
There is a certain level at which the snow that falls in winter is just melted at the close of the summer. This is the snow-line. Its level necessarily varies with the season and is usually different even in countries in the same latitude. The fierce winds of winter allow but little snow to rest on exposed ridges, while in sheltered valleys its thickness may be great. On slopes bathed in sunshine snow melts much, more quickly than in ravines darkened by perpetual shade.
Snow slides off the rocky slopes and accumulates in the valleys. In summer, therefore, tongues of snow creep far down the slopes between bare spurs; yet it is very easy to decide on some elevation
Fig.28.—Upper Part of Tasman Glacier, with the Hochstetter Dome at its Source.
Mount de la Bêche is between the Rudolph and Tasman Glaciers. Medial moraine on Tasman distinct. Photo from 8,000 feet on slopes of Mount Cook.
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GEOLOGY OF NEW ZEALAND.
Fig. 29. —Glacial Region of the Southern Alps.
The great Tasman Glacier is 18½ miles long.
Ft. 1. Hochstetter Dome.. 9,258 2. Elle de Beaumont .. 10,200 3. Mount Green .. 9,325 4. Mount Darwin .. 9,715 5. Malte Brun .. 10,421
I Ft. Ft. 6. De la Bêche .. 9,815 7. Mount Haidinger 10,107 8. Mount Haast .. 9,835 9. Mount Tasman .. 11,475 10. Mount Cook .. 12,349
Ft. 11. Mount Hicks .. 10,410 12. Mount Stokes .. 10,101 13. Mount Sefton .. 10,359 14. Mount Sealey .. 8,631
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whose contour-line would traverse equal breadths of bare rock and snow. In the North Island the snow-line is about 8,000 feet above sea-level, and in the extreme south of the South Island about 6,000 feet.
Above the snow-line the accumulation of snow takes place winter after winter, and from time to time the gathered snow falls in slides or avalanches down the slopes. In the valleys partial thawing at the surface is followed by renewed freezing. This, combined with pressure, gradually converts the snow into ice and forms a glacier (Fig. 28). The ice slowly moves down the valley, but is gradually thinned by melting. Finally it reaches a level at which further advance is prevented by the thawing of the terminal face. The movement of ice has been ascribed to the effect of expansion and contraction as it is alternately warmed and cooled; but it is generally thought that it merely flows as a viscous fluid. The ice is veined with alternate blue and white layers. These probably mark the snowstorms and intervening periods of quiescence. A glacier flows with majestic and ponderous deliberation. The actual rate is slowest near the terminal face, and is faster in the centre-line than near either margin, where it is retarded by friction (Fig. 29).
The imperfect fluidity of the ice causes it to become fissured. The crevasses thus formed are at first transverse, but the outward movement of the ice being greater in the middle than at the sides causes them to become nearly longitudinal near the margin of the glacier. The main cause of crevasses is an irregularity in the bed of the glacier. Notwithstanding the onward flow of the ice, therefore, it is the same region of the glacier that is crevassed year after year (Fig. 30).
The avalanches bring down into the valleys rock-fragments as well as snow and ice. In the upper portions of a glacier the rock is soon covered with fresh falls of snow, but lower down the covering melts, and the rocks become exposed, forming a surface moraine. The moraine-matter is naturally greatest near the side of the glacier. Where two glaciers unite, moraine extends down the centre of the united stream as a streak of medial moraine. As the melting of the glacier becomes more and more complete, moraine is exposed all over the surface of the ice, and completely hides it. This is most conspicuous on the glaciers on the eastern slope of the Southern Alps. Where the ice melts, at its sides and at its terminal face, it deposits the load of
GEOLOGY OF NEW ZEALAND.
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Lent by Department of Mines, New Zealand.
Fig. 31.-Striated Surface or Ice-worn Boulder, Arrow Flat, Otago.
Fig. 30. -Mount Cook from Tasman Glacier.
Showing Hochstetter Icefall. Surface moraine in foreground.
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GEOLOGY OF NEW ZEALAND.
moraine, and huge unshapely masses of lateral and terminal moraine are thus formed. The material of these consists of large and small boulders, all having angular forms. Such material as this is readily distinguishable from other fragmentary matter, even in the earliest geological deposits.
Glacial Erosion.—From time to time boulders fall down the crevices, and finally reach tie bottom of the ice. They are frozen hard into the icy mass, and while firmly ield act as tools-scoring, grooving, and polishing tie rocks over which the ice flows. This action takes place in a vertical and not in a lateral direction (Pig. 33). The rapidity of the action depends upon the weight of the ice, and consequently upon its thickness. This is greatest at a spot rather more than half-way down the length of the glacier. Here the work is most rapid, and a deep basin is worn out.
Fig. 32. —Valley of North Clinton, Lake Te Anau.
A typical glacial valley.
Towards the terminal face, where the ice becomes thinner and thinner because of the melting of its surface, the erosion of the bed is less rapid, and for this reason the floor of the valley rises towards the terminal face. The flow of the ice may be maintained even after this reverse slope is established, for it is the slope of the surface that chiefly affects the flow. So long as there is a surface slope the ice will flow onward.
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GEOLOGY OF NEW ZEALAND.
Fig. 33.—A Striated Glacial Boulder near Queenstown.
Lent by Department of Mines, New Zealand.
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GEOLOGY OF NEW ZEALAND.
Fig. 34.-Boulder of Greywacke showing Scratches characteristic of Ice-work Boulders, Queenstown.
Lent by Department of Mines, New Zealand.
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GEOLOGY OF NEW ZEALAND.
If a large glacier melts so rapidly that its basin is not filled with moraine as the terminal face retreats, a lake will take its place. Lakes Wakatipu, Te Anau, Manapouri, Monowai, and many others in the south-west of New Zealand lie in such basins.
The erosive power of a glacier is greatly increased below the junction of a tributary. This fact emphasizes an important difference between river and glacial erosion. The junction of a tributary with a river does not increase its rate of erosion, for this depends more upon the velocity of the water than upon its quantity. Again, the tributary supplies much detritus to the river it joins, and the
Fig. 35.—Upper Rangitata Valley.
In the foreground a glaciated rock-surface; in the middle distances roches moutonnées.
energy of the stream is partly expended in reducing this material, instead of eroding the bed of the stream. On the other hand, when a tributary glacier joins the main stream the mass and weight of the ice in the main stream are increased, and the valley in which it flows is thereby deepened. Thus steps are often prominent in glacial valleys, and the steps are always found immediately above the junction with a tributary. Such a step is seen in the zigzag of the Routeburn Valley and, to a less-pronounced extent, in many other valleys that
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GEOLOGY OF NEW ZEALAND.
radiate from the lakes and fiords. The vertical erosion of a glacier causes the valley it forms to be trough- or U-shaped, and all prominences on its floor are smoothed and rounded off. They are called rohes moutonnées.
A tributary glacier has its surface in accord with that of the stream that it joins; but, since the thickness of ice in the two streams is very different, the beds over which they flow are at different levels. Hence, when a glacier melts, the valleys of its tributaries will have beds high above that of the main valley. Their waters will tumble in waterfalls into the valley of the main stream. Such valleys are hanging or perched valleys. They are very abundant in the fiord and
Fig. 36.—Upper Rangitata Valley.
Showing roches moutonnées of the Jumped-up Downs.
lake districts. The Bowen Falls, the Stirling Falls, and the Sutherland Falls are excellent examples. Such hanging valleys must not be confused with those that are formed in the margins of youthful river-gorges or with streams that fall over a hard band of rock. Of these, the Niagara Falls, in America, afford an excellent example; and in New Zealand may be mentioned the Waitangi Falls, in the Bay of Islands. The main features of the grand scenery of our lake and fiord region are ascribed to the work of glaciers. The magnificent cliffs that hem in the valleys, the flat floors of the valleys themselves,
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GEOLOGY OF NEW ZEALAND.
the steps that are formed in these floors, and the lofty waterfalls that leap over the sides all appear to be necessary results of glacial erosion.
The heads of glacial valleys have a most characteristic form. They have extremely steep terminations, semicircular in form. Such forms are called cirques (Fig. 38). They are common throughout south-west Otago. A good example is to be seen in the usual photograph of Mitre Peak, Milford Sound. Sinbad Gully, to the left of the peak, ends in a typical cirque (Frontispiece). Lakes frequently occupy the floors of cirques. The basins they occupy owe their erosion to the downward pressure of the ice flowing down the steep slopes from which the supply of ice for the former glacier was derived.
Fig. 37. —Rock-basin formed by Glacial Erosion occupied by Iceberg Lake, North Clinton.
The origin of cirques is not yetfully understood. It is generally believed that ice exerts a sapping action at the foot of steep slopes that are connected with the surface of the ice by crevices, or bergschrunds.
It must not be supposed that the explanations of glacial phenomena here offered have been adopted without question by all observers. There are some who believe that the deep valleys are downthrown areas of the trough-fault type. Some even believe that the action of ice is protective rather than destructive, and that the valley of a hanging stream has, in consequence of its
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GEOLOGY OF NEW ZEALAND
Fig. 39.— Cirque, Head of North Branch of Clinton, near Lake Te Anau.
Fig. 38.— Cirque, North Clinton.
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GEOLOGY OF NEW ZEALAND.
Fig. 40.—Erratic Boulder of Greywacke transported by Ice, Arrow Flat, Otago.
Lent by Department of Mines, New Zealand.
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GEOLOGY OF NEW ZEALAND.
altitude, been filled with and protected by snow, while the valley at a lower level has lacked this protection, and has been worn lower and lower by the water that has flowed along it.-
In New Zealand we have good opportunities for instituting a comparison between the effects of ice and water. In all parts of the country the mountain-ranges have for a long time been subject to destructive agencies. In the North Island, where glaciers have not existed, the valleys are V-shaped, the spurs overlap, and there are no falls in the valleys of the streams, and no hanging valleys. In those districts of the South Island that have been glaciated the valleys are U-shaped, the spurs do not overlap, there are falls or steps in the valley-floors, and there are numerous hanging valleys. These differences cannot be ascribed to differences in rock-structure or in rainfall; nor in the absence of definite evidence is it wise to state that in the one district complicated faulting has disturbed every detail of the physiography, while in the other it has had little, if any, effect. All these facts strongly support the theory that the striking scenic features of the South Island are the results of glaciation.
In the South Island, glaciers in the Pleistocene period were far more extensive than now. On the surface of these glaciers enormous boulders were sometimes transported. When the ice finally melted they were left on the surface of the ground far away from the source from which they were derived, and sometimes in exposed places. Such large boulders are erratics. In many cases the surface of such erratics is often characteristically polished or striated.
6—Geology.
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GEOLOGY OF NEW ZEALAND.
CHAPTER VIII.
SEA-COAST.
The destructive action of the sea at its margin always attracts attention. As the shore-line is neared, the lower parts of the waves raised by the action of the wind on the ocean-surface are retarded. The upper portions, however, maintain their rate of movement, and therefore lack the support that they had in deeper water before their lower parts were retarded. Consequently, the tops of the waves fall forward or break —a movement that is often helped by the pressure of the wind from behind. When the break or fall of the top takes place there is a mad rush of water, local currents of great strength but of short duration being formed. In these currents are suspended pebbles and sand that are dashed against the rocks or beach. The work they do is the same as that done by the matter suspended by running water. Rocks are gradually worn away, cliffs are undermined, landslips fall, and the fallen matter is in time worn to fragments. The process is repeated over and over again. By this action exposed headlands are gradually worn back and the coast-line becomes more regular. The rate at which the coast-line recedes is on an average about one foot in a century. This accomplishes a smaller destruction of land than the action of running water, which reduces the level of the surface about one foot in four thousand years. The latter acts all over the land-surface of the globe, while the former is confined to the coast-line.
The action of the sea is much influenced by the hardness of the rocks that form the cliffs facing it. The softer rooks are worn back into caves, for the upper part of the cliff is supported by the harder rocks on either side. The differential action is still more marked when a series of jointed rocks occurs with compact strata on either side (Fig. 41). When a wave breaks against the cliff, the pressure of the water in the joints soon forces the blocks asunder, and the jointed rock is worn back, forming deep cuts in the cliff. On the coarst-line north of Manukau dykes of volcanic rocks penetrate conglomerates. The latter are solid, because natural cements have
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GEOLOGY OF NEW ZEALAND.
united the pebbles together. The dykes, however, show the usual contraction joints that are formed when heated matter cools. The result is that the solid dykes have been worn back much more rapidly than the conglomerate, and deep caves or tunnels have taken their place. Sometimes a tunnel extends from the cliff-face to a dip in the country beyond, or perhaps a little way back from the cliff-face the undermined rock falls in and a natural bridge results. Such bridges are not uncommon features on the New Zealand coast; there are examples near the Manukau, on Otago Peninsula, and near Catlins. When the tunnels are low, the breaking waves may fill the
Fig. 41. —Cliff of Horizontal Strata of Jurassic Rocks.
A shelf of marine erosion forms the top of the cliff.
entrance; and if there is a hole at the farther end the pressure of the wave may force out air and water in a high spout. Such blowholes, as they are called, are conspicuous on the coral reefs in the northern islands of the Dominion, and at various places on the coast of the mainland.
The steepness of the cliffs that front the ocean is dependent upon the nature of the joints that traverse the rocks. The cliffs that rise vertically to a height of 300 feet at Sea View, Dunedin, have
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GEOLOGY OF NEW ZEALAND.
well-formed vertical joints. Sometimes these are so far distant from one another that the tunnels which the sea wears out along them may separate projecting points from the mainland. When the rock above these tunnels falls in and is washed away, small islets or stacks become separated from the cliff. More frequently small islets consist of hard and resistant rocks that are left behind as the sea wears away the softer rocks that surround them. Such islands as the Cavallis and the Chickens represent the higher points of ridges which have been submerged by land-movement. Other islands, such as Rangitoto, are volcanic mountains that may have been separated
Fig. 42.—Brighton, Otago: A Small Plain of Marine Erosion at the Foot of the Hills.
from the mainland since their formation. When rocks are traversed by numerous joints, the cliffs, except in the most exposed headlands, lose their usual vertical form, for the rocks break down in talus slopes. These are conspicuous at Sinclair Head, in Cook Strait. Except for the small cuts where soft or jointed rocks traverse the cliffs, there is a tendency for the sea to straighten up the coast-line. The harbours along the coast and the deep inlets of the Fiord and Sound Districts have been formed not by the action of the sea, but by the depression of the land-surface, which has changed stream-valleys or other scars of the land into inlets of the sea.
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GEOLOGY OF NEW ZEALAND.
The action of the sea does not usually extend to a greater depth than 10 or 12 fathoms. The retreat of the coast-line is therefore associated with the erosion of a rock shelf or platform which extends to the level of 10 fathoms. Subsequent movements may elevate such a platform, which then constitutes a plain of marine denudation. A small shelf of this nature was elevated at Wellington by the rock-movement that caused the earthquake of 1855. There is a larger one in Otago, on a part of which the township of Brighton has been built. These are only a few feet above high-water level, but traces of more-elevated plains are found at various places on the coast-line. Such plains can be distinguished by the noticeable cliffs that bound them on the landward side, and by the gravel composed
Fig. 43.—Diagram or Gravel-Bank across Inlet.
The gravel is first deposited on the sheltered side of a; the growth continues until b is reached, where the deposit soon becomes heavy, and an outlet finally opens near a.
of the flattened pebbles and marine shells that are found in hollows of the rock-surface (Fig. 42).
Deposition. — Most of the material worn and torn away from the cliffs and beaches is deposited within a little distance of the shore. The finest material remains longest in suspension, and may be carried far out to sea by tidal currents. It finally settles in the deep water, and forms a deposit of fine mud. Coarser matter, such as fine sand, does not remain so long in suspension, but is deposited just beyond the region where the fierce breaking of the waves commences. During heavy weather, when sand, silt, &c., are produced and removed in large quantities, the shells of organisms that have died on the sea-floor are buried. Their remains form a component part of the strata
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that are being built up, and from their presence as fossils in any group of rocks a knowledge of the life of the period when the sediments were being deposited can be obtained.
Beaches.—Coarse sand and gravel are not removed from the beach, or, at any rate, not from the shallowest water near the beach. They are, however, moved along the beach. Every wave that breaks suspends some sand momentarily, and if the wave is moving along the coast with its own velocity, or with that of an ocean or tidal current, the matter that is for the moment suspended moves with it. In this way the material of the beaches is marching along the coast in a series of short steps, but whenever any obstruction breaks the force of the waves or lessens the velocity of the current the material is deposited, and its march stops. All projecting headlands constitute obstructions of this kind, and so on their leeward sides material is deposited, which gradually stretches out across the bay as a tongue of gravel or sand, and in time extends right across it (Fig. 43). Along this tongue the gravel continues its march, and, banking up against the opposing headland, forms a beach along, its base and another tongue on its leeward side. Until the gravel-bank has completely closed in a bay, the tidal flow
Fig. 44. —Wakapuaka Bay, Nelson.
The gravel-bank deposited by north-east seas and currents completely closes the arm of the inlet and connects Pepin Island with the mainland.
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and the water that enters the bay from the land will pass through the opening that remains. When the bank is complete, a huge mass of sand and gravel is soon built up against the opposing headland, if it is noticeably projecting, and the water that is enclosed will overflow alongside the projecting headland to windward (Fig. 44).
The Otago Peninsula offers excellent illustrations of this. The north-east side of each headland is protected from the southerly swell and the southerly current. Here the sand is deposited; but the banks are complete, and are far wider than elsewhere at their north-east end, against the weather side of the farther headland. The outlet is always on the south-west end. The same features are conspicuous at Banks' Peninsula. A huge mass of gravel has banked up against the Peninsula, while it gradually tapers away towards its southern extremity, where from time to time the Selwyn River breaks across it.
The movement of the sand along the coast often blocks the entrances to the smaller streams, especially in heavy weather, or during a long spell of dry
Fig. 45.—Totara Lagoon, near Hokitika.
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weather. After a heavy rainfall the flooded waters will cut a deep channel across the enclosing bar. The bar is most formidable on the side from which the drift moves, and the channel that is made in flood-time is usually on the side to which the drift moves. The bar is thus gradually built up farther and farther along the coast, and in time compels the river to flow for some miles parallel to the coast (Fig. 45). When such extreme results have been caused a stream may, in a period of heavy flood, once more break across the long barrier that has thus been built up. Such conditions are well seen at the mouth of the Totara Stream, to the south of Hokitika.
The New Zealand coast is most favourably situated for the effects of coastal drifts to be developed. On the east coast the oceanic current, the flood tide, and the prevalent swell all act together in the same direction. From Nugget Point to Cape Campbell the only projecting point that opposes an efficient barrier is Banks Peninsula.
The rate at which material drifts along the coast is considerable. The Timaru Breakwater, commenced in 1875, has almost completely prevented the drift, and 1,500,000 tons of gravel have been banked up behind it since that time. On the west coast the current of the ebb tide moves with the drift and the swell, and the effect is almost as pronounced as on the east coast. Farewell Spit, which is composed entirely of drifted sand, is a monument to the moving-power of the drift. In the North Island the general result is the same. The formidable bars of the west coast all attest the mighty volume of drifting sand, while from Ahipara to Cape Maria van Diemen the whole breadth of land is formed of sandhills that unite together rocky hills that were once separate islands (Fig. 46). Inlets remain open only where the drift is arrested by projecting buttresses of land (as at Banks Peninsula), or where there is no range of coast-line to windward to provide the gravel (as at Preservation Inlet and Wellington), or where projecting islands and capes protect the land from currents and swell (as at Russell and Whangaroa).
It can be clearly seen that the action of the sea on the coast-line is to even up any irregularities that may exist. The projecting headlands are opposed to the most violent action of the waves, winds, and currents, and are therefore worn back most rapidly. In all the sheltered inlets or across their entrances, on the other hand, beaches and sandbanks are formed, and the coast-line is built up and extended. Finally the coast-line becomes straight, or consists of long sweeping curves leading up to rounded capes. This mature form is well shown
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Fig. 46. —Chart of Manukau Harbour.
The light part on the periphery dry at low water; the two shaded portions from 0 to 5 fathoms and from 5 to 10 fathoms; over 10 fathoms, black. The material of the bar is entirely supplied by the drift along the shore from the south-west. The outflow from the harbour keeps the bar three or four miles seaward.
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along almost the whole line of the west coast from Milford Sound to Cape Maria van Diemen : everywhere the sea is heavy, the currents are strong, and the rivers supply enough material to enable the sea to build up the coast rapidly. The features of the coast-line are repeated on a smaller scale along the shores of the lakes. Here, however, there are no strong currents, the direction of the waves is less constant, and their action less vigorous. The material brought down by the streams is not marched along the shore, but forms flat areas or deltas at the mouth of the rivers. The Dart River has formed a delta where it enters Lake Wakatipu, and the Tongariro River one at the south of Lake Taupo.
Notwithstanding the straightening action of the sea on the coastline, it is a familiar fact that portions are indented with deep and steep-walled inlets. From what has been said it is obvious that these have not been formed by the action of the sea. In most cases they are old river-valleys that were eroded when the land was at a higher level and were submerged when the land sank.
The nature of the sand on a sea-beach will obviously depend mainly upon the nature of the material that is brought down in suspension by rivers, as well as the nature of the rocks that border the sea-coast. To some extent also it will depend upon the force of the waves that beat upon the coast.
On the east coast of the South Island the sand consists mainly of small grains of quartz, for this is the hardest of the minerals of the mica-schist and greywaekes of which the country is mainly composed. Besides being hard, quartz is also particularly resistant to the chemical attacks of water. Locally, where the action of the sea is relatively violent the light quartz may bo floated away, and heavier minerals, such as magnetite, which are present only in small quantity in the ordinary sand, become concentrated. Then the beach will, over a small space, consist mainly of magnetite, and the sand will be black. Other heavy minerals are also associated with the magnetite in these local patches, and in some instances gold can be obtained from them in payable quantity.
Occasionally garnet is a component mineral of the rocks. Its hardness, chemical inertness, and moderately high specific gravity cause it to accumulate on the beach, to which it imparts a conspicuous red colour, and is often called "ruby sand." Patches of this are found near the mouth of the Kakanui River, close to Oamaru.
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GEOLOGY OF NEW ZEALAND.
On the west coast of the North Island the sands are derived from the waste of the great volcanic cones. Magnetite is by far the hardest and most resistant mineral in these rocks, so the sands on that coast are almost entirely black sands, and it has been attempted to use them as iron-ores. With the grains of magnetite are small crystals of hypersthene and augite, which also have a black colour. Farther north, crystals of hornblende largely take their place, for this mineral is abundant in the rocks of Mount Egmont.
On some of the quiet beaches near Auckland fragments of shell form the greater part of the sand.
The rougher beaches consist of masses of gravel arranged in slightly inclined layers composed of flat rather than round pebbles. Few organic remains are found on them, chiefly because the shells or bones cannot withstand the action of the waves that cast them with such force upon the boulders. On the lower portions even of gravel beaches sand accumulates, and extends some distance seawards. Organic remains here are more abundant, except where the sea is heavy and frequently disturbs the sand to such an extent that any organisms are smothered.
In the deeper water there is a deposit of mud extending at least as far as the 100-fathom line. The mud accumulates slowly, and is little disturbed by the waves even when the weather is heavy. Organisms are here relatively abundant, and, as the mud gathers but slowly, their remains often constitute an important part of the deposit that is formed. The mud consists of the finer and lighter mineral grains that are torn from the rocks by wave and stream. Mica, being almost indestructible, and separating in flakes that float readily, is the most frequent constituent. The mud deposited round the New Zealand coast is blue in colour.
The relative depths at which material is deposited vary greatly in accordance with the extent to which the area of deposition is sheltered or is open to the action of heavy seas, tides, or currents. For instance, in many of the sheltered inlets the beaches are muddy, and wide mud-flats are exposed at low water. On the other hand, where the tidal currents are strong, gravel may cover the bottom even where the water reaches a depth of 100 fathoms or more. In Cook Strait, where the flood and the ebb tide run four or five miles an hour, the bottom is covered with gravel, or is even rocky, to a depth of over 100 fathoms.
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Oceanic Deposits.
The area of deposition of mud seldom extends far from the shore-line, and in the New Zealand area the mud is succeeded by a deposit of Globigerina ooze. This consists of the remains or tests of Foraminifera—minute thin-shelled organisms of simple structure that float in the surface waters of all marine areas. When they die their shells sink to the bottom of the sea, and, where they are not mixed with and concealed by sediment, they form a fine-grained deposit of limestone, called ooze. This is found all round New Zealand in the ocean-depths between 500 and 2,000 fathoms. Farther to the south, beyond 50° latitude, the water is too cold for the Foraminifera to live, and Diatoms take their place. These are minute vegetable organisms with tests of silica. Far to the south of New Zealand diatomaceous ooze therefore accumulates all over the seabottom.
In the deep water or abyssal areas of the ocean the bottom is covered with red clay. It appears that the calcareous tests of the Foraminifera gradually dissolve as they sink through the depths of ocean. In these great depths fragments of decomposed pumice, and perhaps meteoric dust, constitute the material that covers the bottom. With the red clay are often found nodules of manganese-oxide, sharks' teeth, and certain bones of whales. The first is probably derived from the decomposition of volcanic material. The fact that the remains of marine creatures are found to an appreciable extent is an indication of the very slow rate at which the red clay accumulates. Red clay covers the bottom of the Pacific Ocean to the east of New Zealand, and is found all over the deeper part of the Tasman Sea.
Owing to the movements of elevation and depression to which all parts of the earth's surface are subject, the coast-line never retains its position for long. An advancing coast-line is always shown by raised-beach terraces at a little distance from the shore, by lines of marine cliffs inshore, and by rivers piercing the coast-line direct instead of through deep inlets. A retreating coast-line, on the other hand, is shown by the small and narrow beaches and sand-dunes that front it, and by its irregular nature —projecting promontory and farreaching inlet succeeding each other over leagues of rough and varied coast-line.
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Rocks.
When the materials deposited by water are acted upon by those forces that cause folding and compression they are changed into hard rocks, the different grains and pebbles becoming welded together. The solidification may often, however, be effected without pressure, for percolating water usually deposits natural cements that unite the grains together into a solid mass. Of these cements, silica, calcite, and limonite are the most important.
Gravels form rocks that are called conglomerates, irrespective of the amount of solidity that they possess, or of the agent by which this solidity has been imparted. Conglomerates occur in many beds in the Hokonui Hills, of Southland. They occur also in the coal-measures in Otago, Westland, and elsewhere, and form large rock-masses near Cape Farewell and Wanganui.
Sand is simply compacted into sandstone (Fig. 47). When it contains a noticeable quantity of mica it is called arkose, and when feldspar is an important constituent it is called greywacke. Sandstones are the most important rocks in New Zealand. In the North Island nearly all the mountain-ranges are composed of them, and in the South Island all the ranges eastward of the main divide and northward of the Waitaki River are formed of the same materials. Though hard, these rocks are so traversed by irregularly intersecting joint planes that they are quite useless for quarrying. The hardness has been caused by welding rather than cementing.
Fig. 47.—Microscopic Section of Maitai Gkeywacke.
Mud forms rocks that are called mudstones, or shales when they split readily parallel to the planes of deposition or stratification. These rocks are common in the mountain-ranges that are mainly formed of sandstone. When a mudstone contains a large quantity
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of calcium-carbonate it is called a marl. The papa rock of the North Island is a true marl, in which the stratification is not stronglymarked. A bed of jasperoid shale is a common feature in the mountain-ranges of many localities of both Islands. The fine-grained structure, the red colour, and the frequent association with manganeseoxides have given rise to the suggestion that these rocks represent red-clay deposits of abyssal areas. Microscopical research does not support this idea, for the jasperoid shales are found to consist almost entirely of minute quartz grains mixed with haematite plates of small size.
There are no deposits of any importance that can be said to represent G-lobigerina ooze. Diatomaceous earth forms a well-known deposit near Oamaru which contains a very finely preserved series of species of diatoms.
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CHAPTER IX.
VOLCANIC ACTION.
At certain, points on the earth's crust materials in a heated condition are emitted from the orifices of channels that communicate with its hot interior. Solids, liquids, and gases are expelled, and the first two named collect in greatest quantities around the point of emission, and gradually decrease at greater distances from it; hence a conical elevation, or volcano, is gradually built up. At its summit there is a cup-shaped depression which marks the place from
Fig. 48.—Interior of Crater, Mangere, Onehunga.
The small cone within the crater was formed during the final eruption.
which the material is ejected; this is the crater. The chief gaseous substance emitted is steam; but sulphur-dioxide, hydrogen, hydro-chloric-acid gas, carbonic-acid gas, and sulphuretted hydrogen are present in relatively small quantities. Certain radio-active gases are present in still smaller quantities. These gases escape into the atmosphere, but the steam is soon returned to the earth's surface in the form of rain (Fig. 48).
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GEOLOGY OF NEW ZEALAND.
The solid substances consist of rock-fragments. When the fragments are minute they are called ash, but if they are glassy they are termed dust. The rock formed from these fine materials is called tuff. It must be clearly understood that the term "ash" does not imply that any combustion has taken place within the volcano. All the material that is emitted is completely oxidized, and the expression "burning mountain" is quite inappropriate as referring to any volcano. If the solid fragments approach the size of a nut, they are called lapilli; if they are larger, the word scoria is applied to them. It is usually the case that the more violent the activity of a volcano the finer are the rock-fragments ejected. The solid material is ejected by the explosive force of steam.
The liquid substances emitted consist of molten siliceous rock. Its composition varies widely; the percentage of silica may be anything between 40 per cent, and 70 per cent., and of alkalies between 3 per cent, and 15 per cent, The fluidity of this molten rock or lava also varies, for it depends upon the composition and temperature of the lava, upon the quantity of contained steam, and upon the number of crystals that have already been developed in the fluid matter. Fluidity is favoured by a low silica - percentage, a high temperature, a large quantity of steam, and a small number of contained crystals. The opposite conditions favour a viscous state.
The initial cause of volcanic action is not fully understood. The following matters have to be explained by any theory advanced: (a) The origin of the force that causes the expulsion of volcanic matter; (b) the source of the lava; (c) the source of the steam and other gases; (d) the arrangement of volcanoes in lines; and (e) the situation of volcanoes near the sea.
There is little doubt that lava represents a portion of the earth that has not yet cooled below the point of fusion, though at the level from which it rises it may have been kept solid by the immense pressure of the overlying rocks. The steam is probably occluded in the lava. It originates from the time when the surface of the earth was molten, and was absorbed from the dense atmosphere that then contained the greater part of the present surface waters of the earth in the form of steam. The steam probably supplies the force by which volcanic matter is raised to the surface. The arrangement of the volcanoes in lines and near the sea is probably due to these areas being lines of weakness —that is, lines along which the earth's crust
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is yielding to pressure and is undergoing movements of folding and faulting. Such movements may temporarily reduce the pressure on the rooks below, and allow them to pass into the molten state. Many objections have been raised to these theories, and other explanations have been advanced. It is believed, however, that the brief statement given above represents current geological thought in reference to volcanic action.
During volcanic activity steam bubbles through the lava, and as it reaches the surface it expands rapidly or explodes. The explosion ejects fragments of rock out of the crater. Sometimes fragments of liquid lava are thrown out. They rotate rapidly as they pass through the air and acquire ellipsoidal forms. They are known as volcanic bombs, and are particularly common on Mount Eden and the other volcanic cones near Auckland. It usually happens that the lava is too viscous to allow of the ready escape of steam. Expansion of the steam takes place, however, and the lava acquires a vesicular structure, which is particularly common in the viscous acid lavas. The steam-pores may be so numerous that the specific gravity of the rock is reduced to such an extent that fragments will float on water. We then have what is known as pumice. When the rising steam is prevented from expanding by a covering of solid rock it accumulates beneath the resistant mass, and finally explodes with terrific force. The force of the explosion rends the overlying rock into fragments, and explosion craters, sometimes of huge dimensions, are formed.
Fig. 49. —Volcanic Bomb, Mount Eden, Auckland.
An explosion may take place at the commencement of volcanic activity, as was the case when the tuff craters of the Auckland District were formed. It may occur after a period of relative inactivity, during which the throat of the volcano has become choked. An explosion may truncate the summit of the volcano, as has probably happened in the case of Ruapehu. At Tarawera the violent explosion of 1886 formed a deep chasm ten miles long in places and a mile and a half wide. The more violent the explosion, the more widely will the ejected
7—Geology.
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material be scattered. The tuff cones at Auckland, such as the Panmure and Onehunga basins and the Kohuora craters, have this
Fig. 50.—Panmure Basin Auckland. A tuff crater.
Fig. 51.—A Scoria Cone at Waimate, Bay of Islands.
origin (Fig. 50). Their cones are therefore very low, the angle of inclination is slight, and the craters are large. When the steam
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explosion is less violent the material is coarser, is scattered less widely, and the cone built up has an angle of inclination that is the angle of rest for the material emitted, generally about 30°. Mount Eden, Mount St. John, and the other scoria cones near Auckland have been formed in this way (Pig. 51).
Fig. 52. —Profile Outlines of some New Zealand Volcanoes.
Drawn on true scale to show actual slope and relative size.
1. a, Ruapeku; b, Egmont; c, Ngauruhoe; d, Rangitoto.
2. a, Rangitoto; b, Mount Eden, Auckland; c, Panmure Basin, Auckland.
N.B. —Egmont and Rangitoto rise from sea-level; the others rise from plateaus.
From time to time during the period of activity lava-streams issue from the crater and flow down the sides of the cone. The lava has a temperature of about 1,500° C., and is therefore white-hot. If it is very fluid it flows over wide areas of the surrounding country, and builds up a cone with a low angle of inclination. Rangitoto,
Fig. 53.—Section of Tuff Cone with Scoria Cone in its Crater.
a, a, Walls of tuff cone; b, scoria crater; c, volcanic pipe; d, country rock.
formed of basaltic lava, is an example of such a cone (Fig. 52). The more viscous the lava, the steeper is the cone that it forms (Fig. 53). The steepness of the cones of Egmont and Ngauruhoe is due to the viscous nature of the andesitic lavas of which they are formed. Being fluids, lavas naturally flow down valleys that have
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been eroded by streams. This is very noticeable at Auckland, where the volcanic lavas lie in valleys previously eroded through Miocene sediments. It is even more noticeable at the Bay of Islands, where a lava-stream from Te Puke has flowed down the course of the Kerikeri Stream and formed dangerous reefs in the bay. In some instances the streams of water continue to flow beneath the lavas that have usurped their valleys. Their waters issue as springs when they reach sea-level or the end of the lavaflow. The Western Springs, which for many years supplied Auckland with water, have this origin, and the River Waihohonu commences as a large clear spring that issues from beneath one of the lavas of Ngauruhoe. Lava often issues from a fissure at the base of a volcanic cone.
During the eruption of Savaii, in Samoa, during 1906, 1907, and 1908 lava never flowed over the side of the cone, but always from a fissure at its base. This was probably the case also at Mounts Eden and Rangitoto.
Fig. 54.—Hills of Volcanic Scoria at Base of Ngauruhoe.
The rate of flow of lava is slow. Down the steep sides of a volcano a velocity of six or eight miles an hour is not unusual, but on the flatter ground extending round its base 100 yards a day is not
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exceeded in some places. The lava takes a long time to cool; but whilst its interior is fluid a crust forms on its surface, and affords a safe footing. During the great eruptions in Savaii (Samoa) in 1906, a stream of basaltic lava five miles wide flowed between two villages, but at no time did it prevent daily communication between the villages by barefooted Natives. If the supply of lava ceases after a solid crust has been formed, the fluid matter may pass entirely from undernaeth the solid crust, and a long tunnel or cave will be left. The lava caves at Auckland, and especially at the Three Kings, are examples.
Fig. 55.—Interior of Crater of Ngauruhoe.
Within historic times no lava-stream has issued from any volcano in New Zealand. During the Tarawera eruption hypersthene andesite was ejected in the form of bombs. The statement that a lava-stream flowed down the northwest side of Ngauruhoe in 1866 is not supported by historical evidence or by observation (Fig. 55). Most of the New Zealand volcanoes are, like those of other parts of the world.
Fig. 56.—Volcano formed of Alternate Sheets of Lava and Beds of Scoria.
d, dyke; s, sill; X, batholite.
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composite cones. They consist of alternate layers of lava and scoria, braced up with dykes. This composite structure is often very apparent in older volcanic areas where denudation has laid
Fig. 57. —The Volcanoes of the Centre of the North Island.
Note the low watershed between the Waikato, Wangaehu, and Moawhango (tributary of Rangitikei).
bare the alternate beds of lava and scoria (Fig. 56). The latter, being composed of fragmentary material, allows of the ready percolation of water, and is therefore often completely oxidized or
93
reddened, while the lava-streams above and below appear perfectly fresh. This is strikingly shown at Dunedin and at Lyttelton, where the composite structure of the old volcanic country is well displayed.
Molten rock often fails to reach, the surface of the earth. It may gradually cool and solidify at great depths in the crust. It then forms batholites, and the rocks that result from its cooling are termed plutonic. Some of it may be forced into crevices that intersect surrounding stratified rocks, forming dykes. Sills are intrusive masses that lie parallel to the stratification. Laccolites are masses that have bent up the overlying rocks into domes. Dykes are common at Dunedin, Moeraki, and the west of Auckland, as well as at many other places; but the occurrence of good sills or laccolites in New Zealand has not yet been recorded.
The distribution of volcanoes in New Zealand is of much interest.
Fig. 58.—Ngaubtjhoe in Eruption.
Clouds of volcanic dust reached a height of 3,000 feet above the crater. —21st March, 1907.
Those that still retain some activity or possess characteristic conical forms are restricted to the North Island. Ruapehu (9,175 feet) (Fig. 59) is the most southern, and from it a line extends through Ngauruhoe (7,515 feet), Tongariro (6,400 feet), and Pihanga (4,200 feet) to Lake Taupo. Westward from Ruapehu volcanic cones are found as far as Kakepuke, while still farther to the
GEOLOGY OF NEW ZEALAND.
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westward, and separated from the central volcanic area by sedimentary rocks, are Mount Egmont (8,200 feet), Pirongia, and Karioi. Northwards from Ruapehu volcanic cones extend as far as White Island. Farther north basaltic cones are abundant at Auckland (Fig. 60), and still farther north at Whangarei and the Bay of Islands. In both Islands there are many other districts where volcanic activity has been pronounced within Cainozoic times, but the characteristic form of volcanic cones has now been almost completely destroyed by atmospheric erosion. Outside the mainland, volcanoes occur at the Kermadecs. In the Cook Islands the form of the volcanic cones is not now recognizable, though the islands are mainly composed of volcanic rocks.
When lavas cool they contract. As the surface cools most rapidly, cracks are formed that separate it from the hotter rock beneath. As a consequence, lavas that have cooled a series of horizontal joints. If the lava was in motion whilst solidifying, the joints are irregular and curved—curvilinear joints. If complete rest had been attained, and the cooling was slow, vertical crevices intersecting at angles of 120° are formed. The intersections of these joints mark out hexagons, and give the rock a perfect columnar structure (Fig. 62).
Fig. 59.—Hot Lake in Crater, of Ruapehu.
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Fig. 60.—Volcanoes of the Auckland Isthmus.
Twenty miles from Manurewa to Rangitoto. The scattered arrangement of the cones is noticeable. The tuff craters are readily distinguished at Koheroa and Panmure. Sometimes there is a scoria cone within a tuff cone, as at the Three Kings.
GEOLOGY OF NEW ZEALAND.
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Rocks formed from the Cooling of Molten Matter.—The large mass of molten rock that forms a batholite cools with extreme slowness, and during the cooling is subject to a high pressure. Such conditions favour complete crystallization. These plutonic rocks are therefore coarsely crystalline. The material of a dyke or other intrusive mass cools relatively quickly, but usually it has formed a portion of a batholite before its intrusion. It contains, therefore, an older generation of large crystals formed before the intrusion took place. These are embedded in a much finer crystalline groundmass, often composed of the same minerals as the larger crystals. The groundmass solidified after the intrusion, when the cooling was relatively rapid and the pressure was reduced. The crystals of the groundmass constitute the second generation. A structure of this kind is called porphyritic. Volcanic rocks often have a similar structure, but more frequently the crystals of the earlier generation of minerals are embedded in glass, for the final cooling under surface conditions was so rapid as to prevent crystallization.
Fig. 61.—Columnar Basalt, Black Head, Duhedin.
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Rocks —whether plutonic, intrusive, or volcanic—are classified according to their chemical corn-position. The composition is, however, so clearly indicated by the nature of the minerals that have crystallized that analysis is seldom necessary. It cannot be said that the classification of these igneous rocks has yet reached a satisfactory finality. In the first place, there are in nature no well-marked groups distinctly separated from others by unvarying chemical composition, for each group has a perfect series of connecting-links with groups on either side of it, and often with others. In the second place,
Fig. 62. —Columnar Structure in Dolerite, Sandymount, Dunedin.
The cliff-face is 400 feet high.
it often happens that rocks of closely related composition consist of widely different mineral aggregates. And, thirdly, rocks that are different from one another in chemical composition may be formed of somewhat similar mineral aggregates.
A classification has lately been proposed by eminent American authorities. It is based on chemical composition alone. For classification purposes no importance is attached to the nature of the minerals that occur in the rock. The disadvantage of the system is
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Fig. 63.—Granite Gorge of Kakapotahi River, Westland.
Lent by Department of Mines, New Zealand.
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that a complete analysis of the rock must be made before it can be named, and no place is given to the nature of the minerals that compose the rock—the one character that expresses the physical and chemical conditions during crystallization.
For the purposes of this book it is sufficient to divide igneous rocks into six groups —Acid, Intermediate Acid, Alkaline Intermediate Acid, Intermediate Basic, Basic, and Ultra-basic. Each of these groups has plutonic, intrusive, and volcanic representatives.
1. ACID GROUP. Contains 70 to 90 per cent, of Si0 2. Minerals — orthoclase, quartz, mica, and various accessories, such as sphene, zircon, and apatite.
A. Plutonic rocks. Structure, coarsely crystalline or granitic.
1. Granite of typical composition and structure. Occurs at Port Pegasus and Paterson Inlet, Stewart Island ; Preservation Inlet; Dea's Cove ; Thompson's Sound ; and at intervals from the Haast River along the western slopes of the Southern Alps to Karamea and Separation Point, the largest mass forming the Gouland Downs.
2. Graphic granite. The quartz and feldspar are so intergrown as to give rise to an appearance that bears a fanciful resemblance to Hebrew characters. The structure points to simultaneous crystallization of quartz and feldspar. Occurs at Paterson Inlet in Stewart Island, and at Golden Bay.
3. Greisen. Orthoclase is absent. Greisen is often associated with stockwerks of cassiterite.
4. Granitite. Orthoclase is largely replaced by oligoclase or a related species of triclinic feldspar. Occurs at Separation Point and other granite areas.
5. Aplite. Mica is absent, and the rock is usually fine-grained. B. Intrusive rocks. Any of the three characteristic minerals— quartz, orthoclase, or mica — may belong to the earlier generation.
1. Quartz porphyry. Usually light-coloured. Occurs as dykes in the coal-measures near Greymouth.
2. Granophyre. A fine-grained intergrowth of quartz and feldspar. Common in boulders in conglomerates of Triassic. age, especially near Gore; also at Ruggedy Point. Stewart Island.
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C. Volcanic rocks. Structure varies from porphyritic to completely glassy rocks.
1. Rhyolite. White rhyolites with large quartz and feldspar crystals (and often with garnets) occur at Mount Somers, and at the head of Lyttelton Harbour (without garnets). Glassy varieties occur in the same areas. In the North Island rhyolites occur from Taupo to Coromandel. The following are the most abundant types of rhyolite :—
(a.) Lithoidal or stony rhyolites. Large crystals of feldspar of the earlier generation are present, but all other crystals are small. Contain no quartz, but tridymite common in the groundmass. Common from Ongaruhe to Te Kuiti.
(b.) Pitchstone. Crystals embedded in a dark glass. Occurs at Taupo and Rotorua ; also at Mount Somers.
(c.) Spherulitic rhyolite. Minute feldspar needles are arranged in spherical masses with radial structure. It is most common at Tairua, where the spherulites are an inch in diameter, but in other localities are much smaller.
(d.) Banded rhyolites. Microscopic spherulites are arranged in rows. These are dark-coloured, and give a noticeably banded appearance to the rock. Occurs at Taupo, Ngongotaha, and the Coromandel Peninsula.
(e.) Spherulitic pitchstone. Spherulites embedded in glass. Occurs at Wairakei and Rotorua.
(f.) Wilsonite. Irregular patches of dark glass embedded in stony matter. Occurs at Waihi.
(g.) Obsidian. A pure dark glass. Occurs at Mayor Island.
2. INTERMEDIATE ACID GROUP Contains 62 to 70 per cent, of SiO2. The typical minerals are orthoclase and hornblende; but quartz, mica, triclinic feldspars, sphene, and other minerals occur more or less frequently.
A. Plutonic rocks.
1. Syenite. This is an uncommon rock in New Zealand. McKay's Bluff and the Boulder Bank at Nelson have often been called syenitic, but the large quantity of quartz found in the rock necessitates its classification as a granite. There is an occurrence of syenite at Akaroa.
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B. Intrusive rocks.
1. Porphyry. This rook again is very poorly represented. Examples are found near Dunedin.
2. Bostonite. Hornblende is practically absent, and the rock is perfectly white. Occurs at Portebello, Dunedin.
C. Volcanic rocks.
1. Trachyte. The rocks of Mount Egmont, Ruapehu, and many other volcanoes of the North Island have often been called trachytic, but they all appear to be andesites. A trachytic rock occurs with the bostonite at Dunedin ; it is practically pure feldspar.
3. ALKALINE INTERMEDIATE ACID GROUP. Contains from 50 to 60 per cent, of Si0 2 , and from 10 to 15 per cent, of alkalies. Minerals—nepheline, orthoclase, aegerine or hornblende, and various accessories. When powdered, these rocks gelatinize readily with dilute HCL.
A. Plutonic rocks.
1. Nepheline syenite. Occurs very sparingly near Dunedin.
B. Intrusive rocks.
1. Tinguarte. Generally a pale-green rock, and often very finegrained. Numerous dykes occur near Port Chalmers.
C. Volcanic rocks.
1. Phonolite. A dark-green dense rock that rings distinctly when struck with a hammer; but this property is by no means confined to phonolites. It is plentiful near Dunedin, and occurs sparingly in the Port Hills, Christchurch.
4. INTERMEDIATE BASIC GROUP. Contains from 55 to 65 per cent, of SiO 2, and from 4 to 6 per cent, of alkalies, but lime and magnesia are high. The typical minerals are triclinic feldspar and hornblende, but augite is very common. Quartz occurs in the more acid types only.
A. Plutonic rocks.
1. Diorite. This is the most abundant rock in Stewart Island and in the region of the West Coast Sounds. In the North Island diorite occurs at Mongonui and a quartz diorite near Coromandel.
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B. Intrusive rooks.
1. Porphyrite. Dykes of this type are not common, but occur in the Coromandel region. There is also a dyke at Nugget Point.
C. Volcanic rocks.
1. Dacite. Contains quartz in addition to triclinic feldspar and hornblende —the typical minerals of this group. The rock occurs in the Coromandel Peninsula, the Hen and Chicken Islands, and at Tauhara, near Lake Taupo.
2. Andesite. An igneous type, widely spread in both Islands; there are many varieties.
(a.) Hornblende andesite and hornblende augite andesite. Occur at Whangaroa, the Thames, Mount Egmont, and Dunedin (Fig. 64).
(6.) Augite andesite and augite hypersthene andesite. Found at Whangaroa, Whangarei, Little Barrier, Great Barrier, Coromandel Peninsula, and in many cones of the central volcanic region, especially Ruapehu, Ngauruhoe, Tongariro, and White Island. The bombs ejected during the eruption of Mount Tarawera in 1886 were hypersthene augite andesite. In the South Island, found at various places between the Rangitata and Waimakariri Gorges.
(c.) Olivine andesite. Very little olivine is present. Occurs as the typical rock of the Port Hills, Christchurch.
5. BASIC GROUP. Contains from 40 to 55 per cent, of Si0 2. Minerals —augite, triclinic feldspar, usually olivine, and in the finely grained rocks much, magnetite.
A. Plutonic rocks. Diallage in place of augite.
1. Gabbro and olivine gabbro. An olivine gabbro occurs over a wide area at Ahipara, in the North Island. Normal gabbros are found at Campbell Island and Auckland Island.
2. Norites. Contain hypersthene with, or in place of, diallage. The rocks occur at the Round Hill, near Orepuki, at the Darran Mountains, near Milford Sound, and at the Bluff.
B. Intrusive rocks.
1. Diabase. Not well represented. Some dykes occur close to the auriferous veins at Reefton. Occurs also at Oamaru and Mocraki.
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2. Camptonites and monchiquites differ from diabases in the reduction of the feldspar and the large amount of hornblende that takes its place. There are many dykes of these rocks in the western portion of the Southern Alps, especially near the Taramakau River, and boulders are common in the gravels of the Shotover River.
G. Volcanic rocks.
1. Dolerite. Coarse-grained, even-grained, or porphyritic. Is abundant near Dunedin, at Timaru, and at the Port Hills, Christchurch.
Fig. 64.—Microphotograph of Hornblende Andesite, Mount Egmont.
Black grains magnetite; dark crystals hornblende paler crystals augite; colourless crystals feldspar. Crystals are embedded in glassy base, x 30.
Fig. 65. — Microphotograph of Hypersthene Andesite, Ruapehu.
The large grey crystals are hypersthene, the smaller clear crystals feldspar, and the small black grains magnetite, x 30.
2. Basalt. A fine-grained rock, in colour black because of the large amount of magnetite that it contains. Is widely distributed. Occurs at Clarendon, Dunedin, Oamaru, Banks Peninsula, the Clarence Valley, Bay of Islands, Pirongia, and Karioi.
3. Basanite. Contains some nepheline in addition to olivine, feldspar, and other minerals. Porphyritic types occur
8—Geology.
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at Dunedin, and a fine-grained rock of this class forms the volcanic cones near Auckland. It is the material of the "basalt plateau" in the Waikato.
4. Melaphyre. An ancient basalt ; the olivine and some of the augite have been changed into serpentine, and so the rock is green. Occurs at Riverton, the Green Hills near the Bluff, and Nelson.
6. ULTRA-BASIC GROUP. Contains about 40 per cent, of Si O2. Feldspar is nearly or quite absent. Most of the rocks have olivine as the characteristic mineral.
Fie. 66.—Microphotograph of Biotite Norite, Milford Sound.
On right, diallage with included crystal of hypersthene; on left, biotite; lower portion, feldspar. X 30.
Fig. 67.—Microphotograph or Dolerite from Dunedin.
Crystals on right and left near top are augite ; striped crystals, triclinic feldspar with albite twinning; groundmass, feldspar and augite. X 30.
1. Pyroxenite. A rock composed of diallage occurs at the Dun Mountain, and at the Red Hill, in the west of Otago. Most specimens of this rock contain some enstatite or bastite, and should be called websterites.
2. Amphibolite. Composed entirely of hornblende. Is common as basic secretions in the diorite areas of the West Coast Sounds.
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3. Harzburgite. Composed of olivine and enstatite. Occurs south of tie Dun Mountain, at Anita Bay, Milford Sound, and at the North Cape.
4. Lherzolite. Contains olivine, enstatite, and diopside, with some chromite. Abundant in the Red Hill district, in the west of Otago.
5. Dunite. Olivine with a little chromite. Occurs at the Dun Mountain and in the Red Hill district in Otago.
6. Rodingite. A variety that contains a high percentage of lime. A white variety of garnet (grossularite) forms the greater part of the rock. Occurs in the Dun Mountain region.
The causes of the differences in composition of volcanic rocks have given rise to much speculation. Some authorities believe the differences to be due to original heterogeneous composition of different parts of the earth's crust. Others think that within the earth's crust there were two or three original magmas, or molten masses of rock, and that intermediate types have been formed by mixtures of the magmas in different proportions. It is more usual to ascribe the variation to differentiation. This depends upon differences between the specific gravities of different compounds in the magmas, and also upon the different specific gravities of the crystals that separate out. The heavier crystals fall to the bottom, the lighter float. If before differentiation has commenced an eruption takes place from the underlying magma, lavas of intermediate types (andesite) would be expelled. After differentiation had separated the magmas into an upper light or acidic portion, and a lower heavier or basic portion, the acidic rocks would be ejected first, and afterwards the basic. This normal order of emission has been observed in several countries, notably by Richthofen, in Western America. It does not seem possible to account for the differentiation of alkaline rocks from the same magma that has allowed of the separation of andesites or of basalts.
It cannot be said that as regards New Zealand any researches have as yet been published showing satisfactorily the effects of differentiation. At Mount Somers there is a complete series from rhyolites to andesites, but the details are not known. On the Otago Peninsula there is a large assortment of alkaline and basic rocks, but the lavas do not lie in any regular order of increasing basicity, and there is even an alternation of alkaline and basic lava-flows. In the North Island volcanic region—extending from Ruapehu to the North Cape,
GEOLOGY OF NEW ZEALAND.
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and from Egmont to White Island—there is considerable variety of rock, though the whole district appears to be a single petrographical province—that is, the rocks throughout the area are, with the exception of some basalts, clearly related to one another, and have probably been erupted from the same magmatic reservoir. In this district the earliest rocks appear to be andesites of varied character (augite, hypersthene, quartz, and hornblende types are known). They are the gold-bearing series of the Thames and Coromandel fields. Similar rocks occur widely at Waitakerei, Whangarei, and Whangaroa. Rhyolites succeed them in the Thames district, and probably the rhyolites of Taupo, with the pumice, are of equivalent age. Afterwards hypersthene andesites were again emitted at numerous points through the rhyolite plateau. Tauhara, Edgecumbe, Kakepuke, &c., were then formed. Eruptions were most violent in the south-west, where Ruapehu and his neighbours were built up, and far to the westward, where Egmont was piled up from hornblende-andesite eruptions. Farther north other eruptions were in progress, and the basanites and basalts of the Waikato volcanic plains, of the Auckland cones, and of the Bay of Islands were emitted. Whether this activity was simultaneous with that which gave birth to Ruapehu and other andesite cones is still unknown, as also is the relative period of eruption of the older basalts of the north and of Pirongia and Ivarioi.
The activity that still exists is confined to areas of augite hypersthene andesite at Ruapehu, Ngauruhoe, Tongariro, Tarawera, and White Island. On the whole, this succession appears to support the differentiation theory, though necessarily much research is still required to establish without question the true order of succession of these lavas.
The difference between the volcanic areas of Auckland and Taranaki is very striking. In the former are found many minute cones ; in the latter is a gigantic cone whose base covers an area greater than that on which the sixty-five Auckland cones are situated. When compared with volcanic districts elsewhere, it will be found that the condition of Egmont represents the normal condition of volcanic areas —that is to say, there is generally a tendency for volcanic matter to build up large and sometimes isolated cones rather than a nest of miniature mountains. The exceptional condition of Auckland can be best explained by adopting the hypothesis that an intrusive sill of basanite underlies the Cainozoic deposits. Whenever the molten matter which formed this sill encountered fissures, eruptions took
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place through, them until they were stuffed up. This idea is supported by the occurrence of Rangitoto, the largest of the cones, at the place where the slates, above which the sill must have been intruded, rise above the Cainozoic sediments.
Fig. 68. —Distribution of Deposits from Tarawera Eruption.
The dark portion shows where dust was deposited. The inner line 1 encloses the area where the deposit was more than 1 foot thick ; between 1 and 2 the deposit was over 6 inches deep ; between 2 and 3 it was more than 2 inches ; between 3 and 4 more than 1 inch. (From Professor Thomas's map.)
By far the most startling evidence of volcanic activity was afforded in 1886 by the Tarawera eruption. Though formed of volcanic
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rock, the mountain was flat-topped, was composed of rhyohite, and showed no signs of a crater. Slight earthquakes occurred at 11 p.m. on the 10th June. They continued to increase in intensity, and at 2 a.m. on the 11th there was a deafening roaring sound, and a fissure was formed along the summit of the mountain. From it issued a huge volume of steam, with clouds of dust and fragments of volcanic rock. The clouds were illumined by reflections from the molten rock within the fissure, thus producing the appearance of flames. About an hour later there was a still more violent explosion, and a huge rent was formed extending seven miles from the base of Tarawera in a south-west direction. Over the area of Lake Rotomahana the rent was wide, and constituted an explosion crater 500 feet deep. Throughout the length of the rift small craters were formed, and maintained some activity for several months. The lava was andesite, a rock that is abundant at intervals throughout the volcanic region.
The eruption was the result of the ascent of heated rock through a fissure in Tarawera. The second explosion at Rotomahana was caused by the lateral intrusion of some of the molten matter beneath the region of the lake, where smaller pressure allowed the steam contained in the rock to explode more violently.
The area covered with volcanic ash extended from Rotorua to Tauranga and to Gisborne, a total area of 4,000 square miles. The thickness of the material added to Tarawera was 170 feet, and a depth not much less covered the ground round the great rent. The depth decreased rapidly as the distance from the points of eruption increased. The total volume of material ejected was about one cubic mile. The rent that was formed is now nearly filled by the water that has drained into it from the surrounding land, and the new Rotomahana Lake thus formed has a depth of 500 feet and an area of seven miles by three. The deposition of material also blocked up the outlet of Lake Tarawera to such an extent that the water of the lake rose nearly 40 feet. The outlet has since been eroded again, and the level of the lake is now but little higher than it was before the eruption. The sites of the Pink and White Terraces were thickly covered with fragmentary matter, and the amount of volcanic activity on their sites showed that during and after the eruption they must have been totally destroyed. They are now at some depth below the level of the lake, which has risen to a height far above its previous level, though it will fall again somewhat when its outlet to Tarawera has become eroded again.
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GEOLOGY OF NEW ZEALAND.
The relation of the New Zealand volcanic areas to those of the rest of the world as a whole is not very well defined. It has been suggested by Prior that these Islands are a meeting-place of two volcanic influences. New Zealand is part of the so-called "girdle of fire" that surrounds the Pacific Ocean. The character of the rocks of
Fig. 69.—Effect of Taraweba Eruftion on Rotomahana.
The darkest areas within the boundary of Rotomahana show the position of the old lakes Rotomahana and Rotomakariri before the eruption of 1886 ; the lighter areas the position of the lakes directly after the eruption.
Ruapehu and the central district generally is in consonance with this idea, for the "girdle of fire" which extends, though with many interruptions, through the Andes, the Rocky Mountains, Alaska, the Kuriles,
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Japan, Java, Sumatra, and the volcanoes of New Hebrides is formed almost throughout of andesitic matter. The alkaline rooks of Dunedin appear to have some relationship with those of East Africa, Madagascar, Kerguelen Land, and eastern Australia. The basaltic rocks of the North Island appear to be of a type similar to that of the volcanoes that in so many places dot the surface of the Pacific, though similar types cover a large area in Victoria.
Fig. 70.—Twenty-five Years' Growth of Vegetation on Tuff of Tarawera Eruption.
Hot Springs.
In many volcanic districts in which the main activity has ceased or is dormant there are minor escapes of steam or hot water. Sometimes these are emitted from the actual craters of the once-active volcanoes., A volcano which has declined to this phase of activity is a solfatara. White Island, in the Bay of Plenty, and the Red Crater, on Tongariro, are examples. Although Ruapehu, Te Mari, and Tarawera at the present time are not showing activity any greater than that which is properly displayed by a solfatara, they have within historic times been far more violent, and are not properly classed with solfataras.
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In addition to steam, various gases are emitted by solfataras. Of these, sulphur-dioxide and sulphuretted hydrogen are the commonest. A reaction takes place between these gases ; sulphur is liberated and deposited round the vent from which the steam escapes.
H 2 + 2S0 2 = 2H 20 + 3S.
In many countries it has been proved that helium and other rare elements are emitted, but it has not yet been shown that such gases escape from the New Zealand solfataras.
Jets of steam issue from various other localities as well as from the craters previously mentioned. A steam-jet is called a fumarole. There is a group of large fumaroles at Te Ketetahi, on the north flank of Tongariro, and there are others at Tokaanu. Karapiti, a fumarole of great constancy and power, is situated near the Waikato, a few miles north of Taupo. There are other fumaroles at Wairakei, Orakei Korako, Waiotapu, Rotorua, Waimangu, and elsewhere. The acid gases that escape from a fumarole in time decompose the rock that borders the crevices through which they pass. Some of the steam condenses, and, mixing with the decomposed matter, forms a thick mud through which steam continues to bubble with a "flopping" noise. These are porridge-pots, and are common in all the thermal districts mentioned above; but they are especially large and active at Tikitere, on the eastern shore of Lake Rotorua, where the weird and hideous appearance of one of the larger examples has given rise to the name of "The Inferno." The constant bubbling of the steam scatters some of the mud over the sides of the porridge-pot, and a cone of small height is built up. The "Mud Volcano"at Wairtapu has been thus formed.
Hot springs are found in most of the thermal regions alongside the fumaroles and porridge-pots. The water is perfectly clear, and has a sky-blue colour. In some cases the springs are simply hot, with no appearance of ebullition ; in others there is a constant but gentle boiling ; others again show periodic ebullition, which in some instances is so violent that a column of water is thrown up to a height of 60 feet in the ordinary manner of a geyser. In nearly all instances the water is alkaline, and contains much silica in solution. As the water cools, its solvent power is diminished, and some of the silica is deposited. As the cooling is most rapid on the margin of the pool, deposition is also most rapid there, and a basin of white silica is gradually built up. If the spring is situated at the summit of a slope, basins
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are formed wherever the water collected into small pools when it first flowed down the slope. In this way were formed the terraces of Te Tarata and Te Otuaporangi—the White and Pink Terraces —which constituted the most attractive sight of the hot-lakes district. The disastrous eruption of Tarawera in 1886 completely destroyed these terraces, and covered up their remnants with a hundred feet of scoria and dust. There are other terraces, but of far smaller size, at Waiotapu and Orakei Korako, and they are not fed by the waters of large geysers, as were the White and Pink Terraces. Some of the hotspring waters contain so much silica in solution that in a day or two an encrustation is formed over any object immersed in them. As has been explained elsewhere, the deposition of silica is not entirely due to the mere cooling of the water, but is aided to a considerable extent by the activity of certain fresh-water algae which are able to live even in the hot water.
A geyser is a periodic hot spring from which boiling water is ejected to a considerable height at regular intervals. The geyser in Iceland has been carefully examined in order to ascertain the cause of this extraordinary action. It has been clearly shown that the periodic action depends mainly upon the long geysertube that ends in the crater at the surface and is fed by springs or steam at the lower end. In the Iceland geyser the tube is 80 feet long. The water that fills it is gradually heated by the steam that enters below. At the surface the water is under the atmospheric pressure only, and boils at 212° Fahr. At the bottom of the tube there is the pressure due to the weight of a column of water 80 feet long, in addition to the atmospheric pressure. Under such a high pressure water must be raised to a temperature of 278° Fahr. before it boils. The steam gradually heats the water throughout the tube to the temperature of the boiling-point, which varies at different depths of the tube, according to the distance from the
Fig. 70a.—Section across Great Geyser, Iceland.
The figures in the middle represent the depth ; the right-hand figures the temperature of the boilingpoint at the given depth; the figures on the left give the actual temperature at the different depths.
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surface. When throughout the length of the tube, the water is heated almost to the boiling-point, convection currents may raise water from one level to another where its temperature is above the boiling-point at that depth. Some of it at once changes into steam. The steam expands, and forces some of the water out of the tube. By this action the pressure throughout the tube is reduced, and at other levels the temperature is above the boiling-point under the reduced pressure. More steam is formed, and its expansion drives the boiling water out of the tube with considerable force. Contact with the air soon reduces the temperature of the water to that of the boiling-point at the surface. The eruption then ceases until the water in the tube is again heated throughout. At Rotorua geysers have been made by thrusting a pipe down the throat of a fumarole, and allowing its upper end to terminate below a small basin of water.
The great geyser of Waimangu, which has now been quiescent for five years, was probably of the typical nature of geysers, though the tube was not well defined. The escape of steam appears to have been prevented by the muddy material that filled the passage. An eruption could not take place till sufficient steam had accumulated to force the muddy material out and clear the passage. The explosive force of this geyser was enormous, and it sometimes expelled mud and water to a height of 1,500 feet. Since it has become quiescent some fumaroles of the neighbourhood have emitted larger quantities of steam than they did formerly. This goes to prove that much of the steam that formerly issued from Waimangu has now found other passages through which it can escape.
In the thermal valley at Wairakei there are geysers that well illustrate two main causes to which activity may be due. The activity of the Prince of Wales's Feather Geyser is controlled by allowing a small stream of water to enter its crater. When an eruption is required the stream is blocked up, and the steam gradually heats the water in the geyser-tube. In this case it is evident that the temperature of the water is the main cause of activity. The Lightning Geyser, on the other hand, boils gently in ordinary circumstances; but if an outlet is opened to such an extent that the level of the water is lowered by a few inches the boiling becomes much more vigorous, and small eruptions take place. The activity of this geyser evidently depends upon the pressure of the water, which is just able to prevent the formation of much steam until a release of some of the water reduces the pressure so far that the steam is formed again in some volume.
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Besides the silica, sulphur, steam, and other gases, there are many other substances in solution, as analyses of the spring-waters show. In some instances deposits of other substances are formed round the springs. At Waiotapu and Orakei Korako, for instance, there is a considerable deposit of alum. The very vivid colour of such lakes as Tikitapu is almost certainly due to mineral substances, such as alum, in solution. The green colour of other lakes in the hot-springs district certainly depends upon some other substance in the solution. The following are typical analyses of the dissolved material in some of the well-known springs of the thermal district. The results are expressed in grains per pint.
A. Priest's Bath, Rotorua; temperature, 98° to 110° Fahr.
B. Postmaster Bath, Rotorua; temperature, 98° to 110° Fahr.
C. Painkiller, Rotorua.
D. Waikiti, Ohinemutu.
E. No. 1 Spring, Te Aroha; temperature, 135° Fahr.
Discussion still takes place about the origin of the water that escapes from the hot springs. There are yet some supporters of the view that
A. B. C. D. E. Na 2S0 4 .. .. 2.6 4.7 4.2 0.4 5.5 K 2S0 4 .. .. 0.2 .. 0.4 .. CaS0 4 .. .. 1.1 0.7 .. .. .. MgSO4 .. .. 0.4 0.2 .. .. .. Al 2 (S0 4 ) 3 .. .. 3.1 4.8 .. .. .. FeSO 4 .. .. 0.2 0.6 .. .. .. H 2 S0 4 .. .. 3.2 4.6 .. .. .. HCl .. .. 0.5 0.9 1.0 .. .. Si0 2 .. .. 2.3 2.5 2.3 3.5 1.1 NaCl .. .. .. .. 6.6 5.5 8.6 KCl .. .. .. .. 0.2 .. 0.25 CaC l2 .. .. .. .. 0.4 .. MgCl 2 .. .. .. .. 0.2 .. .. A l2C l6 .. .. .. .. 0.6 .. .. Fe 2 Cl 2 .. .. .. .. 0.6 .. .. NaHC O3 .. .. .. .. 3.0 65.9 CaH 2 (C O3 ) 2 .. .. .. .. 0.1 1.5 MgH 2 (C O3 ) 2 .. .. .. .. 0.1 1.0 Total .. .. 13.4 19.2 15.5 13.0 84.1
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the water is derived from rain and streams that sink into the rocks and become heated by the volcanic material which in these regions is near the surface. When the percolating water reaches some major fissure which has an orifice below the level where the water soaks into the soil, it is returned to the surface as a spring. The opposing theory is more strongly supported. The water is regarded as "juvenile"— that is, it is supposed to have been contained in the volcanic material since the earliest stages of the earth's history, and is now reaching the surface as the volcanic matter cools and can retain it no longer. This theory depends on the well-known fact that many molten substances can retain in occlusion gases that are given off when the melted substance solidifies. When the surface of the earth was hot and molten some of the steam, which would then constitute the most important part of the atmosphere, would be occluded, and disturbances of the molten surface would end in some of the upper layers, with their occluded gases, passing to regions at some distance below the surface. During the lapse of ages the earth has been gradually cooling, and the molten matter is now so cold that it can no longer retain the steam, and is now gradually liberating it. Through any lines of weakness that may exist the steam rises towards the surface, and is partly or wholly condensed, forming geysers, fumaroles, or hot springs, as the case may be. The occurrence of hot springs in volcanic districts where there is evidently molten material that is gradually cooling, the fact that volcanic rocks do contain much steam, and the large amount of dissolved mineral matter in the water of hot springs are all facts that strongly support this theory.
The hot springs that are found far away from a volcanic district may have a similar origin. At the Hanmer Plains, in the valleys of the Waiho and Karangarua Rivers on the west coast of the South Island, hot springs are well known. At Hanmer the waters appear to rise along a fault plane, and may come from some cooling mass of rock lying at a great distance from the surface. On the other hand, all of these localities are adjacent to high mountainous country, and percolating water flowing through the rock of the mountain would acquire a high temperature before it reached the level of the valley. It is well known that during the boring of the Simplon Tunnel a stream of water was encountered that had a temperature as high as 120° Fahr. If there was an outlet for such internal streams at the level of the valley-floor, the water would issue as a hot spring.
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CHAPTER X.
MOUNTAINS.
Mountain-ranges of complex structure form the most striking feature of the country. Though the culminating peak of Mount Cook rises to a height of little more than 12,000 feet, above the snowline there is a large area of snow-covered country, and ranges with an average height of nearly 6,000 feet extend from Puysegur Point to Separation Point. In the North Island the ranges are nearly as continuous from Cape Terawhiti to East Cape, though their altitude is much less. Nor are these the only mountainous districts. The mountain-masses of central and eastern Otago, the Kaikouras, the Tasman Mountains, and, in the North Island, the country round Hokianga, as well as the volcanoes of the central district, are all more or less distinct from the main mountain-backbone.
When first deposited in a river-valley or along the sea-coast the sediments rest almost horizontally. Volcanic rocks, too, except on the actual slopes of a volcano, have no high angle of inclination when formed, for all the large lava-streams extend to the flatter country round the cone, and there solidify into gently inclined rock-masses. The lines that mark different kinds of sediment or different conditions of deposition are called stratification planes. They can still be seen even when the soft sediments have been hardened into rock, unless metamorphism has produced a complete change in the composition and structure. In many rock-exposures, especially in mountainranges, the originally horizontal stratification planes are now inclinedin some cases are even vertical. Since there is no reason to suppose that the conditions of deposition have in the past been markedly different from those of the present day, we must conclude that the inclination of the strata has been caused since they were deposited.
The actual appearance of the stratum at the surface is called its outcrop. In ordinary cases the outcrop of a stratum on a level surface would be a straight line. This line is the strike. The dip is at right angles to the strike, and is therefore in the direction of the steepest slope down the surface of the stratum. The dip is measured by the value of the angle between the line of dip and the horizontal plane.
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Fig. 71.—Diagram illustrating Normal Faulting.
A, B, C, Three rectangular surfaces of a model; x, y, the direction of the strike of the strata; t, v, direction of dip of the strata; t, v, w, angle of dip of strata; q, r, s, fault plane; q, p, o, angle of hade of fault; p, o, throw of fault; m, n, stratigraphic throw of fault; h, j, and k, I, two components of heave of fault; r, t, displacement along fault plane. The surface B is supposed to have been levelled down by erosion after faulting had taken place. The diagram also illustrates the apparent different thickness of strata when seen on different surfaces. The true thickness of each stratum is seen in the surface A, which is at right angles to the strike x, y. For the sake of distinctness the strata are not characteristically marked on all the surfaces.
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It is usually found that when a traverse is made at right angles to the strike the dip gradually changes as successive strata are passed. Occasionally it changes rapidly from one direction to its opposite, and the strata may form an arch or anticline, or a trough or syncline. The curve of the arch or trough may vary widely : sometimes it is sharp and acute, at others rounded or obtuse. The general term fold is used for both anticlines and synclines. When a fold is upright its axis is vertical; but the axis of an inclined fold is itself inclined, while that of a recumbent fold approaches the horizontal.
The width of the outcrop of a stratum varies in accordance with the inclination of the surface of the ground to the dip of the stratum. When the two are parallel, the outcrop of a single stratum will cover the whole area. The wider the angle between the two planes the
Fig. 72.
Plan and Section of a Geological Dome.
Plan and Section Of A Geological Basin.
narrower is the outcrop, until, when the two planes are at right angles to each other, its extent is exactly equal to the thickness of the stratum.
The direction of the line of outcrop in a horizontal surface is a straight line when the strike does not change. In the more usual case, when the surface is undulating, the line of outcrop becomes a complicated curve, the nature of which depends on the relations between the form of the surface and the dip of the stratum. The following general rules can be applied : —
(1.) When the stratum is horizontal, the outcrop has V-shaped curves pointing up the valleys and down the spurs.
(2.) The same form is found when the dip is in the same direction as the valley-floor, but the slope of the stratum is less steep than that of the valley.
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(3.) If the stratum slopes more steeply than the valley-floor, but in the same direction, the outcrop will have a curve convex to the entrance of the valley.
(4.) A dip that is opposed to the slope of the valley-floor will have a curved outcrop concave to the entrance of the valley—that is, of the same nature as in (1) and (2).
(5.) When the strike is known as well as the outcrop at one point, another outcrop of the same stratum will be found on the line of strike at the same level as that of the first outcrop.
Fig. 73.—Diagram Showing Relation of Contours to Outcrop.
a, b, c, d, e, f, g, contour-lines, h, k, l, m, n, streams. In the valleys the contour-lines are convex to those of higher value; on the spurs they are concave to those of higher value when the point of view is the high ridge. o, p, outcrop of a stratum more highly inclined than the valley-floor; dip, 70°. q, r, outcrop of a stratum less highly inclined than the valleyfloor ; dip, 5°. The points where the outcrop crosses a contour-line all lie in a line in the direction of the strike.
The strike, too, may change in direction. If it is so curved that it encloses a circle, and the dip is inwards, a basin is formed. In a dome the strike is circular and the dip outwards.
A mountain-range is composed of a succession of anticlines and synclines, but usually forming, on the whole, a bulge on the earth's surface. This is an anticlinorium. The anticlines and synclines mentioned before form the sides or limbs of this anticlinorium. The limbs of these anticlines and synclines have secondary folds on their flanks and on their limbs. Again, folding is continued on a smaller
9 —Geology.
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Fig. 74.—Finely laminated Scuist Rock crumpled by Pressure, Central Otago.
Lent by Department of Mines, New Zealand.
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and smaller scale until the final smallest series is to be seen only in a hand-specimen of rock (Fig. 74) or even in a microscopic preparation.
It would be expected that the anticlines would form the summits of the mountain-ridges, but usually this is not the case ; the reverse is often found. The crests of the mountains are often structurally the bottoms of the troughs of synclines. This apparent contradiction of reason by fact may be explained by the consideration that at the crest of an anticline the strata are stretched, perhaps even fractured —at any rate, they are not hardened or compressed. These portions, therefore, are most readily attacked by the destructive action of the atmosphere. The anticlines are therefore rapidly worn away. On the other hand, the strata in the syncline are compressed and hardened ; they become relatively resistant, and in the course of time stand out prominently when the anticlines have been worn down below their level. It is not unusual to find the river-valleys running along the crest of an anticline, while the mountain-ridges are in structure the troughs of synclines (Fig. 76).
Fig. 75.—Contorted Triassic Sandstone, Russell, Bat of Islands.
In many cases mountains are no more than the remains of horizontal strata once continuous over a large area, but largely removed by denudation. These are residual mountains. The hilly
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country between Wanganui and Ruapehu is an example on a small scale.
Even in mountain-ranges it is not always the case that the rockfolds are of the complex nature described. Especially in regions where the action of pressure has not been particularly intense, the folds sometimes remain simple, or there may be a series of parallel and equally inclined folds. Such a series constitutes isoclinal folds (Fig. 78).
In addition to undergoing the bending movements that have produced folds, rock-masses are from time to time subjected to fracture. When not immeasurably slow, the bending of the rocks must cause fracture ; thus arises a series of fracture planes or joints at right angles to the stretching-force in an anticline—that is, parallel to its axis. As an anticline is usually pitched —that is, it changes in altitude from point to point—there must be stretching, and consequently fracture, transversely as well as longitudinally. By this
Fig. 76. —Diagram of Fold Mountains.
a, a, surface before denudation; b, b, surface after denudation.
Fig. 77.—Diagram of Residual Mountains.
a, a, surface before denudation b, b, surface after denudation.
action a series of transverse joints will be formed at right angles to the longitudinal series previously mentioned. A rock-mass thus gently folded will be split up by joints into more or less rectangular portions, and if stratification planes also are well marked it may break out on any exposed surface into cubical blocks. If the folding is so intense that the anticlines become steep and inclined, the joints are less regular but more numerous, and the rock breaks out into angular fragments of all shapes. Irregular jointing such as this is a marked characteristic of the hard sandstones and shales of which the mountainranges of New Zealand are formed. It is this, combined with the action of frost, that promotes the formation of the huge talus slopes so noticeable on the eastern portion of the mountain-range of Canterbury.
In many cases a mere jointing does not entirely relieve the tension to which the rocks are subjected—a differential movement of the
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rocks may take place on either side of a fracture plane. Such a fracture plane is a fault. In all ordinary instances the fault plane is nearly vertical. Its actual inclination is recorded by a measurement of the angle that its plane makes with the vertical plane. This angle is termed the hade of a fault. In normal faults the hade is always towards the downthrow side of the fault—that is, towards the side on which the
Fig. 78.—Diagrams Of Folds.
Illustrating the occurence of parallel inclined strata throughout a mountainrange.
strata rest at the lower level. The throw of a fault is the vertical distance between the broken ends of the same stratum on opposite sides of the fault plane (Fig. 71).
In many cases it is probable that the movements along fault planes produce no visible effects at the surface of the ground, for the actual movement usually takes place at some distance from the surface, and
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adjustment between the strata minimizes the surface effect. Each individual movement along a plane is slight, and any scarp that may be caused on the surface is usually soon obscured by the action of denudation. The Basin Ranges of North America, however, form a remarkable exception, for in this dry region, where denudation is at a minimum, large fault scarps are visible. Mr. McKay has described and photographed several scarps of this nature in the Amuri country, but they are all relatively small.
It is evident that faults must have a considerable importance in mining operations, for when one is encountered a rock formation that is being excavated will suddenly terminate, and, unless the hade and throw can be ascertained, great difficulty will be experienced in finding the same stratum on the other side of the fault plane. The following general statements will explain simple difficulties that may arise : —
1. If the fault is parallel to the strike, a stratum may be picked up on the downthrow side by sinking a shaft through the depth equal to the throw of the fault and then driving horizontally.
Fig. 79.—Fault At Landguard Bluff, Wanganui.
a, a, blown sand (Recent) ; b, b, Younger Pliocene ; c, c, bed of conglomerate ; d, fault; x, y, z, hade of fault.
2. If the fault is parallel to the dip, and the stratum has been worked on the upthrow side, a drive should be made at right angles towards the rise of the stratum, and the distance that has to bo driven can be calculated if the throw of the fault and the dip are known ; or a shaft may be sunk through the depth that can be calculated if those data are known.
3. If the fault has an intermediate direction between that of the strike and that of the dip, a combination of these methods may be employed.
The effect of faulting on an outcrop is very deceptive also. A fault plane parallel to the strike causes a repetition of outcrop of one or more strata ; a fault parallel to the dip causes a horizontal displacement of a stratum, which is termed the heave.
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It is generally believed that movements along fault planes are such that the upthrow side remains stationary, while the rocks on the downthrow side sink down. The force of gravity is believed to cause the movement, and the downthrow side is thought to be insufficiently supported. For this reason normal faults such as those described above are often called gravity faults.
It appears that occasionally the movement along a fault plane is horizontal. Fault planes of this nature are called blatt planes by Suess. The movement that took place along the plane of fracture that was formed at the time of the San Francisco earthquake in 1906 was mainly horizontal.
Another type of fault is only slightly inclined. Such faults are often found along the axis of a recumbent fold (Fig. 80). After the
Fig. 80. —Diagram Of Reversed Fault.
Illustrating a thrust plane or reversed fault which has been developed along the axis of a recumbent fold, a, b, thrust plane ; c, d, amount of displacement.
fracture has been formed along this axis a mass of rock is thrust upwards along the plane, and if the movement is of sufficient magnitude rocks of great age may finally rest on others of much less antiquity. Such planes of fracture are called thrust planes or reversed faults. The structure of New Zealand is not yet sufficiently well known to enable us to state exactly where such structures are to be found. Mr. McKay has described thrust planes of great magnitude in the southern part of Marlborough ; one of these occupies the valley of the Awatere, and another that of the Clarence. In both cases a mass of rocks of Maitai
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age constituting a mountain-range has been thrust over a younger series of the later Cainozoic age (Pig. 81).
In all known cases where disturbances have taken place along fault planes there has never at any time been a great movement, and disastrous results on the surface have not been caused. The total effect of numerous small movements may be so great that the fractured ends of a stratum on opposite sides of a fault are sometimes thousands of feet apart. It is seldom that such a movement has caused a permanent submergence of low-lying tracts of land, or has permanently raised the land-area above sea-level. The latter, however, did take place in Wellington in 1855, when the sea-coast of Palliser Bay was raised 9 feet and the bed of Wellington Harbour itself rose 5 feet. At the same time a scarp was formed extending north-east from Wellington along the base of the Tararua Mountains. Prom the fact that the land was raised, it appears that the rock-movement took
Fig. 81.—Section Across Kakikoura Mountains.
a, Amuri Bluff; b, Cook Range ; c, Green Hills (Marken Face) ; d, Seaward Kaikouras ; e, Middle Clarence Valley; g, Inland Kaikouras ; i, Schooner Range; k, Waihopai Valley; l, Wairau Valley; m, Red Hills; F, F, thrust planes ; 1, Miocene ; 2, Triassic.
place along a thrust plane. At the time of the great Mino Owari earthquake in Japan a fault scarp 20 feet high was formed across some roads.
The actual effect produced by earth-movements is an earthquake. The friction between the moving masses of rock generates waves of transverse and of longitudinal vibration. Passing through the crust of the earth these cause a vibration of all objects situated on it, and constitute an earthquake. The waves have a long period but small amplitude. As the distance from the earthquake-centre—that is, the fault plane —increases, the period of the waves becomes greater and greater, and their passage cannot be felt. On the other hand, when such vibrations leave solid rock for loose strata at the surface their period decreases and their amplitude becomes greater. The waves are then so changed that in very severe shakes their passage along the
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surface can be clearly seen. Changed waves of this nature cause the disastrous effect of an earthquake, but are, of course, most severe near the fault plane along which movement has taken place. The positions of the centra, or points of origin, of all the more important earthquakes in New Zealand have been admirably recorded by Mr. Hogben in " The Geography of New Zealand." It will be noticed in the map there given that the earthquake-centres do not appear to have any relation to the centres of volcanic action. The region of most intense seismicity is in the neighbourhood of Cook Strait, and this indicates that in that region the earth's crust is subject to strains from which relief is from time to time afforded by rock-movements along fault planes. These statements must not be taken as an assertion that earthquakes are never associated with volcanic action, for it is well known that volcanic activity is always accompanied by earthquake shakes. These are caused by the sudden expansion of the steam as it rises from the heated interior of the earth. The earthquakes, however, caused by such explosions are local, and of small intensity outside the immediate neighbourhood of the volcano. This was clearly shown in 1886, when the earthquakes caused by the terrific explosions at Tarawera caused no disastrous effects even at Rotorua, which was only twenty miles from the centre of activity.
The structure of the New Zealand mountain-ranges is not yet sufficiently well known to allow us to say exactly the nature of the synclines and anticlines that occur in them. Sections have been drawn by Hutton, Haast, and Hector to represent their structure in various places, but they are very generalized, and do not pretend to accuracy. There is no doubt that the mountain-ranges throughout the country are composed of much-folded rock, and there is a remarkable uniformity in strike and dip over a large distance on the eastern side of the main divide. This appears to indicate that this portion of the country is composed of a series of isoclinal folds; but, as fossiliferous strata are almost entirely wanting, much detailed work is necessary to establish the exact nature of the folds.
The distribution of mountains in New Zealand allows of a simple general description. In the North Island a prominent chain of steeply folded rocks extends from Cape Terawhiti to East Cape. The chain consists of several nearly parallel ranges of relatively low altitude, from 5,000 to 6,000 feet. The rocks of which the ranges are formed are of Maitai age, their strike is parallel to the direction of the range,
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Fig. 83.—Section of Western Part of Southern Alps.
From Taipo Eiver north-west to Westland Plains. (After Bell and Fraser.)
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and they are steeply folded throughout. In his sections Sir J. Hector represents the strata as almost vertical, but does not indicate the nature of the folds of which the strata now exposed form a part. In various portions of its extent the chain is known by different names —the Rimutaka, Tararua, Ruahine, Kaimanawa, and the Huiarau Ranges all form portions of the one chain. Several relatively low passes cross the chain, and are used for road-lines. The gorge of the Manawatu separates the Ruahine from the Tararua Range, and has been corroded by the river almost to the sea-level.
Whether this chain is a complete mountain-chain, or whether it is only a portion of a much more extensive structure of which an eastern part now remains in the chain mentioned and a western part in the Hauturu, Hakarimata, and Wairoa Mountains, and the hills of Waiheke Island, is not definitely known. If the latter theory is adopted, the mountainous masses south of the Bay of Islands, with the Raitea and the Maungataniwha Mountains, must be considered as still more westerly portions of the same mass. At present definite information is lacking in regard to the strike of the Maitai rocks of which these northern hills are formed, and authorities are not wanting who regard all these northern elevations as belonging to a separate and older axis of rockfolding which was directed to the north-west. Suess, however, in his "Face of the Earth," adopts the former explanation, and says, "These are only isolated fragments of the sunken range." If this idea be accepted, it is evident that in the North Island there is a continuation of the mountain-chain of the South Island, though volcanic action and submergence have at different times accumulated so much later material round the spurs and peaks of the older range that the continuity of the mountain-mass is now concealed. It is noticeable that the part of the mountains that still exists is apparently a continuation of the Kaikoura Mountains, of the South Island.
Besides these folded ranges of Maitai rocks there are a large number of volcanic mountains that rear their summits above the volcanic plateau. So far as is known, nearly all these mountains are composed of andesite rocks. Few of those in the central parts of the plateau have summits more than 4,000 feet above sea-level. Tauhara, Karangahape, Mangatautari, and Edgecumbe are among the bestknown. Besides these cones there are some flatter-topped rhyolite mountains that appear to be remnants of extensive flows. Horohoro and Ngongotaha are examples. To the south and west of the volcanic
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Fig. 85.—Section From Stewart Island to Waitaki River Across The Anticlinorium of Otago. (Adapted from Hutton's "Geology of Otago and Southland.")
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plateau the volcånoes attain their most majestic proportions. In the south-west corner the impressive group of Ruapehu, Ngauruhoe, and Tongariro still display an activity that may yet burst out into furious eruption. These volcanic masses are formed entirely of hypersthene andesite. Sixty miles to the west rises the cone of Egmont. It is quite separate from the rest of the volcanic area, and is composed of hornblende andesite.
Fig, 86.—Section From Maunganui Bluff To Three Gables. (After Hector : "Handbook of New Zealand Geology.")
The linear arrangement of the mountains of the South Island is even more pronounced than in the North Island. The axis of greatest elevation extends from the north-west through the Anatoki, Tasman, Marine, Lyell, and Victoria Mountains to the Spenser Mountains, where there is a union with an axis that runs southwest from D'Urville Island. From this junction the axis passes continuously south-west at a distance of less than twenty miles from the coast for the greater part of its length. On the western side of this axis there are few mountains that can be regarded as more than spurs of the main axis. The Paparoa Range and the Darran Mountains are in some measure an exception, and attain the dignity of mountainranges. On the eastern side of the axis prominent and important spurs extend nearly continuously for thirty or forty miles from it. Separated by broad river valleys and basins for the greater part of their length, the spurs often expand towards their distal extremities, and form a forbidding and almost unbroken barrier fronting the level or undulating country of the east. In the north the Kaikoura Ranges, situated near the east coast, run nearly parallel to the main axis. In the southern districts there are many independent ranges with a north-south trend. The directions of the mountain-ranges are clearly shown in the diagram on the next page.
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Fig. 87. —Map Showing Mountain-ranges of New Zealand.
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The axis of elevation does not correspond with the structural axis. Hutton has pointed out that the structural axis extends north-east from the west of Mount Cook to Cook Strait. South of Mount Cook the axis bends sharply to the south-east, and ends on the east coast at Dunedin. In general this axis is much nearer to the west coast than is the axis of elevation. The reason for this being called the structural axis is to be found in the rocks of which it is composed. These are schistose, and the metamorphism becomes less and less as the rocks are followed eastward. In the north-west of the Nelson Province schistose rocks lie below sediments that contain fossils of Ordovician age. Hector and Hutton both correlated the series of schists on the western slopes of the Southern Alps with those of proved age in the Nelson District. Hector extended this correlation to the Otago schists ; while Hutton, for reasons considered under the chapter on metamorphism, classed these as of Archaean age. For these reasons the structural axis as here described was thought to be formed of rocks of extreme antiquity.
It is evident that the occurrence of the oldest rocks on the west coast implies that in this region the folding action was most intense, and elevated the oldest and deepest rocks. Hence the centre of the range was originally there. Hutton has strengthened this explanation of the structure of the central alpine district by comparisons with Nelson and with Otago. In the former area there is a considerable extension of the folded rocks to the west of the schistose band. In Otago, where the structural axis traverses the breadth of the Island, the schists are flanked by Mesozoic sediments on either hand; and on the southern side in particular the folds become less and less sharp, and gradually the rocks flatten out to horizontal strata between Waikawa and Fortrose. A consideration of these facts led to the supposition that in the central districts between Nelson and Otago there had formerly been a westerly extension of the folded rocks which the prolonged action of the violent westerly weather had in the course of geological ages removed. The idea is further supported by the occurrence of less-metamorphic and apparently unaltered rocks in some localities, as at Ross between the schists and the coast.
From time to time unconformities have been mentioned as occurring in the rock series between the Mesozoic rocks on both sides of the Otago axis and the schistose rocks of the axis. Investigation has, however, up to the present wholly failed to establish the
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occurrence of such structures. Repeated traverses of the Southern Alps, too, have shown that the rocks across the range form a completely conformable series. A recent bulletin of the Geological Survey, descriptive of the Hokitika "sheet," has failed to reveal anything more than a "possible unconformity." As stated elsewhere, the metamorphic character of the rocks on the western slope may be due to the action of the intrusive granites; and, if this is the case, there is no reason to suppose that they represent sediments of older Palaeozoic periods. Lastly, it must be remembered that Suess has insisted upon the unilateral structure of mountain-ranges. If this is the structure of the Southern Alps, they may be folded against a foreland that now lies submerged below the waters of the Tasman Sea.
Whatever may be the structure of the main chain of the Southern Alps, there are some mountainous regions that have an independent origin. The mountainous block between the lakes and the fiords is formed of ancient crystalline rocks, and the mountain-summits in this district are simply rock-masses that have escaped denudation more than have the rocks around them. They are residual mountains.
Fig. 88. —Diagram Of Block Mountain. F, F, fault planes.
In Otago the mountain-ranges run generally north-and-south, crossing the schist structural axis obliquely. In "The Geography of New Zealand" this area has been described as an ancient peneplain elevated and dissected, while the ranges have become isolated in consequence of the interference of streams. Professor Park has more recently described these mountain-ranges as "block mountains," owing their relative elevation to the subsidence of the country round them. This explanation is in some measure supported by the upturning of the Pliocene strata in the basins, though it is strongly opposed by the very youthful nature of the river-valleys that pass through the country. Almost everywhere, except in the flat basins, these are narrow deep gorges. This points to elevation of the mountains rather than depression of the basins.
The isolated Kaikoura Ranges have still another origin, according to McKay. This observer has described profound thrust planes in
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the valleys of the Clarence and Awatere. His sections indicate that he explains the elevation of the Landward and Seaward Kai-
kouras by the action of a gigantic thrust from the westward. This lateral thrust appears to have moved masses of Maitai rocks up the reversed faults. This movement has elevated the rock-masses to such an extent that they now form lofty moun-tain-ranges. The thrust planes .as represented in McKay's diagrams are exceedingly steeply inclined for such planes (Fig. 81).
Fig. 89.—Section From Cape Egmont to Porangahau. (After Hector.)
In addition to these main mountainmasses formed of Maitai and older rocks, and the volcanoes of relatively youthful age, there are many hilly districts that might almost be called mountainous. The country extending westwards from Ruapehu to within fifteen miles of the coastline is in many places more than 1,000 feet high, and occasionally, in more inland localities, is 2,000 feet and more above sea-level. Though the strata tilt outwards gently from the central area, the whole country is here composed of Miocene and younger rocks that have been elevated without any folding action. The hilly structure is entirely due to the corroding action of running water. It is otherwise in the Puketoi Hills, which form the eastern limit of the basin of the Manawatu River. Here the Cainozoic rocks are highly inclined, and have been compressed into a series of steep folds. In other localities—e.g., the upper Buller, Poverty Bay, and elsewhere—the Cainozoic rocks often occur at high levels. They are usually horizontal, or are at most tilted rather than folded. Morgan has recently contended that the Southern Alps owe their elevation to the movement of rock-masses from the east along thrust planes.
10 —Geology.
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CHAPTER XI.
ROCKS OF NEW ZEALAND.
The origin of a rock-mass does not, in the majority of cases, endow it with qualities that adapt it for use in any special direction. Rocks suitable for building purposes may be formed by any of the various processes that have been described in the preceding pages. The qualities that must be possessed by building-stones are durability and characteristics that allow of easy working ; the durability depends on the chemical composition, on hardness, and on the absence of crevices and pores.
All silicate rocks have a chemical composition that renders them exceedingly durable, when the probable length of time that any building will be required for use is taken as a basis of measurement. It is true that, when exposed at the surface of the ground under conditions of nature, such rocks show corroded and decomposed surfaces, and that in many cases the decomposition extends for 20 or 30 feet into the rock-masses. It must, however, always be remembered that an enormous lapse of time has been necessary for the weathering-agents to effect these changes. The observation of these facts need cause no apprehension as to the durability of the rock. Even the volcanic rocks that contain nepheline or olivine will not undergo any appreciable deterioration within an historic period. Compact carbonate rocks are sufficiently durable for the ordinary purposes of construction, but when they are porous in some cases their durability is greatly lessened.
To some extent the hardness of a rock depends upon the hardness of its component minerals, but in general the hardness is more particularly dependent upon the coherence of the minerals or rockparticles of which it is formed. A rock composed of original molten matter is usually extremely hard, for the individual crystals that occur in it are either embedded in glass or form a dense felt or mosaic that effectually resists strain in every direction. When originally fragmentary matter forms a rock, the hardness must evidently depend upon the extent to which the fragments have been welded together,
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or upon the extent to which the different particles have been cemented together by the deposition of secondary material. Such rocks are usually of sedimentary origin, though an important group of this class consists of volcanic material originally in the form of scoria or lapilli. The extent to which rocks are traversed by crevices is generally evident on a weathered surface. If large boulders of solid appearance are found at the outcrop, the rock must be free from crevices that would render it unsuitable for building purposes. If an outcrop shows small angular fragments, the rock-mass is probably traversed by joints that render it unsuitable for use in building. The sedimentary rocks of New Zealand are generally traversed by crevices to such an extent that they break down when an attempt is made to quarry them.
Porosity is a character usually opposed to hardness. It is greatest in those rocks that are least coherent. The porosity of a rock is easily measured by immersing a weighed block in water, and weighing it again when it has absorbed the water that it will contain.
The characteristics that allow of easy working are to some extent the opposite of those that lead to durability. Obviously the softer rocks are more easily quarried than the harder rocks, and limestones will answer this requirement to a greater extent than silicate rocks.
The presence of crevices that divide the rock into fragments also greatly increases the ease of working. The most satisfactory condition is found in those rocks that are divided by joints into rectangular blocks. Though such a condition is rare, it is quite a frequent thing to find sedimentary rocks that have at any rate one prominent series of parallel joints. This, of course, reduces the amount of time and labour that has to be spent in dressing operations.
New Zealand Rocks used for Building.
Sedimentary Rocks.—These form large rock-masses in both Islands of New Zealand. Of the older sediments, only fine-grained types are certainly known to exist. They are found in the north-west of Nelson, but have such a pronounced stratification that they cannot be quarried in blocks large enough for use in building.
Sandstone.—The greater portion of the mountain-ranges of both Islands is formed of a sandstone, thoroughly welded and cemented into a hard compact rock, which, because of the presence of much feldspar, is called greywacke. The age of much of the sandstone is
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still doubtful, but it is certain that a large proportion is of early Mesozoic age. This rock is useless for building purposes, for it is traversed by many series of crevices that divide it into irregular angular blocks of small size. It is at Waikawa only that the sandstone is lying in horizontal beds and can be quarried in large blocks. Even here quarrying has proved unsatisfactory. The fine-grained rocks of the same series are just as useless. The dominant separation planes are stratification surfaces. They are usually intersected by other joints that divide them into small and irregular slabs. In a few districts, especially near Otepopo, the slabs can be obtained of large size, and with the necessary thinness to serve as slates. They appear to lack the required toughness to render them of much use for roofing purposes. Limestone rocks are somewhat more satisfactory. At Oamaru a bed of Cainozoic limestone attains considerable thickness, and, as it is practically free from joints and stratigraphical divisions, it can be sawn out in large blocks. It is used extensively throughout the Dominion for facing stone buildings, and sometimes for whole structures. The rock is creamy-white. At Mount Somers a similar rock with a pink tinge has been quarried. Similar stone occurs in various places, notably at Raglan and Whangarei, but it appears to be wanting in the uniformity of structure that is possessed by the Oamaru stone.
This limestone is admirably suited for carvings, mouldings, and other fine work to be placed in situations where the atmospheric conditions are not too harsh. Though very porous, and having a relatively low breaking-strain, it resists weather sufficiently well to render it suitable for ordinary building purposes, even in exposed situations, if kept from contact with the earth. When first quarried, limestone is relatively soft, but it hardens somewhat on exposure to the weather.
Lithographic limestone occurs at the Abbey Rocks in Westland, and near Mangonui in the extreme north, but the deposits have not yet been worked.
Metamorphic rocks are not much used.
A large deposit of marble is situated in the Pikikiruna Range, between Motueka and the Takaka Valley. It is coarsely crystalline, and varies in colour from pure white to grey. An attempt Las been made to quarry it, but in the place where the attempt was made it was found impossible to obtain large blocks sufficiently free from division planes for ordinary purposes. Marble also occurs in Caswell Sound, but its quality appears to be highly variable.
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GEOLOGY OF NEW ZEALAND.
Mica-schist has a wide occurrence in Nelson, Westland, and Otago. In many townships in Otago it is used as building-material. The readiness with which it breaks parallel to the foliation, and its relative softness, make it an easy stone to work. On the other hand, it does not break readily across the foliation, and therefore even surfaces in two directions are difficult to obtain.
Igneous rooks occur widely throughout New Zealand, and several different varieties have been used for building purposes.
Granite is practically confined to the South Island, and, although good outcrops occur, it is little used. A pink variety that is found at Preservation Inlet, and another found on the north-west coast of Nelson, would make a highly ornamental stone. Some of the granite at Ruapuke Island has been quarried and used as foundations of buildings in Dunedin. The diorites and other varieties of plutonic rocks of the south-west of the South Island do not appear to have been used for building. The granite of the Separation Point district is porphyritic, and disintegrates readily. Attempts have been made to quarry this rock and place it on the market, but the curious lack of cohesion between its component minerals has up to the present time rendered all attempts unsuccessful. Very recently a quarry has been opened at Tonga Bay, and good stone is obtained from it. The granite of Kahurangi Point and the Gouland Downs is of a pink colour and strongly porphyritic. It is admirably adapted for building and ornamental purposes, but the inaccessible nature of the country has hitherto prevented its use.
In the North Island the diorite of Cabbage Bay has been quarried and put on the market for ornamental purposes, but it appears that the hornblende crystals it contains prevent it from taking the high polish that is required for such a purpose.
The great variety of plutonic rocks often found near the sea-front in sheltered harbours of the south-west makes it probable that the handsomer kinds will be more extensively used in the future.
Volcanic rocks are used for building in Dunedin, Christchurch, and Auckland.
An andesite has been most extensively used in Dunedin. It has a natural horizontal jointing, which saves much labour in quarrying and dressing operations. The Otago University is built of this rock.
A phonolite has been used for other buildings, and recently the railwaystation has been built of dolerite that was obtained near Hyde.
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In Christchurch the Canterbury College is built of a coarse olivine andesite. Trachytes have been used for other buildings, and basalt is the material that has been utilized for the Museum. All of these stones have been obtained from the Port Hills.
At Auckland the basanite that forms the material of the small volcanic cones has been but little used for building on account of the labour that has to be expended in dressing the material. Some of the rock has been obtained from the Mount Eden lava-flows, and some from those of Rangitoto.
A rock composed of fragmentary volcanic material—Port Chalmers breccia—has been largely used for foundation-blocks and for bridges. The advantages it possesses are —it is near the railway-line, it breaks out in large blocks without natural divisional planes, and it is relatively soft and therefore easily dressed. On the other hand, it disintegrates readily unless it is obtained from places where natural weathering has not developed its weakness. The term breccia is commonly employed for any rock formed of fragmentary material if the different fragments have a marked angular form. The angular form of the component fragments distinguishes a breccia from a conglomerate. Volcanic scoria when it has become solidified, or cemented into a hard rock, becomes a breccia. Glacial moraines may also develop into breccias. The rocks alongside a fault plane are sometimes so shattered as to justify the use of the word breccia in describing them.
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CHAPTER XII.
DEPOSITS OF ECONOMIC IMPORTANCE.
From an economic point of view, deposits may be classed according to the purposes for which they may be used. On this basis the deposits that are of value may be classed as fuel, building-material, deposits from which non-metallic products are prepared, and deposits from which metallic substances are prepared.
I. Fuel.
The most important is coal. A description of the New Zealand coal, with the rock formations in which it is found and its distribution, will be given later. Peat also occurs in many places in the South Island, but there are very few places where it is used as fuel.
Petroleum has been obtained from bores in Miocene rocks at New Plymouth, in Poverty Bay, and near Greymouth. In none of these localities has any great quantity been obtained, though the prospects at New Plymouth are encouraging. The origin of petroleum is ascribed to the natural distillation of the remains of organisms, especially marine animals. The distillate condenses and collects under some pressure in porous sandstones, especially in the anticlines, if the sediments have been folded and are covered by impervious rocks.
II. Building-materials
have also been dealt with so far as rocks are concerned. Many of the limestones of Miocene age are found suitable for use in the preparation of cements. At present the limestones of Milburn and Green Island (Otago), Limestone Island (Whangarei), and Pohara (Golden Bay) are being used for this purpose. Materials for use in preparing concrete are found in great abundance.
Clays suitable for brickmaking are abundant in all parts of the country. In Dunedin, clays formed by the decomposition of volcanic rocks are used for this purpose ; in Chirstchurch, loess is employed ; in Auckland, marine clays of Miocene age are used. In many places the clays are found to be suitable for the preparation of pottery, notably
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at Auckland and at Stirling. These clays are not suitable for the preparation of finer kinds of earthenware, though a deposit near Milton was for some time used for this purpose. Good fireclays have not yet been found in any quantity.
III. Materials prom which Non-metallic Substances Are PREPARED.
Sulphur is obtained in some quantity from Tikitere, near Rotorua. It usually contains some earthy matter, and requires purifying. For some time sulphur was obtained from the crater of White Island, in the Bay of Plenty. There are many other deposits from which a considerable quantity of sulphur can be obtained.
Kauri-gum, is found in large quantities in the recent deposits of the northern part of the Auckland Province.
Petroleum-shale is present in considerable quantity at Orepuki, and on Flagstaff, Dunedin. In both localities the shales contain a high percentage of petroleum, and an extensive plant has been erected at Orepuki.
Phosphate Rock.—This material is found at Clarendon, near Milton, where it occurs in association with limestones of Miocene age. The rock occupies relatively small localized areas, and it changes laterally into the limestone. The phosphate has probably been deposited from solution, and is of secondary origin. The limestone contains some phosphatic substances, such as bones of whales and teeth of whales and sharks. Percolating water has dissolved these, and deposited them in those places where it issued from the ground.
IV. Metallic Substances.
These are found in four conditions —(1) in river and beach sands and gravels ; (2) in old gravel-deposits ; (3) in lodes or veins in rockmasses ; (4) in impregnations throughout rock-masses with local concentrations of a rich nature.
1. River and Beach Sands and Gravels.
The substances which are found in river-gravels are those minerals that are tough or hard, or have a high specific gravity, or are resistant to chemical actions, or possess a combination of these properties. The toughness or hardness prevents the minerals from breaking up into minute particles when submitted to the constant wear-and-tear to which water-borne material is subjected.
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Fig. 90.—Auriferous Gravels at Ross, Westland.
The gold is obtained by hydraulic sluicing.
Lent by Department of Mines, New Zealand.
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The high specific gravity prevents them from being suspended or floated by the water.
Precious metals are the minerals which are mainly obtained from such deposits. Their toughness prevents them from breaking up, and their high specific gravity prevents any but a very slow movement along the bed of a stream. For these reasons the gravels of any stream that flows through rocks that contain even a minute quantity of gold are auriferous. The older the river and the greater amount of rock that has been worn away, the richer the gravels. Thus the last work on the Klondyke deposits in Canada accentuates the fact that the gravels of phenomenal richness have been derived from rocks that contain a small amount of gold, and their remarkable richness is due to the enormous amount of rock-waste that has taken place. The rich auriferous gravels of the Clutha and other rivers of Otago also fails to imply the previous existence of auriferous reefs of great richness, but rather that an enormous quantity of schist containing veins of low value has been worn away by the river and its tributaries.
In Westland the beach sands are generally richer than those of the rivers. This is due to the fact that the rivers are extremely rapid, and have short courses ; much of the gold is carried to the coast.
The turmoil of the waves breaks the other minerals to pieces, and floats them away, and the gold is concentrated. The beaches are especially rich after heavy weather, for then the waves remove large quantities of lighter material, but the gold previously distributed throughout much sand is not removed from the beach. The oldergravels owe their richness to the application of the same principles. In them it is usually found that the gold runs in " leads." These simply mark the course of the currents of the stream by which the gravels were deposited. It is always found that the gravels are richest near the bottom of older rock on which they were deposited.
The remarks made here about gold apply equally to all metalliferous gravels. The valuable substances found in them in New Zealand are, —
Gold. —Round Hill, Orepuki, in gravels throughout the schist districts of Otago, and in gravels and beaches from Milford Sound to Westport, In the North Island some gravels in the Coromandel Peninsula are auriferous.
Platinum .—At Orepuki, Waikaia, and Golden Bay.
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GEOLOGY OF NEW ZEALAND.
Fig. 91. Gold-dredge at Work on Alluvial Gravels, Golden Bed, Central Otago.
Lent by Department of Mines, New Zealand.
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Native silver has been obtained in some gravels at Shotover River. The nickel-iron alloy awaruite is found in gravels of the Gorge River, Westland. Cassiterite in gravels at Port Pegasus, Stewart Island. Magnetite is very abundant in sands on the west coast, especially in the North Island. At New Plymouth and Onehunga it is believed yet that the sands may prove to be iron-ore deposits of great economic value. The magnetite is derived from the denudation of the rocks of the great volcanoes Mount Egmont and Mount Ruapehu. It contains a large amount of ilmenite, and some ferro-magnesian minerals such as hypersthene and augite.
The size of gold-grains in the gravels is sometimes considerable, and much speculation has taken place as to their origin. It is maintained on the one hand that the nuggets are sometimes larger and are always purer than any masses of gold that have been found in lodes. It is also maintained by some writers that the heavy masses of gold should occur only in gravels consisting of boulders much larger than the gold nuggets, for the streams that can roll only small boulders along their bed could not move the much heavier masses of gold. For these reasons it has been maintained that the nuggets have grown in situ. It is known that gold is very slightly soluble, and it has been urged that precipitation will take place from a solution of gold even when it is extremely dilute if it washes over a gold surface. This theory does not appear to rest on very solid foundation.
The nuggets are generally regarded as extremely rich portions of lodes from which the quartz has been removed by attrition, while the purity of the gold is due to the oxidation of the sulphides originally present with it.
2. Old River-gravels.
There are few auriferous deposits, of this kind in New Zealand. The famous Blue Spur Claim, near Lawrence, is, however, an examle. The gravels here appear to be of an early Cainozoic age. Such deposits as those of the Arrow and Shotover valleys, though now high above the river-beds, are yet of quite young geological age.
3. Lodes or Veins.
Metalliferous minerals are generally found in veins, lodes, or reefs, which are bands of rock that may traverse the surrounding material or country rock in any direction. The veins consist of much worthless mineral or gangue in which relatively small quantities of valuable
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minerals occur more or less irregularly. The material of the vein is often markedly different from that of the country rock. It usually consists of comparatively few minerals in a state of some purity. In many cases the lode-matter is distinctly crystalline, and empty spaces —vugs or druses —may occur into which the crystals project. Sometimes the vugs are continuous, and the lode has a comb structure. When different gangue minerals are found in a lode they are usually arranged in layers which are similar on the two sides of the lode, which is then said to be symmetrical. More rarely this symmetry is wanting, and the mineral layers are different on the opposite sides of the lode. Lodes occupy planes of fracture that have originated in many different ways ; sometimes they are fault planes, sometimes joint planes, and at other times fissures that appear to have no relation to structural planes in the country rock, and along which no movement has taken place.
In many instances the country rock has been much altered in the neighbourhood of the vein, and such minerals as epidote, chlorite, and garnet may have formed in large quantities. So great is the alteration that it is believed that many veins represent nothing but altered portions of the country rock. To such action as this the term metasomatism is applied, and many lodes are now said to be wholly metasomatic.
Variation in the width of lodes is found both vertically and horizontally, but it is often the case that expansion is again found when a lode has been almost completely pinched out. So far as is known, there is no limit to the depth to which lodes may descend. The oreshoots are even more variable in richness, direction, and size than the lodes in which they occur, and it is to this fact that much of the uncertainty of mining is due.
A lode is never of equal richness throughout its depth, nor are the minerals the same at all depths. Near the surface the sulphides have all been oxidized and the metal is in the free state, while there is a great deal of iron-oxide. This portion is called the "gossan." The material of lodes is generally somewhat less compact than the surrounding country rock, and therefore admits percolating water readily. This water carries some of the precious metals in solution to lower levels, and there, as a result of chemical actions, deposits them, and a bonanza zone is the result. Below the zone where the native metals are deposited other matter crystallizes in the form of sulphide, and an
GEOLOGY OF NEW ZEALAND.
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enriched sulphide zone is the result. Beneath this the sulphides are unchanged.
This regular arrangement of zones in the lodes is not always found. Thus in the Waihi lodes the only difference between the lode-matter
In the background the great "open cut" on the quartz lode of the Martha Hill, Waihi.
Fig. 92.— Martha Lode, Waihi.
Lent by Department of Mines, New Zealand.
in the upper and that in the lower portion was the free state of the gold : here was nothing of the nature of enriched zones, and the change from the oxidized portion to the unaltered sulphides was sudden. In some of the Thames lodes, on the other hand, notably
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the Caledonian and Waiotahi Mines, large bonanzas have been found, and it is possible that they owe their origin to this process of secondary enrichment, as it is called.
The gangue mineral in New Zealand is generally quartz, as in the Waihi and Thames Mines and the auriferous lodes of Reefton and Otago. In many places calcite is common, and fluorite occurs at the Baton River. Many other minerals may occur as gangue ; barite and dolomite are examples.
The valuable minerals are of very many kinds. In the lower parts of veins, at any rate, they are almost invariably sulphides; but sulphates, carbonates, and many other compounds are found near the outcrop.
Gold often occurs in the free state, but it is generally associated with pyrite, either mechanically mixed in minute particles in the pyrite, or combined with sulphur or other elements as a sulphide or other compound.
Many other sulphide minerals may occur in lodes. The following are the more important examples in New Zealand : —
Arsenopyrite is common in the veins at the Thames and at Coromandel.
Cinnabar is found at Waitahuna, near Dunedin.
Fig. 93.—Diagram of Lode.
Galena and sphalerite are found at Te Aroha. The presence of these minerals greatly increases the difficulty of extracting the gold.
Argentite occurs at Waihi and probably throughout the Thames district, but it is mixed with other sulphides in very small particles. The bullion extracted from these lodes contains silver and gold in the ratio of 6 to 1.
Stibnite is found in some mines at the Thames, and constitutes all the metalliferous mineral at Endeavour Inlet and in a lode at Alexandra.
Chalcopyrite is found in lodes at Moke Creek, at Kawau Island, and at Mount Radiant.
Scheelite occurs in some quartz lodes in the schist rocks of Otago, especially at Glenorchy and Macrae's. The lodes contain also a small amount of gold.
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GEOLOGY OF NEW ZEALAND.
Origin of Lodes.
Much discussion lias taken place as to the origin of lodes. Four theories may be said to be most deserving of consideration —(1) lateral secretion ; (2) deposition in fissures by ascending water of meteoric origin ; (3) deposition by magmatic water in fissures ; and (4) metasomatism.
1. The theory of lateral secretion is based upon the occurrence of minute quantities of precious metals which are known to occur in all country rock. It is supposed that percolating water dissolves these substances, and carries them in solution to the major fissures, through which the water rises to the surface in the form of springs. On its passage to the surface the pressure on the water is decreased, its temperature diminished, and metallic matter is deposited. To the obvious objection that precious metals are insoluble, it is replied that insolubility is merely relative, and that pressure and a high temperature endow water with solvent powers in a high degree. To the equally obvious objection that the more soluble substances in the rock are often but slightly attacked, whereas the richness of the lodes demands the solution and subsequent precipitation of metals from huge rockmasses, it is replied that water can only dissolve a certain amount of a compound, and its solvent power for the common minerals might be satisfied long before any gold was dissolved. For some time it was supposed that the precious metals of the lodes of the Coromandel Peninsula owed their richness to lateral secretion, but Morgan and Park have both stated that the quantity of metal present in the andesite which is there in the country rock is not sufficient to account for the rich lodes. It has been proved by the action of hot-spring waters upon Roman coins in France that such water can dissolve metals and can deposit them again as sulphide compounds.
2. The second theory ascribed great percolating-power to water, which may dissolve out metals from deep-seated rocks and deposit them when from any cause it rises to the surface. Objections to this theory are based on the well-known fact that deep mines are relatively dry, and that theoretical reasoning shows it is unlikely that the percolation of water at any depth in the crust is sufficiently rapid — if it actually takes place—to allow of the action stated.
3. The deposition by magmatic waters, on which the third theory is based, depends on the belief, now very generally credited, that all igneous magmas are saturated with water, and that they contain also
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small proportions of precious, metals. As the cooling of the magma proceeds, the water is liberated and rises through any fissures that lead to the surface. On the way it deposits the metallic contents. The theory is supported—(a) By the general association of lodes with districts in which volcanic action has taken place—e.g., Thames area. (b.) By the frequent presence of metals, though in minute proportion, in the waters of the springs of the thermal districts. Gold, for instance, has been found by Dr. Bell in the siliceous sinter at Whakarewarewa, and mercury has been found in the hot-spring deposits at Ohaeawai, near Recent basaltic rocks, (c.) Instances have been reported from America in which there is apparently a relation between dykes that radiate from granitic masses and the auriferous lodes in their neighbourhood. No such instances have yet been found in New Zealand.
4. There is no doubt that metasomatic action does take place in the case of almost every lode, as demanded by the fourth theory; but the origin of the solutions that have caused the action still remains obscure, and it has not yet been proved that the whole of any lode is due to such action.
Within New Zealand auriferous lodes are found in (a) schist rocks in Otago—e.g., Barewood and Arrowtown ; (b) sedimentary rocks of Maitai age at Reefton ; (c) sedimentary rocks of Aorere (Ordovician) age at the Golden Ridge, West Wanganui; and (d) to the most important extent in andesitic rocks of Oamaru age in the ThamesCoromandel Goldfield.
Besides these true lodes, there are other ore-bodies of a slightly different nature. Stockwerks are large ore-bodies in which the greater part of the lode is country rock, with small veins penetrating it in all directions. Such formations are almost restricted to a granite country rock, and cassiterite is the valuable mineral found in them. It is generally believed that these, at any rate, are due to metasomatic action of the magmatic water from granite expelled during the final stages of solidification. This action has been called pneumatolysis.
Some ore-bodies are interstratified with the strata of the country rock. They occur most frequently at the anticlines into which the rock is bent, and are called saddle lodes. The lodes at Bendigo are said to be of this nature.
Gash veins are irregular ore-bodies very variable in size and extent, but generally parallel to the stratification. They have been described as occurring at Cape Terawhiti.
11—Geology.
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GEOLOGY OF NEW ZEALAND.
Fig. 94.—Outcrop of Iron-ore, Parapara, Golden Bay.
Lent by Department of Mines, New Zealand.']
GEOLOGY OF NEW ZEALAND.
153
4. Impregnations.
These deposits consist of material minutely disseminated throughout a large mass of rook. All basic igneous rocks, for instance, contain impregnations of magnetite. In ordinary instances the magnetite is not present in sufficient quantity to justify any attempts to extract it on the commercial scale. Occasionally it may be concentrated locally, and form important ore-bodies, but no instances of this are known in New Zealand.
Chromite is a mineral that is found as an impregnation in ultrabasic rocks. It is found throughout the olivine rocks of the Dun Mountain, in Nelson. Local patches of the rock contain it in relatively great quantity, and several of these have been mined.
5. Masses.
There is a mass of limonite at Parapara which forms an important iron-ore, though it has not yet been worked. Dr. Bell regards it as a secondary deposit. The iron is supposed to have been derived from iron-bearing minerals of the surrounding rocks, and to have been deposited on the crystalline marble as a consequence of a reaction between calcium-carbonate and ferrous sulphate.
Phosphate rock is found in masses at Clarendon, near Milton, in Otago. It has apparently been concentrated from the associated limestone by percolating water.
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CHAPTER XIII.
The larger animals are of little importance from the point of view of geology. The burrowing habits of some demand notice, for they expose soil to the action of wind, sun, and rain, and thus a series of events of importance may be commenced. The sand exposed by the burrowing of rabbits is spread by the wind over the vegetation close at hand ; this may be killed, and more sand exposed, until in a windy and dry area a sandy waste results. It is said that this is the cause of the formation of the "wilderness" on the west side of the Mararoa, in Southland. Among smaller animals the action of earthworms is also highly important, for the small heaps of castings that are exposed on the surface of grass-covered fields provide much of the material that is carried away by rain-water, which starts it on its journey to the sea.
Plants are of greater importance, for their roots penetrate into the crevices of rocks, and as they grow they force the rocks asunder. They secrete organic acids that act chemically 011 the rocks and decompose them. When the plants die, still more organic acids are produced by their decay. These are carried by the percolating rainwater into the rocks below, and thus the action of the rain-water already described is greatly accelerated. The material thus supplied by plants, added to the oxygen and carbonic-acid gas already contained in the rain-water, has such a destructive action upon rocks that they are completely changed into clay.
Organisms are of greater importance from a constructive point of view, but even in this respect the higher animals are comparatively of little moment. Here and there bones have collected in sufficient quantity to form notable deposits, though in New Zealand, where terrestrial reptiles and mammals have never been numerous, such bone-beds are not found, except in the swamps, where moas apparently died in hundreds.
The lower animals are of much greater importance. Shell limestones composed almost entirely of remains of Mollusca are abundant
GEOLOGICAL ACTION OF ORGANISMS.
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in many of the younger rocks, and are found in rocks even as old as Triassic sediments at the Nugget Point and at Nelson. These have been formed in shallow water where sediment was not too plentiful, though the motion of the tidal waters was sufficient to bring to the molluscs an abundance of food, which enabled the animals to cover the floor of the sea.
Still more important are the coral polyps. At the present day these minute coelenterates are widely distributed, though it is only in regions where the temperature of the water exceeds 62° Fahr. that the reef-building forms exist. In many tropical seas they abound, and build up hard branched colonies of tree-like forms. These growths unite to form reefs, which fringe the shore, especially where no streams enter the sea and sully the purity of the water with muddy sediment. Most reefs lie only a short distance from the land, with shallow water outside them. Their nature suggests the name of fringing reefs, which has been given to them. Other reefs are often at a great distance from the land—a hundred miles or more —and have shallow water within them, but deep water without. Their continuous and massive character has gained for them the name of barrier reefs. Most singular is the circular type of reef, which is sometimes found in lonely state in mid-ocean, unrelated to any other land-area. Such a reef is an atoll. Many are to be found in the "other islands" that have lately been added to the Dominion of New Zealand. Palmerston Island is perhaps the best known, though there are many others— amongst them Penrhyn and Suwarrow.
The fringing reefs, which are well represented in the Cook Islands, are easily accounted for. They rise from a floor of no great depth— not too deep, in fact, for the coral polyp to live. The animals grow and multiply, and the calcareous skeletons that the colonies build up gradually increase in size, and finally reach the surface. It must not, however, be supposed that the reefs are entirely composed of coral. It has been recently recognized that other organisms are even more important than coral polyps in building up the reefs. Of these, algae, such as Halimeda, which have calcareous cell-walls, are of great importance, and the sand that is so attractive on the coral beaches is composed almost entirely of tests of Foraminifera. Barrier reefs and atolls are not so simply accounted for as fringing reefs, since they rise from water far too deep for the existence of coral polyps and calcareous algae. It is generally accepted that 50 fathoms is about the limit at which ordinary reef-building corals can live.
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Two main theories have been proposed to account for these reefs. The first was propounded by Darwin. The barriers and atolls were supposed by him to have been fringing reefs originally, and that the land around which they were formed was slowly depressed. The corals were able to grow upwards as rapidly as the land sank. In
Fig. 95.—Diagram illustrating Darwin's Theory of Coral Reefs.
1, original rock-mass. 2, first sea-level, with fringing reefs at a, a. 3, subsequent sea-level after depression; the growth of fringing reef has kept pace with depression. 4, final sealevel ; rook completely submerged, and a lagoon occupying the position of its apex.
time, as the downward movement continued, the coast receded far backwards, or the island entirely disappeared, and the coral reefs alone remained to mark its former limits. The theory was strongly opposed by other observers, who found elevated reefs on islands close to barriers and atolls. It appeared impossible that elevation could
Fig. 96. —Diagram illustrating Murray's Theory of Coral Reefs.
a, a, original sea-floor; b, b, level to which floor is raised by deposition of ooze ;c, c, coral growth ; d, d, outward expansion of coral.
account for the raised reefs while depression was needed for the atolls in regions so close together. Sir John Murray was so strongly impressed with this and other difficulties that he proposed another theory. The coral reefs were supposed to have grown on submarine banks, which, in consequence of the constant rain of tests of Foraminifera
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which, fell on them from the oceanic water above, gradually reached the level at which the corals would grow. When the growth of coral began, the reef would soon reach the surface. It would then grow most rapidly where it was exposed to the break of the ocean-waves, which supply food to the polyps. The inner portions, deprived of food, would soon decay and dissolve away, and a lagoon would be formed inside the reef. The ring of coral would extend outwards slowly, for pieces would break off and form a talus slope, upon which the outward growth would be supported. More recently Professor Agassiz has put forward a theory to the effect that barrier reefs are situated on the margins of old plains of marine erosion. They should, therefore, rest on volcanic rock at a depth of a few fathoms.
It was always supposed that a crucial test would be applied to these theories if a bore could be drilled through the rim of an atoll. Accordingly in 1899 a bore was successfully drilled to a depth of 1,145 feet by a party from Sydney University led by Professor David. The atoll chosen was Funafuti, in the Ellice Group. Throughout this distance the bore traversed coral material only, and the organisms were found to be in the position of growth. No ooze or other material was met with. The lagoon had a growth of Halimeda on its floor, and a bore showed that this continued for some depth, a sufficient proof that the lagoon was not due to solution.
The result of the bore showed that Darwin's theory was applicable to the Funafuti atoll. Apart from the light that this bore shed on theories as to the origin of atolls, it gave most important information as to the origin of certain large masses of dolomite rock. While the material of the coral skeletons is almost pure calcite, it was found that below the sea-level the percentage of magnesian carbonate increased, and in the deeper portions of the bore the material was almost pure dolomite. This is held to prove that the magnesium of sea-salts is able to displace calcium in calcite to such an extent that the compound is gradually changed into dolomite. There are in New Zealand no masses of dolomite rock, and the Oamaru limestone and similar deposits throughout the country do not truly represent fringing reefs of Miocene age.
Such organic deposits as the ooze which covers so much of the ocean-floor, and is composed of countless millions of skeletons of minute marine animals, are described elsewhere.
Coal, the most important of all geological formations from an economic standpoint, consists of plant-remains. Special conditions
158
are necessary to allow of the accumulation of sufficient vegetable matter to form a seam of coal. In some low-lying swampy districts vegetation grows rankly, and when it falls it is covered with water, and combination of its carbon with the oxygen of the atmosphere is thus prevented. Lowly forms of fungi and algae cause it to rot and decompose. A slow depression of the land-surface may take place. Generations of plants succeed generations, until their rotted remains form a bed of vegetable matter tens of feet in thickness. If the depression continues, the plant-remains may become buried beneath sand, gravel, or other sediment. The material becomes compacted by the weight of the sediment resting upon it, and changes into lignite, in which some of the vegetable structure can still be seen. If the thickness of sediment becomes considerable, the temperature of the vegetable material rises considerably, for it must be remembered that the average temperature rises 1° Fahr. for every 60 feet of descent into the earth's crust. The higher temperature makes the lignite give off some of its water, it becomes more compact, and changes into brown coal. With still deeper burial and at a higher temperature most of the remaining water is given off, and new combinations take place between hydrogen and carbon, and so-called bituminous coal is the result. With still more extreme conditions anthracite, which is nearly pure carbon, is formed. Finally, in the most extreme cases, carbon alone—graphite—remains.
A series of analyses of New Zealand coals has recently been made, and the following are typical examples :—
GEOLOGY OF NEW ZEALAND.
Fixed Carban. Hydrocarbons. Water. Ash. Sulpher. Paparoa (semi-anthracite) 76.02 16.93 0.51 4.40 0.29 Denniston (bituminous) 55.73 40.08 2.37 1.82 0.55 (Coalbrookdale) Brunner (bituminous) .. 54.70 39.03 0.47 5.80 2.31 Hikurangi (glance coal) .. 51.42 43.43 3.84 1.31 5.62 Kawakawa (pitch coal) .. 45.93 46.46 4.17 3.44 5.67 Taupiri (brown coal) .. 42.11 43.57 12.24 2.08 0.26 Kaitangata (brown coal) .. 36.81 39.29 16.50 7.40 0.45 Nightcaps (brown coal) .. 31.04 39.24 24.80 4.92 0.23 Gore (lignite) .. 25.34 40.42 30.92 3.32 0.45 Alexandra (lignite) .. 18.78 43.10 32.62 5.50 0.84
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GEOLOGY OF NEW ZEALAND.
Varieties of Mineral Fuel.
Peat. —Texture fibrous. Found in swamps in various localities, especially in the South Island.
Lignite. —Vegetable structure still visible ; colour brown ; streak brown, burns with a long flame. Occurs in Central Otago, Manukau, and elsewhere.
Brown Coal. —No vegetable structure visible ; colour black ; streak brown ; burns with long flame ; conchoidal fracture. Occurs at intervals throughout the eastern districts of the South Island, and also at Mokau, Hikurangi, the Waikato, and many places north of Auckland.
Bituminous Coal. —Colour black ; streak black ; cakes when it burns ; straight fracture. Occurs at Westport, Brunner, and in adjacent localities.
Anthracite. —Colour metallic grey ; streak dark grey or black ; requires special grate for burning. No good occurrence in New Zealand, though the Paparoa coal is anthracitic.
Graphite. —Colour metallic, grey ; leaves a lead-grey streak on paper. No good occurrence in New Zealand.
Other classes of coal have been mentioned in New Zealand. Pitch coal and glance coal are superior varieties of brown coal, and possess some of the properties of bituminous coal, but they contain more water than it, and do not cake when burning.
Coal always contains some pyrite. The mineral is rather frequent in New Zealand coals, and would render them unsuitable for some metallurgical uses. Coal may be formed from any type of vegetation, and its presence in any geological formation does not imply a tropical climate at the time that the coal-bed was formed. Coal is not restricted to any geological period, for there are seams in rocks of every geological age from the Devonian to the Miocene. The greatest coaldeposits of the Carboniferous period in Europe and America were formed from remains of gigantic club-mosses and allied plants— those of New South Wales mainly from remains of ferns, those of New Zealand from wood of flowering-plants. It is probable that accumulations of vegetable matter that may afterwards change into coal are now being formed in lagoons in tropical forests, in mangrove swamps, and in peat swamps in temperate climates.
A great deal of discussion has taken place on the question whether the vegetable material that has changed into coal grew where the coal-beds are now found, or whether it was transported by running
160
GEOLOGY OF NEW ZEALAND.
Fig. 97.—Coal-seam, Nightcaps Colliery, Southland.
Lent by Department of Mines, New Zealand.]
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water. It is probable that in some localities tie one explanation is correct, while in others tie material was floated to the lagoons, where it accumulated. Tie widely extended and uniformly thick coal-seams of Europe and America imply the growth of the coal-vegetation in place, and this is in some cases proved by tie occurrence of stumps and roots of trees below the coal. The smaller and more variable seams of France were probably formed from transported material. This explanation probably applies also to most of the New Zealand seams, which vary in thickness, and often contain pebbles that may have been carried attached to the roots of trees.
Partings of shale usually occur in the coal-seams of the east of the country, and the rapid thickening and thinning that is found in them supports the belief that the vegetable material accumulated in stagnant lagoons, into which it was floated by streams.
Other plant-remains are of little importance. It has been stated previously that an important portion of the coral reefs consists of calcareous algae. Remains of these have not yet been recognized in the coral limestones of Miocene age in New Zealand.
At the present day diatomaceous ooze is being deposited over a wide area in Antarctic waters, but no important deposits have been found in the rocks, though there are thin seams at Oamaru. In the Bay of Islands a deposit of diatomaceous earth has been recorded.
162
GEOLOGY OF NEW ZEALAND.
CHAPTER XIV.
METAMORPHISM.
Minerals are merely chemical compounds, and, though they seem hard and resistant, their chemical stability is only a matter of comparison. If the action of weak acids is continued for a sufficient time, if the temperature changes to a certain degree, or if the pressure acting on the rocks becomes notably greater or less, the minerals will undergo changes more or less profound in their nature. Nor is it necessary for all these different variable conditions to act on rocks simultaneously, for any one of them by itself may effect considerable changes in the nature of a rock-mass.
It is near the surface of the rock-masses that the action of weak acids and of oxygen is most noticeable. In such a situation the feldspar is changed into kaolin, silica, and alkaline carbonates if it is orthoclase, albite, or oligoclase ; or into kaolin, silica, and calcite if it is andesite, labradorite, or anorthite. Augite is changed into ironoxide, serpentine, and calcite. Olivine becomes serpentine and ironoxide. Calcite dissolves completely. Other minerals may remain little affected. Quartz and mica in particular are highly stable under the conditions that prevail at the surface of the earth. Some of the changes that minerals undergo at or near the surface were described when the action of percolating water was considered. All of these changes are in reality examples of metamorphism, though the alteration is in such cases due entirely to the action of the water and the substances it holds in solution. The word metamorphism is wide in its scope, and includes all those changes that minerals and rocks undergo after their original formation, 110 matter by what agent the changes are effected.
It is not always easy to trace the alteration to its cause, and it is usually the case that two or three agents have combined together to produce any particular change. In the instances of weathering cited above the actual cause is so clear that it is not unusual to class all the effects together as the result of so-called hydro-metamorphism —that is, the metamorphic changes that have been caused by water.
163
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There are many instances in which, we find that the metamorphism is certainly not a result of the action of percolating water, for the altered rocks contain no more water, but generally much less, than the sediments from which they have formed. If, for instance, the Southern Alps are crossed from east to west near Mount Cook, where the change in rock-type is in general the same as in other traverses of the great range, it is found that on the eastern slopes of the main divide the rocks are hard compact sandstones and greywackes. The component grains have been firmly welded together, and the rocks are often traversed by small thread-like quartz veins, but the component mineral grains are nearly fresh and unaltered. Soon after the divide has been passed a change becomes noticeable —silvery flakes of mica make their appearance, and the fractured surfaces of the rock become distinctly bright. Farther westward the mica becomes more abundant, and it is clearly seen that its plates are arranged parallel to the stratification of the rock, which is here nearly vertical. Soon irregular plates of quartz appear, and the rock is seen to be made up of irregular alternating thin layers of quartz and mica. This is mica-schist. On the lower western slopes there is a still greater change, for, though layers of different minerals give the rook a foliated appearance, the mica becomes less conspicuous, and feldspar and garnet take its place to some extent. Green actinolite is found in some quantity, and the rock is almost coarse-grained enough to be called a gneiss. In many places near the foothills of the western slopes there are massive outcrops of granite. In this instance it is evident that percolating water has not caused the change, for the variation of the rocks takes place in a horizontal, not in a vertical, direction, and the thickness of the rock series (twenty miles) is so great as to preclude the idea of percolating water with its small quantity of acids causing such a change. Again, the feldspar, actinolite, and garnet are all minerals that do not contain water. It will, however, be noticed that in the above description the alteration of the rock becomes more and more profound as the granite-masses are approached, and the idea is at once suggested that the intruded mass of granite may have caused the whole change. The granite must have been forced into the rocks when it was at a high temperature, and it must have been under great pressure at the time of intrusion.
The metamorphic effect of the intrusion of plutonic rock is called contact action. Where contact action has affected rock-masses it is
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GEOLOGTY OF NEW ZEALAND.
usual to find a large development of silicates of aluminium, such as andalusite and staurolite. Such minerals appear to be conspicuously absent from the Westland area. This is easily explained if the nature of the greywackes, which probably represent the rock before it was metamorphic, is borne in mind. These greywackes contain a large amount of feldspar, and contain, therefore, a moderate percentage of alkalies. There is no excess of alumina to allow of the formation of the simple aluminium-silicates; but, on the contrary, there is sufficient alkali and lime to combine with the silica in the formation of muscovite and actinolite and other complex silicates. The best example of contact action in New Zealand is found on the eastern side of the great granite intrusion of which the Gouland Downs are mainly formed. Here many of the sedimentary rocks adjoining the granite contain chiastolite, cordierite, and staurolite.
In the great schist district of Otago there appears, again, to be no distinct unconformity between the Maitai slates and the schists. Wherever the area is entered from the north or south there is a gradual change in the mineralogical character of the rock. Gradually it becomes metamorphic, and no distinct boundary between sandstones and schist can be found. The schistose structure is most pronounced in the centre of the area, but there appears to be no formation of feldspar and no approach to the structure of a gneiss. Here there is 110 mass of granite that may be pointed to as the probable cause of the metamorphism. Sections show that the rock has been deeply buried, and, as a consequence, the temperature to which it was raised must have been high and the pressure that acted on it intense. These two conditions are sufficient to cause the complete mineral change that is indicated by the term metamorphism.
From these two examples it is evident that there are at least two causes—a high temperature and a great pressure—that are of great importance in causing metamorphic changes, and it is probable that neither would be effective unless aided by the presence of water. Experiments and researches of recent date have clearly shown that water under these extreme conditions becomes a chemical reagent of great potency, and dissolves and decomposes minerals that under ordinary conditions are stable and refractory. The chemical composition of a rock as well as its mechanical structure have great influence upon the readiness with which it yields to metamorphic agents. Limestones are particularly subject to change,
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and if they are pure simply recrystallize into granular marble ; but if impure a great variety of complex lime-silicates may result. Of these, the lime garnet (grossularite), actinolite, and diopside (the lime pyroxene) are most frequent. The large mass of marble in the Pikikiruna Mountains, west of Motueka, represents rather a pure limestone of Ordovician age ; but there do not appear to have been any examples of impure limestone in the metamorphic districts, unless the actinolite schists of Westland represent them.
Fine-grained sediments are far more prone to change than coarse ones, and the changes are seen chiefly in the formation of aluminiumsilicates and of mica. As stated above, the relative abundance of alkalies and lime in the schistose rocks of New Zealand has prevented the crystallization of the simple aluminium-silicates. An exception is found in the west of the Nelson Province, where chiastolite is found quite frequently in the altered rocks of the Aorere Valley, close to the large mass of granite.
The effect of mechanical structure and of chemical composition is well seen in the Pikikiruna Range. Here the limestones have been changed into marbles, and fine-grained sediments into garnet micaschists, while the coarser sandstones have simply become quartzites.
Structural changes accompany those of a mineralogical nature. In a fine-grained sediment mica is first developed, and the flat basal planes of the crystals are arranged at right angles to the direction of pressure. At the same time the very grains of the rock are also flattened, and any pre-existing flakes of mica apparently move until they have the position referred to. Thus the rock in a little while comes to consist of flattened grains and flakes of mica, all with their broad faces parallel. The particles may be extremely minute —so small that they cannot be seen with the naked eye —but they give the whole rock a tendency to split in the plane in which their broad faces lie. This is rock-cleavage, dependent upon the position of the mineral grains, as distinct from mineral cleavage, which is dependent upon molecular arrangement in a single crystal. The fine-grained rock thus altered is a slate, but this term is not properly applied to it unless the pressure was inclined to the stratification so that the cleavage is not parallel to the stratification. If the cleavage is parallel to the stratification, the rock is a flag. True slates occur only doubtfully in New Zealand, though flags have a wide occurrence inland of Oamaru. In many districts fine-grained sediments separate into plates readily
166
GEOLOGY OF NEW ZEALAND.
and evenly along the stratification planes, and this has often given rise to the statement that slates have a wide occurrence.
With a greater pressure and at a higher temperature mineralogical and structural changes become more pronounced. The plates of mica are larger, and display a tendency to arrange themselves in layers. This separation of minerals into layers is called foliation, and the rocks that are foliated are schists. The large schist-area of Central Otago shows very little variation in rock-type ; the whole district is an area of mica-schist, with here and there a band of chlorite or of
Lent by Department of Mines, New Zealand.
Fig. 98.—Folded Mica-schist, Otago.
actinolite schist. The plane of foliation of schist rocks is that of the most prominent rock structure that is strongly inclined to the direction of the pressure which caused the development of schistose structure. The actual cause of the development of the structural changes is a flow-movement of the rock, for intense pressure may exceed the ultimate strength of the rock, and cause it to flow in the direction in
167
GEOLOGY OF NEW ZEALAND.
which pressure is least. The process of recrystallization is chiefly due to the chemical action of water, for under the conditions of pressure and temperature that cause metamorphism water is a strong chemical reagent. It readily dissolves many minerals that have projecting points in the direction of the pressure, and redeposits them in continuation of grains whose largest areas lie at right angles to the pressure.
Various names are applied to rocks that have. been changed to a greater or less extent by metamorphic action.
Phyllite is a slaty rock in w T hich the development of mica has been more complete than in typical slate. These rocks are characterized by a silky lustre on the cleavage surface owing to the reflection of light from the basal planes of the thousands of little mica crystals. There is a large development in the Kurow district, and at Skipper's, on the Shotover River.
Argillites are extremely fine-grained rocks formed chiefly from decomposed feldspathic sediments compacted by the deposition of silica. The term is more commonly used for any hardened mudstone. They occur widely in the mountain-ranges.
Gneiss has the same mineralogical composition as granite, but shows a rude banding. The banding may be original, or may be due to pressure which acted on the rock subsequently to its consolidation. In some instances it has been shown that gneiss is a metamorphic rock formed from sediment under conditions of extreme temperature and pressure near large intrusions of irruptive rock ; these are the paragneisses of Rosenbusch. The term gneiss is often employed in a wider sense, especially with a prefix that indicates the dominant mineral that it contains or the type of rock of which it is the banded form. Thus hornblende gneiss, biotite gneiss, diorite gneiss, or syenite gneiss are terms not infrequently employed. The greater part of the rocks of the Sounds area are gneisses ; they vary much in composition, but diorite gneiss is the most ordinary type. Stewart Island is largely formed of gneiss, and the rock occurs over a large area at Separation Point.
Granulite is rather similar to gneiss, but is mainly distinguished from it by the presence or by the greater abundance of garnet. In many cases a granulite is more finely grained and more compact than a gneiss. If the presence of abundant garnet is taken as a criterion, many of the crystalline rocks of the Sounds area should be classed as
12—Geology.
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GEOLOGY OF NEW ZEALAND.
granulites. The rooks from Dusky to Breaksea Sounds in particular, and from the entrance of Milford Sound to Balloon Peak, belong to this class.
While in some cases the classification of metamorphism as outlined above is satisfactory, there are many instances in which the effect is evidently not due to the action of one of the agents—water, temperature, e pressure—alone- There are even many instances in which the action of any one influence cannot be said to have had a dominant effect in causing the chemical change. In such cases it is evident that any one of the terms hydro-, thermo-, or dynamometamorphism would be misleading, and attempts have been made to remedy this by combining two or more of these terms. This again is unsatisfactory, because in all ordinary cases the three agents have acted together to produce the result, and if a logical practice were adopted all metamorphic results would have to be ascribed to hydro-thermo-dynamo-metamorphism. A more satisfactory classification is clearly desirable.
Recently another method of classifying metamorphic effects has been proposed in America. It is based on chemical effects, which are known, instead of causes, which are largely unknown. All metamorphism is classed as kata - morphism and ana - morphism. The former includes all those instances in which there has been in general a reduction in the complexity of the chemical composition of the rock. Such a change affects those rocks that are relatively near the surface of the earth. Katamorphic changes result in an increase in volume and a decrease in hardness. Within the zone of rocks affected anhydrous silicates become hydrated or are converted into carbonates or into oxides. Most of these changes take place within the zone of weathering, and katamorphic rocks include all of those that have been altered by weathering —that is, rocks that owe their changed nature to the chemical action of percolating water and the oxygen and acids that it carries in solution. Anamorphism is of a precisely opposite nature ; the general change that it causes is in the direction of a greater complexity of chemical structure. The minerals of which the rock is composed decrease in volume and increase in hardness. Water is given off ; silica enters into combination with bases, and replaces carbon-dioxide. The oxides combine with silica. These are changes that are effected at great depths only, where pressure and temperature are both high. While it appears that water is not
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GEOLOGY OF NEW ZEALAND.
necessary to aid in the chemical changes, it is probable that the structural effects, which are always in the direction of producing foliation, require the presence of water, which under these conditions is a strong chemical agent.
From the above statements it is evident that the distribution of metamorphic rocks in New Zealand is by no means regular. In the North Island there are 110 schists. The Maitai sediments are more changed than any other group of rocks, and they are merely flags, or, if coarser, welded sandstones traversed by small veinlets of quartz.
In the South Island metamorphic rocks have a wide extent in Marlborough, on the western flanks of the Southern Alps, and in Otago.
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GEOLOGY OF NEW ZEALAND.
CHAPTER XV.
GEOLOGICAL HISTORY OF NEW ZEALAND.
The facts that are made use of in all attempts to estimate the relative age of rook-masses are of three kinds.
(1.) Facts of Stratigraphy. —It is evident that those strata of rock that are lowest in any series exposed in a natural section are the oldest, while the youngest of any series are at the top. In nearly every exposure of stratified rocks this is a satisfactory way of deciding the relative age of the rock-strata. In those few instances, however, where the strata form a part of the lower limb of a recumbent fold the lowest rocks would be the youngest. Although satisfactory and definite for a single exposure, care must be exercised when any attempt is made to extend the limits of the section to other outcrops, for between the two exposures there may be a fault, or some change in the inclination of the strata, that would properly place one series of rocks far below another which might appear to occupy the lower position.
Care must always be taken to avoid confusing cleavage, joint, or foliation planes with stratification planes.
It often happens that one series of rocks is inclined at a different angle from the series on which it rests. Such a structure is called an unconformity. It evidently indicates a period of unrepresented time during which the older series of rocks was elevated and inclined oxfolded and partly worn away again before the second series of rocks was deposited. There are some structures that may often be mistaken for unconformities. Current bedding, drifted sand, and beach deposits may all form layers of material distinctly inclined to the horizontal plane at the time when they were laid down. The same is true of volcanic rocks, for beds of scoria are formed parallel to the slope of the cone, and lavas often solidify before they reach the bottom of the slope down which they flow.
In stratigraphical work it must always be remembered that when a series of rocks is formed on a coast-line the upper strata will in all
GEOLOGY OF NEW ZEALAND.
171
ordinary instances overlap, or will have a wider lateral extension than the rooks on which they rest.
(2.) Facts of Palaeontology.—Material deposited on the sea-floor covers and entombs the remains of animals that live there. Since the organisms that have lived at different times have been different in appearance and nature, it is possible, by an inspection and identification of the fossil remains in any group of rocks, to state, after comparison with the organisms in well-known districts, at what time the deposits were laid down. In order that this method should be employed with complete success it would be necessary for any group of organisms to have lived simultaneously in all portions of the globe. Very elementary considerations are sufficient to show that this is not the case at the present day. Terrestrial animals in particular are very different in Europe from those in Australia. The vegetation is almost as different in the two continents.
Comparisons of the fauna and flora in the other land-areas show that important differences exist between the inhabitants of all the separate lands. Though less marked, there are still differences, noticeable enough between the marine animals that are living on the shores of various lands at the present day. There is no reason to suppose that the periods of the past have been markedly different from the present day so far as the general distribution of organisms is concerned —that is to say, it is thought that at all times the organisms in each country have possessed more or less distinctive characters.
Yet it is probable that any dominant group of animals, especially those which are able to live in marine areas, will in time spread over the whole of the waters of the earth. This power of dispersal is less marked in groups of animals that have littoral habits, and is relatively feeble in those animal and vegetable forms that inhabit land-areas. It is therefore usually incorrect to say that because a particular organism occurs in certain rocks in all different countries of the world these rocks were formed at the same time in all the countries. For example, marsupials were at one time the dominant animals in Europe. At present they are characteristic of the Australian regions, but had not reached New Zealand until introduced by man. What is true of marsupials is true also, though to a less-marked degree, of all other groups of animals that they attain their maximum development in one country and perhaps become extinct there before they have become numerous in other countries. It is evident, then, that those
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cooks that contain remains of any group of organisms in one country were probably not formed at exactly the same time as rocks in another country that contain remains of the same group of organisms. Although a strictly contemporaneous existence cannot be asserted for a particular group of organisms in different countries, it appears to be true that in all lands animal and vegetable life have gone through very similar stages of development.
(3.) Facts of Lithology. —Any assertion as to the age of rocks that is founded on their mineral characteristics is apt to be extremely misleading, especially if it is used of countries that are widely distant from one another. Coal, for example, is found in greatest quantity in rocks of Carboniferous age in Europe and America ; but it is not correct to assert that because certain rocks are coal-bearing elsewhere they must be of Carboniferous age. Yet within the limits of a small area much use may be made of a lithological character. Throughout New Zealand, for instance, sandstones and greywackes of Maitai age are generally to be distinguished by the greenish-grey colour that they possess. When the area is limited to a small district this criterion of lithological character is more useful, though implicit reliance must never be placed upon it, nor must it be used except provisionally in correlating series of rocks even in the smallest area.
From what has been said it is evident that a stratigraphical unconformity is necessarily associated with a palaeontological break, for throughout the interval during which one series of sediments has been elevated, eroded, and again depressed the development of animals and plants will have continued. When sediments are once more deposited on the depressed surface, a series of life forms will be represented in the fossil remains that they contain which will in all probability be considerably different from the remains of those preserved in the sediments that were formed before elevation had taken place. Such unconformities, and their associated palaeontological breaks, therefore divide a rock series into natural divisions, and were used first in England and afterwards in other countries as the separating planes of the main periods in the deposition of sediments. Those that were supposed to be of major importance separated the primary divisions or eras, but these were divided into minor groups or systems by unconformities of less importance. The terms age and period are usually employed when reference is made to the life forms that existed during the deposition of the rocks of eras and systems,
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GEOLOGY OF NEW ZEALAND.
for these terms are used when the rocks only are referred to. The classification now generally adopted is as follows:-
1. Archaean era or age. Divisions are different in various countries .. .. Manapouri System.
New Zealand Development.
2. Algonkian era or age. Protozoic life forms. Definitely recognized, in America only ; elsewhere called Pre-Cambrian.
3. Primary era or age. Palaeozoic life forms.
A. Cambrian system or period.
B. Ordovician system or period .. Aorere System.
C. Silurian system or period .. ..
D. Devonian system or period .. ..
Baton River System.
E. Carboniferous system or period.
F. Permian system or period.
4. Secondary era or age. Mesozoic life forms.
A. Triassic system or period .. ..
B. Jurassic system or period .. ..
C. Cretaceous system or period.
Maitai System.
5. Tertiary era or age. Cainozoic life forms.
A. Eocene system or period .. ..
B. Oligocene system or period.. ..
C. Miocene system or period .. ..
D. Pliocene system or period .. ..
Oamaru System.
Wanganui System.
6. Quaternary era or age. Anthropozie life forms.
A. Pleistocene system or period ..
Pleistocene.
B. Recent system or period .. ..
Recent.
It was never supposed that the divisions of this table represented equal intervals of time or equal thicknesses of sediment. They merely stand for the different rock-systems that are separated from one another by important unconformities in the district where the rocksuccession was first determined.
Sir James Hector endeavoured, in his important work as Director of the Geological Survey, to correlate the New Zealand deposits with these periods. Such an attempt can never be wholly satisfactory, for the earth-movements of elevation or depression that ended one
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GEOLOGY OF NEW ZEALAND.
period or commenced another were not world-wide, but often only local phenomena. Here the unconformities occurred at different times, and divided the sediments into natural groups that were probably not the time equivalent to those in England or elsewhere. Palaeontological characters are also unsatisfactory, for several forms often lingered on in this distant part of the world, and were living when others of a later development arrived. Hence in a single formation it is not unusual to find fossils characteristic of two different periods in England. Hutton realized this difficulty, and gave local names to his geological periods, which he only tentatively correlated with the geological periods of Europe.
In a country like New Zealand, where all the older rocks are greatly affected by the earth-movements, and have been folded and faulted in a complicated manner, there must always be great difficulty in finding out the relative age of the formations, since stratigraphical evidence is in such cases much confused. The difficulty is greatly increased when the rocks have been submitted to metamorphism to such an extent as have the New Zealand rocks, because their original characters are destroyed, and new ones are adopted in place of them, and these new characters depend mainly upon the chemical composition. Structures are lost, fossils disappear, and rocks of very different age may become essentially similar. Even when not metamorphic, most of our older rocks are wholly unfossiliferous, and ate therefore wanting in the intrinsic characters by means of which their age can be most readily estimated. In the absence of more definite evidence than has at present been obtained it is not surprising that different observers have arrived at very different conclusions.
Manapouri System (Archaean Era).
This includes Hector's gneiss-granite formation. It includes Hutton's Manapouri system of 1885, though in 1899 he relegated the formation to the Triassic period. It does not include the Otago schists, classed by Hutton in 1899 as Pre-Cambrian.
A series of rocks was originally classed by Hutton in this era, though he subsequently saw reason to alter his opinion. The rocks occur over the greater part of Stewart Island, and over the southwest of the South Island from the Hollyford River and Lake Te Anau to Preservation Inlet on the west side of the lakes. It is the district of the fiords and fiord lakes of Otago and Southland. The
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GEOLOGY OF NEW ZEALAND
rocks are all crystalline, but vary much in composition and texture. Diorite is the commonest type of rock, but granite is found locally. Gneisses are common, but granulites not very abundant, and in the northern part of the district there is a mass of norite in the Darran Mountains. In places the rocks vary into hornblende schists, and in a few localities, such as Caswell Sound, there is marble. At Milford Sound a mass of olivine rock—dunite—occurs at Anita Bay, but it is probably an intrusion of younger age.
The minerals of which the rocks are composed are in no way peculiar. Feldspar and hornblende are usually most conspicuous ; pink garnet in large crystals is in many of the granulites, with minute crystals of rutile. At Dusky Sound there are rather large plates of muscovite.
Fig. 99.—Mount Mackenzie, Clinton Valley.
Formed of diorite of Manapouri age; its outline due to glacial erosion.
Economically the rocks are of little importance. A little chalcopyrite has been found at Dusky Sound. Large plates of muscovite have been obtained at the head of George Sound. The rocks have never yet been used as building-stone.
The thickness of the formation is probably very great. The plane of foliation dips uniformly to the north-west, and Hutton estimated the total thickness at 150,000 feet. There is no direct evidence as to the age of the rocks. Very few contacts with other formations
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GEOLOGY OF NEW ZEALAND.
have been observed, but on the east side of Lake Te Anau they appear to be unconformable to the Maitai sediments that rest on them. The supposed Archaean age is based upon the structure, and is supported by the frequent occurrence of rutile in the granulites.
Aorere System (Ordovician Period).
This corresponds with Hector's Lower Silurian. It also corresponds with the Aorere and Mount Arthur series of Hutton's Takaka system, 1885 and 1899.
It is only in the north-west of the Nelson Province that Ordovician rooks have been definitely recognized. They are black carbonaceous slates which contain graptolite fossils. The graptolites, which were minute organisms that floated on the surface of the sea, are somewhat similar to the graptolites in the Ordovician rocks of Australia, though they include forms that belong to widely different zones of the Ordovician rocks in England. McKay has recorded graptolites in slates at Preservation Inlet, but no attempt has been made to classify them. The most important graptolites at Nelson are species of Phyllograptus, Didymograptus, and Tetragraptus. These are stated by Clarke to be characteristic of very different rock divisions or zones in England and America, but very similar to the occurrence of graptolites in Australia. Dr. Shakspear, on the other hand, has lately stated that the New Zealand graptolite-bearing rocks can be divided into the same zones as those in England and America.
Fig. 100. —Graptolites from West Wanganui.
On right, Didymograptus murchisoni; on left, Phyllograptus folium.
There is a marble below the graptolite slates ; it is coarsely crystalline, and white or grey in colour, but up to the present no systematic attempts have been made to quarry it. Mica-schists occur in this
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GEOLOGY OF NEW ZEALAND.
series. The rocks are apparently metamorphic forms of greywackes and quartzites which are found with the graptolite slates. It is only in the Nelson district that these three Ordovician types —marble, schists, and graptolite slates—are found together. The schists, which are associated with the graptolites, were thought to form an outcrop, more or less continuous, with the schists that occur on the western flanks of the Southern Alps and afterwards bend to the eastward in Otago. Hence Hector classed the schists as partly of Ordovician age, though he stated that younger rocks had also suffered the metamorphic changes, and were associated with the Ordovician to form the great schist area of Otago. Hutton at first agreed with this, but afterwards classified the Otago schists in the Archaean age.
Economically the Ordovician rocks are not of much importance. The marble has not yet been quarried. The gold obtained in the north-west of Nelson is derived from quartz reefs in these rocks. The ironstone of Parapara lies on the surface of the marble, but is of much later age. In a bulletin of the Geological Survey that has been issued recently the concentration of the limonite of which these deposits consist has been ascribed to the action of the calcium-carbonate of the marble upon the solutions of ferrous sulphate derived from the weathering of adjacent rock-masses, which contain iron-pyrite.
Cambrian System.
(Here classed with the Aorere System.)
The only rocks that have been referred to this system occur at the summit of an anticline in the Pikikiruna Mountains, near Motueka. The rocks are quartzites. They are not fossilifc-rous, and are conformable to the Ordovician rocks that rest on them, and with which they should be included until more definite evidence is forthcoming.
Baton River System (Silurian Period).
This includes Hector's Lower Silurian system and the Reefton series of his Devonian system. It is the Baton River series of Hutton's Takaka system, 1885 and 1899.
The area over which Silurian rocks occur is perhaps large, but so far there is only one locality where they are known to be fossiliferous. This locality is on the Baton River, a tributary of the Motueka. A few Trilobites, Brachiopods, and Gastropods have been found, and, though they have not yet been described, there can be little doubt
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GEOLOGY OF NEW ZEALAND.
an Upper Silurian age is indicated by their occurrence, for the genera Orthis, Chonetes, and Homalonotus are certainly represented. The typical rock is a bluish slaty limestone, which is associated with schists and quartzites. This has led Hector to state that probably a portion of the metamorphic schists in Otago and Westland is of Silurian age.
Economically the Silurian rocks in the small area in which their age has up to the present been proved are of no importance, though it is possible that they contribute some of the gold to the auriferous gravels of the Baton River and neighbouring goldfields.
Devonian. (Hector.)
Rocks of this system have been identified by fossil remains in one district only, in the neighbourhood of Reefton. The rocks are bluish limestones, and some of the fossils are similar to those in the Baton River rocks, though they are associated with other forms, such as
Fig. 101. —Unconformity- between Baton River and Maitai Systems.
a, Baton River system ; b; Maitai system ; c, Oamaru system.
Avicula and Spirifera, that have led Sir James Hector to adopt the Lower Devonian age for the system. The rocks rest unconformably below the Maitai rocks of the same district, and are not auriferous, though the overlying Maitai rocks in a closely adjacent area are penetrated by rich gold-bearing veins. There seems to be no valid reason to separate this from the Baton River system.
Maitai System (Trias-Jura Period).
This includes the Te Anau series of Hector's Devonian system, his Carboniferous system, as well as his Permian, Triassic, Liassic, and Jurassic systems. It includes Hutton's Maitai and Hokonui systems, 1885 and 1899.
It also includes as a metamorphic series the Otago schists called by Hector the foliated schists, and referred by him to the
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GEOLOGY OF NEW ZEALAND.
Carboniferous, Devonian, and Silurian systems. They were placed by Hutton in the Takaka system in 1885, and were classed as Pre-Cambrian in 1899.
Another large series of rocks has been classed as Devonian on evidence of a very different nature. The Te Anau series, as it is called, consists of greenstone (not nephrite), greenstone scoria, and conglomerates, called at various times Te Anau breccias, greenstone ash, and greenstone breccias. More exactly, the solid rocks of the series are melaphyres, with associated tuff and scoria beds and conglomerates. Their green colour is a result of conversion of the olivine and augite of the original basalt into serpentine and chlorite. The Te Anau rocks have been classed with the Devonian because at Nelson they are said to rest below the Maitai slates, and the Carboniferous age has been assigned to these. At Nelson the Te Anau rocks occur near some ultra-basic plutonic material, which for this reason has been classed as Devonian. For reasons subsequently stated, the Te Anau rocks are here classified as Triassic.
Carboniferous. (Hector and Hutton.)
A very large series of rocks that throughout New Zealand is of extreme importance from a structural point of view was classed by Sir James Hector as Carboniferous. The local name of Maitai was given to the rocks because of their typical occurrence in the Maitai Valley at Nelson. Red and green flagstones and argillites, welded sandstones, and still more commonly greywackes, are the typical Maitai rocks. Their geographical occurrence is important, for they occur in the extreme north, where, as far south as Whangaroa, they are associated with masses of diorite and olivine gabbro; but their relationship to these rocks has not yet been clearly defined, though McKay speaks of the plutonics as intruded sheets. The Maitais form many of the high hilly districts between Whangaroa and Whangarei. Near Auckland they occur on Motutapu and Waiheke, and extend from Wairoa in a narrow belt past the Taupiri Gorge through the hilly land that separates the Waikato basin from the ocean. Finally these rocks terminate in the isolated hills of Tuhua and Hauturu. Another ridge starts from the Great Barrier, and forms the basement rock of the Cape Colville Peninsula. The most important outcrop commences near the East Cape, and stretches south-west through the Huiarau, Kaimanawa, Ruahine, and Tararua Ranges to Cape Terawhiti. From Cape Palliser a ridge extends fifty miles or so north,
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GEOLOGY OF NEW ZEALAND.
and also unites with the Tararua Mountains, through the Rimutaka Mountains.
The extent of the Maitai rooks is as important in the South Island. Commencing near Picton, they constitute the mountain-ranges through the Waiau and Awatere watersheds to the Kaikoura Mountains, which are formed throughout of Maitai rocks.
The typical outcrop at Nelson is continued through the Spenser Mountains and throughout the eastern slopes and crest of the Southern Alps, and, following the easterly bend of the anticlinal, passes into the Kirkliston Mountains. Farther south the rocks occur in the Takitimu Mountains.
This short account of their general distribution demonstrates the great importance of the Maitai rocks in the structure of the country. Unfortunately, these rocks are rarely fossiliferous. At various places the tube of an annelid worm has been found (Torlessia mackayi). At Nelson two true spirifers identical with Australian Carboniferous forms are said to occur, but elsewhere in all localities where fossils have been found in rocks that would be classed as Maitai on their lithological characters they are found to belong to species of much later age. It is therefore evident that palaeontological evidence in favour of the Carboniferous age of these rocks is practically absent.
Nor are stratigraphical results more satisfactory. At Nelson, where stratigraphical evidence has been reported, it was stated that rocks of supposed Permian and Triassic age appeared to lie below the Maitai because of faulting and overthrust. Recently, however, Professor Park has stated that the supposed faults do not exist, and that the Maitai rocks really rest conformably on rocks that contain well-known Triassic fossils. This has caused him to class the Maitai as of Jurassic age. He describes the Maitai rocks as occupying a big simple syncline east of the Triassic sediments.* Careful examination by the writer has shown that the rocks are in reality folded in a complex manner. After a detailed observation of the sections at Nelson the writer agrees in general with Professor Park so far as the absence of faults is concerned, but prefers to call the Maitai TriasJura rather than Jurassic, because typical Jurassic and Triassic fossils have been found in the Maitai rocks, or in sediments quite conformable with them. At the same time, it must be evident that, though the
* More recently he has again classed the Maitai rocks as older than Jurassic, and states that their apparent position is due to movement along an overthrust plane.
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GEOLOGY OF NEW ZEALAND.
Maitai rocks contain Triassic fossils wherever they are fossiliferous, it is somewhat rash to class the whole series of rocks as Trias-Jura. There are large areas where no fossils have been found and the rocks are of great thickness, so it is quite possible that other systems are represented in the Maitai. At present the only evidence available points to the inclusion of all the Maitai with the Trias-Jura. The occurrence of Trigonia and Gryphaea with the typical Triassic fossils Halobia and Monotis may, however, be held to justify the inclusion of the whole series with the Jurassic by those who insist upon the greater importance of the two former genera.
Economically the Maitai is not of much importance; the rocks are too irregularly and too frequently intersected by joints to allow of their use as building-material. Generally they are not metalliferous, though at Reefton they contain auriferous quartz veins; and the same feature is found at the Cape Colville Peninsula; but in this district the lodes are richer where they traverse the hypersthene andesite, which lies unconformably on the Maitais.
The Maitai rocks, as a whole, are much disturbed; they seldom lie horizontally, but are usually steeply inclined. Usually they are folded into what appear to be large isoclinal folds, but are occasionally sharply crenulated and contorted, as at Mount Torlesse and the Bay of Islands. The folding has caused some structural changes in the rocks. The feldspar of the greywackes is usually changed into sericite more or less completely, but seldom to such an extent as to give a pronounced glimmer to the fractured surface. In nearly all localities they are intersected by small stringers of quartz.
In Otago, Maitai rocks lie north and south of the structural axis of schist that runs south-east from Mount Aspiring to Dunedin, and throughout the line of the Southern Alps they occur to the east of the structural axis, and are in places found in coastal ranges near the west coast. In all parts of the country indicated the Maitai rocks pass by insensible graduations into schists. It is impossible to draw any dividing-line that separates the typical Maitai from schists, and there is no abrupt change in strike and dip. One district that has lately been closely examined by the Geological Survey in the North Westland area did not show any structural break that could be regarded as an unconformity, though in the section investigated the rocks changed from typical Maitai to schists of many types. These facts justify us in including the schist areas of Otago and Westland as of Maitai age until facts may be brought
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GEOLOGY OF NEW ZEALAND.
to light that may prove them to be older. It must, however, be clearly remembered that in the north-west of Nelson Province the schists are undoubtedly of Ordovician age.
Permian. (Hector.)
A group of rooks similar to those that have been called Maitai or Carboniferous has been classed as Permian by Sir James Hector. These rocks occur at Nelson, Mount Potts, and over an area stretching from the sea-coast near Nugget Point along the northern base of the Hokanuis. They are typical at Kaihiku. In all these localities the rocks are fossiliferous. In places they contain saurian remains. This is most noticeable at Mount Potts, where the remains have been classed by Hector as belonging to Eosaurus, an amphibian. It is more likely, judging by the size of the remains and by their association with marine fossils, that they represent Ichthyosaurus; but characteristic portions of the skeleton have not been found. Other fossils are Spirifers of several species, Terebratula, and Spiriferina. Associated with these are some leaf-beds that were supposed to contain leafimpressions of Glossopteris, but more recently they have been identified as Phyllopteris, and other impressions with them as Baiera. All of these facts point to the inclusion of these Permian rocks with the Maitai until further clearer evidence is forthcoming. After a close examination of the localities where Permian rocks have been recorded by Hector, the writer concludes that they contain no fossils that are necessarily older than the Triassic, and that there is in none of the localities any unconformity between the Kaihiku rocks and the Triassic rocks that are found close to them. This conclusion agrees with Hutton's views.
At the summit of Mount St. Mary, near Kurow, there is a fossiliferous deposit in sandstones that are partially metamorphic. These rocks have been classed provisionally as Permo-Carboniferous, but evidence in favour of this is wanting. The rocks are probably Maitai, for they are conformable with Maitai sediments alongside of them. The fossils have not yet been identified or described, but there is no doubt that Spirifer and Terebratula are represented.
Triassic.
The occurrence of such fossils as Monotis and Halobia in various localities has caused all workers in New Zealand geology to class certain sediments as Triassic. The special areas of occurrence are
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GEOLOGY OF NEW ZEALAND.
Nelson, Kawhia, and Castle Peak in the Moonlight Hills for Monotis; and similar localities, with the addition of Nugget Point and the Hokonui Hills, for Halobia. The sediments in which the fossils occur are shales and greywackes identical in appearance with those classed as Maitai.
With the fossils named there are many others, such as Trigonia, Ostrea, Gryphaea, Mytilus, Spirifer, Spiriferina, and Ammonites. The occurrence of Gryphaea and Trigonia in these rocks at once suggests a Jurassic age; and it would seem more logical in such an isolated land as New Zealand to take the organisms that are of latest age elsewhere as giving the age to the New Zealand formations, for there is no reason to suppose that any of the genera originated in the southern lands, but rather that all the organisms reached us from northern centres of distribution. It seems more reasonable to suppose that Triassic forms lingered on into the Jurassic, rather than that Jurassic forms of Europe appeared in New Zealand in the Triassic. However, Monotis and Halobia have been used so largely by Boehm as giving a Triassic age to sediments in various western Pacific islands that in the meantime it is desirable not to insist on this probability; but it is obviously advisable to class the whole series as Trias-Jura, though at the same time it is recognized that the lower portion of the series is distinctly Triassic, and the upper distinctly Jurassic, but that there is no unconformity between them.
Fig. 102. —Tkiassic Fossils, Nelson. Slab of rook with oasts of Mytilus problematicus.
Triassic rocks in Nelson and in the Hokonui Hills are richly fossiliferous, and the fossils are of great interest, though, like those of earlier formations, they are almost entirely undescribed at present. Plant beds occur in many places in the South Island associated with the marine strata. They contain abundant impressions of leaves of
13—Geology.
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GEOLOGY OF NEW ZEALAND.
Taeniopteris, Alethopteris, Sphenopteris, and other ferns. The fossils that have been definitely described are, —
Monotis salinaria, var. Richmondiana;
Halobia lommeli;
Mytilus problematicus;
Spirigera Wreyi.
Triassic fossils have been found at Ashley Gorge and at the North Cape, and more recently in the Cape Colville Peninsula; thus all of these localities, formerly classed as Maitai, must, even if the Carboniferous age of the Maitai be maintained, be classed as Triassic. The Triassic rocks have been subject to great disturbances. They are
Fig. 102A.-Triassic Fossils, Nugget Point, Otago. Slab of rock with Spirifers.
seldom or never in horizontal strata, but are more or less highly inclined, and form portions of huge rock-folds.
The material of which all these rocks is composed has been derived from plutonic masses, for they are composed of grains of quartz, feldspar, and hornblende or augite. When they become as coarse as conglomerates the pebbles are found to be composed mainly of granites, though various other varieties of acid rocks are represented, especially granophyres and rhyolites. The particular types of these rocks that are represented as pebbles have not yet been found in situ, unless the granophyre at the Rugged Point of Stewart Island represents on e
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GEOLOGY OF NEW ZEALAND.
example. The great thickness of the sediments shows that the area was one of deposition for a considerable time, though the general coarseness of the material shows that the deposition was relatively rapid, and took place on a coast-line. Presumably the coast-line fringed a large continental area, from the surface of which rivers carried large quantities of sand.
Speculations as to the nature and situation of this continental area are rather indefinite. There is little positive evidence; at the present time granitic areas occur to the west and south of the outcrops of Triassic rocks, but in most cases the granites are younger than the Triassic. There is a granitic mass far to the east in Bounty Island; and the granophyre of Stewart Island, which is intrusive into Archaean rocks, suggests that the old land may have extended south and east. There does not appear to be any important difference in the coarseness of sediments on the eastern and western portions of the Triassic rocks, though it is perhaps true that in general the coarseness is more noticeable in the east. On the other hand, the marine fossils appear to imply a continuous shore-line through New Caledonia, past the north of Australia to Timor, and the plant fossils resemble those of the Australian Trias.
It is also well known that throughout the later Carboniferous, Permian, and Triassic periods the Gondwana continent, which connected together South Africa, India, and Australia, was probably in continuous existence. The Triassic sediments of New Zealand were possibly formed on the shore-line of this continent. The evidence that we have appears to be conflicting—on the one hand, the nature of the sediments and the occurrence of granitic rocks appears to point to the existence of an easterly continent; on the other, the well-known distribution of land at this time in the Australian region, and the extension of fossil forms to the north, points to a westerly continent.
Economically Triassic rocks are not of great importance. Deposits of manganese-ore are found rather frequently in bands of red jasperoid argillites, which are generally classed as Maitai, though they are often found close to Triassic rocks. The association of these rocks has led Hutton to suggest that they represent abyssal deposits of red clay. The general distribution of the rock supports this view, but there is at present no other evidence in its favour. The absence of deposits of foraminiferal ooze above and below it is especially damaging to the theory. It is more reasonable to regard these argillites as fine sediment that was subjected to specially oxidizing conditions.
GEOLOGY OF NEW ZEALAND.
186
Jurassic.
Between Waikato Heads and Kawliia, in the North Island, there are undoubtedly areas of Jurassic rocks. Another area, in the South Island, extends from Fortrose to Catlins, and westward through the Hokonui Hills to Mount Hamilton. Sandstones and marls are the commonest rocks, and the latter are very fossiliferous.
At Kawhia the marls are concretionary, and large Ammonites are sometimes found in the concretions. Belemnites are quite abundant in places in all the outcrops. Aucella and Inoceramus occur at Kawhia and at the Waikato Heads.
Fig. 103.—Curio Bay, Waikawa.
The sea has eroded a platform along a stratification plane of the Jurassic rocks. The resistant silicified stumps of the Jurassic trees stand out as projections still uncovered by the tide.
In the Mataura district plant-remains are abundant. Foliage of Taeniopteris, Alethopteris, Cycads, and Podocarpus have been identified, amongst others, and masses of silicified wood are abundant. At Curio Bay, near Waikawa, there is a fossil forest, where the stumps maintain their erect position and the trunks lie prostrate beside them. Marine fossils are found in the Hokonui Hills; Gryphaea, Ostraea, Spiriferina, Terebratula, and many other genera are repre
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GEOLOGY OF NEW ZEALAND.
sented. The strata lie almost horizontally between Fortrose and Catlins, but farther north become .steeply inclined. There is no unconformity between these rocks and those of the Nuggets, which have been classed as Triassic. The only fossils of which proper descriptions have been given are : Belemnites aucklandius, Ammonites novo-zelandicus, Aucella plicata, Inoceramus Haasti, Placunopsis striatula.
So strong is the evidence of continuity of deposition that Captain Hutton classed the two formations together in his Hokonui system. Professor Park has classed all the Maitai rocks as Jurassic.
No minerals of economic value have yet been found in Jurassic rocks. Small bands of carbonaceous shale and thin ribbon-like seams
Fig. 104.—Jurassic Fossils.
Impression of Ammonite on right; next to it a Belemnite resting on Plagiostoma; on left a Belemnite resting on Aucella plicata.
of coal, but occasionally reaching a thickness of 2 feet, occur here and there in the Waikawa district, but these substances have not yet been found in quantities that have any commercial value.
The Jurassic rocks are generally free from any evidence of volcanic action; but in Canterbury, on the western side of the plains, from the Rangitata to the Rakaia, there is a large series of volcanic rocks that appear to have the same dip and strike as the associated sediments which in places contain plant-remains that indicate a Jurassic age. The volcanic rocks are andesites and rhyolites; hypersthene and
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GEOLOGY OF NEW ZEALAND.
mica andesites are most frequent, and the rhyolites are often glassy. The latter contain large grains of quartz, and usually red garnets.
The Jurassic rocks have been generally regarded as belonging to the early part of the period. After their deposition there followed the critical epoch in New Zealand geology. The effects of a great earth-pressure were experienced, and all the vast series of Jurassic Triassic, and older rock series were folded and elevated. Previous to this time the present land-area was the shore-line of a continent which is now lost to sight. Since this time the general form of the land has been unchanged, though there has been much modification in details. Elevation since then has from time to time linked New Zealand to other lands as a great continental mass. On the other hand, there have been periods of depression when the land has become reduced to a chain of islands ; but its general form has remained constantly long and narrow, for all later formations are found to be marginal on the Jurassic and Triassic rocks that were elevated at this time.
The rock-movements were associated with much metamorphism, for it is probable that the Otago schists acquired their structure as a result of the later Jurassic earth-pressure. These rocks extend from the Clutha and Lake Wakatipu on the south to the Waitaki on the north. They are almost uniformly mica-schist. The earth-move-ments appear to have been associated with little or no volcanic action. Hutton, however, has in his latest work described the plutonic rocks of the West Coast Sounds as intruded at this time. It has already been stated that the rocks of the Sounds bear evidence of extreme antiquity, but there are no reasons to doubt that the intrusion of all the granite-masses between the Haast and Separation Point took place about this time. It is also possible that the great peridotitemasses at the Dun Mountain and the Red Hill of Otago are of Jurassic intrusion, though they are more likely of somewhat later date.
Oamaru System (Cainozoic —Mostly Early).
This includes Hector's Lower Greensand, Cretaceo-tertiary, Eocene, Lower Miocene and Upper Miocene. It includes Hutton's Waipara, Oamaru, and Pareora systems, 1885 and 1899.
Waipara (Cretaceous) . (Hector and Hutton.)
On the east of the Wellington Province there is a large area of rocks that were classed by Hector as Lower Greensand, though all other
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GEOLOGY OF NEW ZEALAND.
ent by Department of Mines, New Zealand.
Fig. 105.—Gorge of Shotover River.
The gorge is cut through the metamorphic rocks.
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GEOLOGY OF NEW ZEALAND
authorities have united in correlating the rocks as Cretaceous. They are best known in the Waipara district of North Canterbury, for, though they have only a small outcrop in that place, they are highly fossiliferous. The Wellington area is not so well known, but it appears to extend from near Cape Kidnappers to Cape Palliser. The rocks are completely unconformable to the deposits on which they lie. They are highly fossiliferous. Baculites and other later Ammonites have been found, as well as Trigonia and Inoceramus and many Belemnites. Reptilian remains are found at Amuri Bluff; Mosasaurs, such as Taniwhasaurus, and Plesiosaurs, such as Mauisaurus, have been described by Hector, who states that they are more closely related to American reptiles than to those of other countries.
Hector tentatively classes the coals of Westport and Greymouth in" his Buller series, and this classification is adopted by Hutton, who includes also the Shag Point coals of Otago and the Pakawau coals of north-west Nelson. The Buller series consists of a great thickness of sandstones and conglomerates that contain much mica, and are therefore arkoses. They often contain plant-remains which usually belong to dicotyledonous genera, though remains of palms and conifers are not uncommon. Marine fossils are very rare. The coal-seams are of great thickness, and cover a large area; the coal is bituminous, and it is not quite clear why it has become so completely changed from brown to bituminous coal. There is no evidence to show that the vegetable matter has been covered by any great thickness of sediment. The coal-seams lie very flat, so the pressure which causes folding cannot be invoked. There are no large masses of igneous material younger than the coal-beds in their neighbourhood. However, the change to a completely bituminous nature has been effected over a large area in the coalfields near Westport and at Brunner.
Fig. 105A.- Uncomformity at Waikato Heads.
a, inclined strata of Hokonui (Jurassic) age; b, horizontal strata of Oamaru (Lower Miocene) age.
Near Gisborne the Waipara rocks include a coarse conglomerate containing huge granite blocks. The origin of these is unknown,
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GEOLOGY OF NEW ZEALAND.
A close inspection of all the typical localities of these rocks has convinced the writer that no unconformity separates them from the Oamaru system in north-west Nelson, Waipara, Amuri Bluff, Shag Point, and Gisborne, as well as the east of the Wellington Province. There is doubtless a great difference in the fossils. It appears quite possible to account for this by the supposition that during the great Mesozoic elevation a barrier to the migration of marine organisms had prevented them from reaching New Zealand, which was then the east coast of a far-reaching continental area. The Cainozoic depression submerged this barrier, and the fauna, which was at first little different from the fauna of the folded and elevated Jurassic rocks, was rapidly changed by the invasion of more specialized forms, and the Cainozoic character of the fauna was soon acquired.
Though nearly all observers have described an unconformity between the Waipara and succeeding rock series, each one has placed
Fig. 106.—Section at Amuri Bluff.
Adapted from McKay's measured section (Rep.G.S., 1874-76).
a, Maitai system. Oamaru system:b, calcareous conglomerate; c, grey sands; d, black grit; e, saurian beds; f, teredo limestone; g, Amuri limestone; h, marls.
it in a different position from the others. After a careful examination of nearly all the localities where unconformities have been described, the writer has come to the conclusion that no unconformity exists.
There is no distinct unconformity between the Waipara rocks and the younger series of sediments that rest on them. Sir James Hector strongly maintained that the fossils of these younger rocks retained distinct Cretaceous characteristics, though these older types were mixed with many other forms characteristic of the Cainozoic age; hence the use of the term Cretaceo-tertiary by him. The rocks are very variable in character, including coal-seams, conglomerates, greensands, and Cretaceous and compact limestones. Wherever the series is in any way complete it is found that the strata occur generally in the order given above, with the limestones at the top of the series.
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GEOLOGY OF NEW ZEALAND.
Fig. 107.—Strata of Middle Oamaru Age exposed in the Bed of the Aorere River near Bainham.
Lent by Department of Mines, New Zealand.]
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This must indicate a progressive lowering of the level of the land during their deposition, for the coal was formed in shallow swampy basins at or near the sea-level. The deposition of greensand takes place only in relatively clear marine areas, while the limestone consists mainly of Foraminifera and corals, and perhaps represents in some localities actual coral reefs.
The distribution of this group of rocks is very general, but they are usually marginal, or lie in deep basins in the interior portions of mountainous country. At the termination of the period the land was much depressed, and, although a large area remained above sea-level in Otago, all the northern portions became changed into a linear series of islands.
Coal-seams referred to this period are found at Kawakawa, Whangarei, the Waikato, and Mokau, in the North Island; and, in the South Island, at Puponga, Pakawau, Springfield, Mount Somers, Shag Point, Green Island, Kaitangata, and Nightcaps. In the majority of instances the coal does not appear to represent vegetation that grew on the localities where the coal is now found, for the seams vary rapidly in thickness. Usually there is no fireclay beneath the coal ; pebbles of quartz embedded in the coal are frequent, and almost certainly represent material that was carried down entangled in the roots of trees.
The rocks are often found at a considerable altitude in the mountainranges, and in such cases as in the Trelissick basin (behind Mount Torlesse) and at Wakatipu the strata are inclined and folded, but elsewhere they are nearly horizontal. Fossils are rather numerous in all the localities where the rocks are found. The species of molluscs are often of great size, and this fact taken in combination with the presence of coral limestone indicates that the climate of New Zealand was much warmer than it is now. Pecten of many species, Ostrea, Lima, Cardiuni, Pectunculus, and Cucullaea are common lamellibranchs. Turritella, Voluta, Scalaria, and Natica are each represented by several species. Terebratula and Waldheimia amongst the brachiopods are abundant. The scapliopod Dentalium is very common.
Remains of crabs have been found, as well as some bones of a large penguin. Moa-bones have been recorded by Forbes from gravels beneath the dolerite of Timaru. Bones and teeth of whales of various sizes are frequent —Squalodon and Kekenodon are the commonest. Teeth of sharks—Carcharodon and Lamna —are abundant in some localities.
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Fossil plants are often very different from the present New Zealand flora, but there is some doubt as to the localities whence the described specimens were derived. Ettingshausen, who examined and described them, has classed them in different geological periods, but so far his classification of the rocks has not been adopted. Quercus, Fagus, and Alnus were recognized by him.
Economically these rocks are of great importance. All the coals of the north and east of the country belong here. The coals are all brown coals, but are of great use for household purposes, as well as for steam. They vary much in quality in accordance with the chemical composition, as previously explained.
The best build-ing-stone of New Zealand is found among the limestones of this period. It is quarried at oama ru, and, though soft when taken from the quarry, it hardens sufficiently for all purposes; the stone is white and fine-grained, and is particularly easy to carve into ornamental shapes for interior decorations.
Fig. 108.—Oamaru Fossils.
The upper fossil, Pseudamusium Huttoni; on left, Pericosmus compressus (Meoma Crawjordi); on right, Cucullaea ponderosa.
At the close of this period the whole area was elevated, and the movement was greatest along the axis of the mountain-ranges and in the interior of the country, for the Oamaru beds there are at a much higher altitude than on the sea-coast. The elevation appears to have been associated with the outburst of a vigorous volcanic activity in both Islands. The alkaline volcanic rocks of Otago Peninsula rest on Oamaru limestones. The vast eruptions at Banks Peninsula appear to have taken place at the same time. The rocks at Timaru,
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Palmerston, and Hyde are of the same age; while the Oamaru basalts were erupted during submarine activity before the elevation had taken place.
In the North Island the andesites of the Coromandel Peninsula and Thames appear to rest on Oamaru sediments. The fact that scoria and tuffs are iterbedded with the sediments of the Witemata system at Auckland proves that the activity commenced before the elevation was complete. The great mass of volcanic lavas and breccias at the Manukau, Whangarei Heads, Whangaroa, and North Cape are similar in nature to the rocks of Coromandel Peninsula, and are probably of the same age.
Fig. 109.—Pecten athleta, Tata Island, Golden Bay.
In the Coromandel Peninsula the volcanic rocks are penetrated by a series of auriferous quartz veins of great richness. The huge lodes of the Waihi Mine are amongst these; they are believed to have been formed by deposition from great quantities of steam and heated water which escaped from the magma below after the more violent phases of volcanic action had terminated.
There is no evidence of volcanic action in other parts of the North Island, except perhaps at Hicks Bay. It is also certain that the eruptions of the Taupo district had commenced, for the papa rocks that extend over such a large area east and west of the Taupo region
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contain some volcanic matter, especially in the Poverty Bay district. The papa is of later Oamaru age, as shown by the high percentage of Recent species of Mollusca represented amongst the fossils contained in it.
The thickness of Oamaru rocks is considerable. In the North Island, near the head of the Wanganui River, they are certainly 2,000 feet thick, for they occur at the tops of the hills that fringe the plains from which the volcanoes rise, and they occur at the bottom of the river-gorges, and even there the base of this rock series is not exposed. The thickness in Hawke's Bay and Poverty Bay is certainly as great as on the west, but in the South Island it is usually less.
In most localities the strata lie almost horizontally, though, as stated before, in basins in the mountainranges, such as those in the Upper Clarence and in the Trelissick basin, they are considerably folded; and the same is true of the sediments that form the Taipo Hills, in the east of the Wellington Province. In general these Cainozoic deposits are usually disturbed where they are in contact with older rock series. The rocks were placed by Hector in four divisions—Cretaceo-tertiary, Eocene, and Upper and Lower Miocene. The first of the divisions
Fig. 110.—Oamaru Strata, Millburn, Otago.
The uneven surface of the Miocene limestone is due to the action of chemical erosion. The overlying sand settles into the depressions as soon as the limestone is dissolved out.
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was adopted because it was supposed that some Cretaceous species were represented amongst the Mollusca. All the coal-formations in particular were referred to the Cretaceo-tertiary. The Eocene and Miocene ages were adopted for the upper series, on account of the large number of Recent species of Mollusca found in them.
The Cretaceo-tertiary, Eocene, and Lower Miocene rocks of Hector were classed by Hutton in the Oligocene because he identified 14 per cent, of the Mollusca as Recent species. He failed to distinguish any Eocene rocks, but stated that there were distinct unconformities between the Oligocene rocks and the Miocene (Pareora).
Park fails to distinguish any faunal differences other than those that may be due to the influence of the station which the fauna inhabited —that is, deep or shallow water. He stated that Hector's divisions of the strata were caused by a failure to distinguish some strata of Waipara age from the Miocene, and by a failure to recognize two limestone strata as distinct in the Oamaru district. The unconformities described by the other authors were found to be absent in all the sections examined, and these included those mentioned by Hector and von Haast. The actual Miocene age is adopted because it is recognized that the molluscan fauna has the characteristics that are associated with that period.
The author's extended observations have failed to reveal to him any unconformity in any part of the Cainozoic series, and he is inclined to believe that too much attention has been paid in the past to the palaeontological evidence. It appears obvious that if a stratigraphical break is present in the series the very numerous exposures of good sections of the rocks would be certain to reveal it to such an extent as to allow of an agreement among observers. The mere fact that in such a well-exposed and accessible series every observer has placed the unconformities in different places in the series must raise doubts as to whether there is any break. The author's work has convinced him that there is none, and in such a case a special explanation must be made of the peculiar palaeontological difficulties. In a region like New Zealand, which is so remote from those districts in which the main advances in organic evolution are supposed to have taken place, it does not appear a very difficult task to do so. A provisional explanation is offered on a preceding page.
The actual period of this great depression is hard to settle definitely. The earliest beds certainly suggest a Cretaceous age; but
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the characteristic Cretaceous forms may have lingered on as a result of the isolation of the sea-basin in which the deposits were laid down. The Mollusca of the upper strata distinctly suggest a Miocene age, but at the same time some typical specimens of Echinoderms from these rocks were stated by Professor Tait to be similar to those of Eocene age in Australia. Special identifications of sharks' teeth, Polyzoa, sponge-spicules, and Eoraminifera support the view that the rocks were deposited in the early Cainozoic.
Wanganui System (Pliocene).
At various places near the coast-line of the North Island there are fossiliferous rocks that contain Mollusca almost identical with those at present existing in the adjacent ocean. A younger Pliocene age has been adopted for these formations by all authorities. They are most typically developed at Wanganui, Petane (near Napier), and Manukau. In the first locality the fossils are associated with micaceous sands, and it is hard to find the source of this material. No rocks are at present exposed in the North Island from which it could be derived. As the land must have been at a lower level during the deposition of the sands, it is hard to imagine how the material could have been transported from the South Island, though the sand is almost identical with that of Earewell Spit. The rocks are not of very great thickness, and do not extend far inland. In the South Island some fossiliferous strata at Awatere have been classed by Professor Park as older Pliocene, but Hector considered them to be Upper Miocene. A large mass of gravels forming the Moutere Hills, near Nelson, is supposed to have been deposited during this period; and the same age has been generally assigned to the Southland plains and the greater part of the Canterbury Plain.
Towards the end of the Pliocene there was a great increase of volcanic action in the centre of the North Island. The first evidence of this on the west side is found in pumice sands near the top of the Wanganui beds, and afterwards there are rhyolite gravels, and lastly andesitic and rhyolitic gravels. Usually these gravels are unconformable to the Pliocene rocks on which they rest, but this is not the case at Wanganui.
Mr. Hill states that pumice sands are found in Miocene rocks at Tolaga Bay interstratified with papa rock; so, as stated before, the
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outbreak of the volcanic action commenced in the Miocene. The facts that at Wanganui rhyolite gravels are the volcanic rocks occurring lowest in the series, and that at Hawke's Bay the Miocene rocks contain rhyolite (pumice), prove that rhyolite was the earliest rock of the great volcanic plateau. It constitutes enormous lava-flows throughout the volcanic plateau, and the lavas rest almost horizontally. The hypersthene andesitic cones of Ruapehu, Tongariro, Pihanga, Tauhara, Edgecumbe, Maungatautari, kakepuke, and others too numerous to mention, which rise above the rhyolitic plateau, appear to be of later date, but in all cases their origin was probably not after the later Pliocene. Mount Egmont, composed of hornblende andesite, must also be classed here. The lavas of this mountain nowhere encroach on the rhyolite plateau, so there is no direct evidence as to its relative age. However, the sea-cliffs near Hawera show clearly that the eruptions of the mountain did not commence till after the Miocene rocks had been deposited; and similar evidence is more plainly offered by the bores that have recently been made at New Plymouth in search of petroleum, for they nowhere show any volcanic material. Farther north the volcanic masses of Pirongia and Karioi are formed of dolerite, which also is younger than the Miocene limestones. The locks bear more resemblance to those of Banks Peninsula, in the South Island, than to other North Island rocks; but this resemblance cannot rightly be used as the basis for any statement as to the age of these volcanoes.
The fossils of the Wanganui system practically all belong to Recent species. Hutton stated that 93 per cent, were Recent, but this proportion has been further raised by subsequent discoveries. There is some doubt as to whether the moa-remains recorded by von Haast from Westland were really of Pliocene age.
Hutton has in several publications stated that the greater extension of New Zealand glaciers occurred during the Pliocene. He mentions the following facts as evidence of the probability that glacial extension was due to a greater elevation of the country: (1) None of the moraines are stratified or contain marine fossils, even where they reach the present sea-level; (2) glacial valleys in the Fiord region now have their floors 1,500 feet below sea-level; (3) there is no evidence of a northerly migration of marine forms; (4) such later Cainozoic and Pleistocene formations that are known do not contain fossils that indicate a colder climate; (5) the fauna and flora of the outlying
14—Geology.
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GEOLOGY OF NEW ZEALAND.
southern islands are a proof that these islands have not had a severe climate since their separation from New Zealand. He mentions the absence of older Pliocene fossils as a proof that the older Pliocene was a period of greater elevation. Lastly, he states that the differences found in the flora and fauna on the opposite sides of Cook Strait, and the differences between the plants and animals of New Zealand and those of outlying islands to the south, prove that the separation of these areas must have lasted ever since the Pliocene period.
Pleistocene Period.
River-valleys, lake-beds, the sea-coast, and volcanic regions are the localities where Pleistocene rocks have been sought. In New Zealand these deposits are extremely local and disconnected, and in the absence of mammalian fossils it is not possible to state with any certainty whether the deposits are really of Pleistocene age. Many of the gravels in the river-valleys of Southland, Otago, and Canterbury; part of the Canterbury Plains; the gravel plains extending from the Manawatu Gorge to the sea-coast; the Heretaunga Plains; and the important pumice area of the middle Waikato, are all probably Pleistocene. In Otago the great inland basins at Maniototo, Idaburn, &c., are partly Pleistocene, partly Pliocene. Hutton has detailed many examples of raised beaches along the coast-line. These terraces are most conspicuous on the north-west coast of Foveaux Strait, where they attain an elevation of 1,000 feet. This altitude gradually decreases as they are traced farther north, and they are not to be found to the north of Auckland.
In all localities the Pleistocene gravels rest uncomformably upon Cainozoic rocks. This is particularly marked near Wanganui, where they lie on the highest Pliocene sediments. It is the general belief that the glacial moraines, which occur m the South Island so widely, were deposited during this period, though, as previously stated, Captain Hutton held a different opinion. Such moraines are conspicuous features at the southern extremities of all the Otago and Southland mountain-lakes. In Canterbury they are found everywhere in the mountain-valleys, and even on the plain at Woolshed Hill and Little Racecourse Hill. Roches moutonnees are not rare, and lateral moraines are to be seen at least 1,500 feet above the valley-floor in the Tasman and Rangitata Valleys. On the west coast the sides of
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Fig. 111. —Map showing Pleistocene Glacial Extension in New Zealand,
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the fiords are worn by ice-action, and moraines practically form the coastal plain between the Southern Alps and the sea. In the Nelson Province they occur at 4,000 feet above sea-level, but are unknown in the North Island. The extremely fresh appearance of these moraines has led to the general idea that their age is Pleistocene; but it is not at
Fig. 112. —Iceberg Lake, Clinton, North Branch, near Lake Te Anau.
The depression occupied by the lake has been formed by glacial erosion.
present supported by direct evidence, and many of the arguments advanced by Hutton for the Pliocene age of the glacial advance remain unanswered.
It is agreed on all sides that there was nothing of the nature of an ice-sheet in New Zealand, and usually the moraines only indicate
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extension of the present glaciers, though often they are found in districts where there is now no permanent snow. There is a large moraine on the eastern side of the Taieri Plain, far away from all existing glaciers, and extending to below sea-level. The irregularities of its surface indicate that it has been subjected to erosion for a long period. It is, however, important to note that the moraine dips at an angle of from 15° to 25° eastwards. The earth-move-ments that caused this dip may also account for its present low position. A full explanation of this is still required.
When considering the probable age of the glacial advance it must be remembered that in the Australian Alps there were Pleistocene glaciers. In Tasmania they extended to within 500 feet of the present sea-level. There were glaciers in the mountains of South Africa, and the Andes of South America supported much larger glaciers than now. In the Antarctic it has been shown that the ice extended farther than now in Victoria Land, at the Gaussberg, and in the South Shetlands. In all except the last three instances a Pleistocene age has been accepted for this glacial extension; so, unless very weighty reasons can be given to the contrary, it is most reasonable to consider the New Zealand glacial extension as Pleistocene also.
The deposits of fine clay that cover the surface of the Canterbury Plains have given rise to a considerable amount of discussion. Hutton has asserted a marine origin for it, but the nearly complete absence of stratification and marine fossils negatives this suggestion. Von Haast described it as a loess, and this opinion has been generally indorsed. If the land were more elevated, the peculiar climatic characteristic of Canterbury would be accentuated. In particular, the nor'-westers would be drier, the glaciers would be larger, and the amount of sediment sent down would be greater. As the mud dried on the river-gravels after floods it would be dispersed by the wind in huge clouds of dust, and would be deposited on the surrounding land.
Within the last year it has been stated that New Zealand was in the Pleistocene period covered with an ice-sheet as far north as New Plymouth and the East Cape. The complete absence of moraines near the coast, except at Taieri, the absence of striated rock-surfaces and boulders, the absence of erratic boulders in all coastal districts, and the absence of glacial topography in these districts, in the author's opinion effectively disprove these statements.
15—Geology.
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The most important fossils of the Pleistocene are the moas. Their remains are found beneath sand-dunes and in caves, but most abundantly in small swamps at the base of the foothills on the western border of the Canterbury Plains. The swamps occupy small basins in the clay. The bones are never associated together in complete skeletons; usually, indeed, no two bones of one skeleton are found together. With them there is a little gravel and bones of other birds, such as Notornis and Harpagornis, now extinct. Bones of the Spbenodon are found in these deposits, though unknown in New Zealand geological deposits of greater age, but the lizard is closely related to reptiles that occur in the Triassic of Europe. It is hard to account for the presence of these moa-bones, but the following theories have been proposed by different authorities. Hector: The moas were driven by fires and Natives until they took refuge in the swamps, and were easily and safely attacked. Von Haast: The swamps lay across migration-routes of the moas, and from time to time birds would stray from the track and become bogged. Hutton: The moas were killed by early and severe winter snows, and their bones were washed down to the flat land by the water derived from the melting snow. To each of these theories there are important objections, but their divergence shows that an explanation of the occurrence of the bones offers considerable difficulties.
Economically the Pleistocene is of interest because of the fertile agricultural land formed during its continuance, and because of the rich auriferous gravels that were deposited during the period.
Recent.
There is no satisfactory distinction between the Pleistocene and Recent in New Zealand. It is certain that the moas continued to live into the Recent period, for complete eggs have been found in Recent gravels. It is equally certain that a race of men fed on them, and perhaps exterminated them. Whether these men were the ancestors of the present Maori race is doubtful. Von Haast has advanced reasons for thinking that the moa-hunter preceded the Maori. He relies on certain paleolithic or chipped-stone implements in the lower kitchen-middens at the Rakaia and in the lower strata of cave-floors at Sumner. The human remains associated with these are said by him to have certain Melanesian characteristics. Certain rock-paintings
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in shelters at Albury are described as possibly the work of, this early race. Maori traditions commonly state that the country was occupied when the Hawaiki canoes arrived, about six hundred years ago. The absence of any mention of the moa, except as a highly mythical bird, is an indication that it had become extinct before the present dominant Maori tribes arrived. The mention of the moa in legends is probably derived from statements made by the race that preceded the Maori. In considering this suggestion it must be borne in mind that the Maori is highly observant, and is extremely accurate in description; that traditions were handed down for centuries with verbal accuracy; and that the moa as an important source of food would arouse immense interest and attention. There is little doubt that New Zealand has never been inhabited by a race other than Polynesian, and that the Polynesians arrived in the land within the Recent period. The people of earlier migrations were in most cases almost exterminated by the later arrivals, and their very existence is now almost mythical.
Volcanic action has been continued into the Recent period in the North Island. The cone of Ngauruhoe is probably of Recent formation, while White Island and Ruapehu have certainly been active within the Recent period. In 1886 Tarawera burst into violent eruption. The mountain is formed of rhyolite, but hypersthene andesite was ejected during the 1886 eruption. Ngauruhoe and the other volcanoes mentioned consist of the same type of rock. Farther north the volcanic cones of Auckland and Whangarei to the Bay of Islands are probably of Recent age.
The rocks at Auckland are basanite, which contains very little nepheline. The extreme youthfulness of these cones is shown by their perfect shape, still unaffected by erosion, and by the fact that the lavas followed the valley-lines in the land, and new valleys have not yet been eroded on the lava-surfaces.
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CHAPTER XVI.
THE OUTLYING ISLANDS.
Chatham Islands: Schists apparently similar to the schists of Otago and Westland constitute the oldest rock in these islands, and they occur over a considerable area. There is also an important mass of volcanic rocks which are described as basalts, but several of them contain a good deal of hornblende. Much of the islands is built up of Cainozoic sediments, which certainly correspond with a portion of the Oamaru system; several of the fossils in the rocks are identical with those in New Zealand in the rocks of this system. There are considerable deposits of sand on the coast.
The Auckland Islands are formed entirely of rocks of igneous origin. Amongst them there are granites and gabbros in the south, but all the other rocks are volcanic. There is an older series of trachytes, but by far the greater part of the islands is formed of basalts, of which there are a very large number of lava-flows.
The Campbell Islands are partly volcanic and partly sedimentary in their origin. A mass of gabbro is the oldest rock, and on it rests a series of sedimentary rocks quite similar to the series of the Oamaru rocks in New Zealand; for there are shales of a carboniferous nature at the base with conglomerates, and on them there is a chalky limestone with flints. There is also a large series of volcanic rock, mostly of a trachvtic nature; the lavas rest on a considerable thickness of tuff.
Antipodes Island consists of basalt that is often coarse-grained. Between some of the lava-flows there is a seam of coal.
Bounty Islands are formed entirely of granite.
Snares Islands: Granite is the only rock that occurs.
Kermadec Islands: Entirely volcanic. At Raoul volcanic eruptions have occurred, the last of them in 1872. The crater on one of the Curtis Islands also is active. The rocks of the Kermadecs are almost entirely andesites and basalts, but a large number of boulders of granite occur on the shore of Sunday Island (Raoul).
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GEOLOGY OF NEW ZEALAND.
Rarotonga, in the Cook Group: The main portion of the island is formed of volcanic rock, basalt, phonolite, and tuff. Round the shore there is a belt of raised coral about 200 yards wide and 15 feet above sea-level.
Mangaia is an interesting island. The flat-topped hill in the centre marks an old sea-level 600 feet above the ocean. This is surrounded by a raised coral reef about 100 feet above sea-level, and between this coral and the volcanic hill there is a swamp 200 yards or more wide. There are two other terraces of raised coral, about 10 feet and 30 feet above sea-level respectively, between the main mass of raised coral and the sea. The first elevation, when the flat-topped volcanic hill was raised above sea-level, was succeeded by a movement of depression, when the barrier reef was formed round it. When this was raised the former lagoon was converted into a swamp. The gradual elevation of the barrier reef was interrupted by two periods of quiescence, during which the two exterior terraces were formed.
Mauke and Atiu are two islands where a fringing reef encircles a volcanic hill of very small elevation.
Aitutaki is a small volcanic island surrounded by a large barrier reef within which there are one or two other small islands. Manuae and To Au-o-tu are apparently slightly raised coral.
Niue is formed of raised coral, in which several distinct terraces can be distinguished. These mark periods of rest separating periods of elevation.
Penrhyn, Manahiki, Palmerston, Rakahanga, and Danger Islands are all atolls.
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CHAPTER XVII.
STRATIGRAPHICAL CALSSIFICATION.
The following table represents the rock-classification here adopted, as compared with those previously used by Hector and Hutton: —
Manapoubi .. Archaean .. Plutonic complex of the Fiord region (Triassic, Hutton).
Aorere .. Ordovician .. Aorere schists Mount Arthur marbles Graptolite shales of West Wanganui}
(Lower Si - lurian, Hector.) (Ordovician, Hutton.)
{Baton River (Silurian, Hector and Hutton) Reefton (Devonian, Hector; Silurian, Hutton).
Maitai
Triassic .. Maitai (Carboniferous, Hector and Hutton). Otago schists (Silurian, Hector; Archaean, Hutton). Kaihiku (Permian, Hector; Jurassic, Hutton). Wairoa and Oreti (Triassic, Hector and Hutton). Te Anau basic lavas (Devonian, Hector). Dunite and other ultra-basic rocks (Devonian, Hector).
Jurassic .. Mataura, kawhia (Jurassic, Hector and Hutton). Intrusions of granite in Westland (Jurassic, Hutton). Rhyolites and andesites of Mount Somers.
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Oamaru
{Cainozoic (mostly _ early)
Waipara (Cretaceous, Hector and Hutton). Cretaceo-tertiary Eocene Lower Miocene Upper Miocene} Hector.
o ligocene Upper and Lower Miocene}Hutton. Basalts and alkaline rocks, Dunedin. Andesites, Banks Peninsula. Dacites, andesites, Cape Colville. Dolerites, Pirongia.
Wanganui .. Pliocene Wanganui (Pliocene, Hector and Hutton).
Pleistocene Moutere gravels, Southland Plains.
Volcanic rocks of North Island plateau.
Recent .. Sand-dunes, river-gravels.
Volcanic rocks at Auckland and Taupo.
The physical history of New Zealand, based on the classification adopted, is as follows : The only information in regard to this area during the Palaeozoic period refers to the north-west of the South Island. There was undoubtedly a marine area in the Aorere district in the Ordovician, and apparently a shallower water-area in the Baton River district in the Silurian. Apparently there were similar conditions at Reefton in the Devonian. The absence of Carboniferous and Permian rocks as here defined may be an indication that during the later Palaeozoic the land was so elevated that no sediments were deposited within the whole area. Of this there is no certainty; we do not even know whether the old Archaean rocks formed a land surface during this time. Information becomes definite with the commencement of the Mesozoic. The general occurrence of sediments throughout the country, and their coarse nature, proves that New Zealand was then practically the shore-line of a continent. The nature of the conglomerates and the grains in the sandstone prove further that the continent was formed of granite and other acidic igneous rocks, though the occurrence of the Te Anau melaphyres shows that the beaches were here and there covered from time to time with basaltic lava and scoria. We do not yet possess a sufficient knowledge of facts to enable us to say where the old continent was situated, though the remains of its flora and fauna, which were covered up by the sands of the beach, show that the land-area was in some way or other connected with
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GEOLOGY OF NEW ZEALAND.
the Gondwana continent, which probably at this time extended over the basin of the Indian Ocean, and united India, Africa, and Australia. During the middle Jurassic there was a complete change. The sediments, which had accumulated into a thick series, were folded and elevated into a long and lofty mountain-range. The new land was perhaps more extensive than it is now, and received some of the flora and fauna of Gondwana Land, of which Podocarpus, some ferns, and Sphenodon may be the remnants. This period of elevation was prolonged, lasting till the early Cainozoic. It is probable that during this epoch the shallower portions of the western Pacific were continuously above sea-level, and thus we were temporarily connected with New Caledonia, and perhaps with Australia. Then came a great consignment of plants and animals : the beech, kauri, and Metrosideros were added to our flora, and many of our land-birds came down from the north. Throughout the Cainozoic there was a general and prolonged depression, followed by an elevation in the later Pliocene which extended the land in a southerly direction, and brought us into communication with the Antarctic Continent. Fuchsia, Sophora, Calceolaria, and Veronica were added to our flora, marine Mollusca to our fauna, and possibly at this time the moa reached our shores. Volcanic action became important when first the elevation commenced, and at Otago Peninsula, Banks Peninsula, and elsewhere prolonged series of eruptions took place.
In the North Island volcanic eruptions began while much of the present area of New Zealand was below the sea-level. But the main eruption of the rhyolites of the volcanic plateau of the north took place later, and was followed by emission of the andesites of Egmont, Ruapehu, and scores of other volcanoes. In the North Island at least there was depression in the later Pliocene, and in the Pleistocene the form of the land was restored, but was somewhat more elevated than now. Glaciers crept out of the mountain-valleys. Over the surface of the newly formed Canterbury Plains blinding dust-storms deposited the material that now forms fertile soil. The volcanic activity of the central district of the North Island continued, and farther north basanites were emitted from sixty-five orifices at Auckland, and from others near Omapere. The eruption of andesitic matter has extended into the historic period, for Ruapehu, Ngauruhoe, and White Island have all been seen in eruption, and may yet exhibit activity even more violent than was shown in the spasmodic outburst of Tarawera in 1886.
INDEX.
Abyss, oceanic, 4, 82.
Acaena, 5.
Actinolite, 18, 163.
Ada, Lake, 42.
Adjusted drainage, 41.
Aegerine, 17.
Aeolian stones, 54.
Agassiz' theory, 157.
Agate, 13.
Age, geological 172.
Aggradation, 30.
Ahipara, 78.
Air-gaps, 40.
Aitutaki Island, 207.
Albite, 16.
Aldrich Deep, 2.
Alethopteris, 184.
Alexandra, 11, 23.
Algonkian, 173.
Alnus, 194.
Alum, 23, 114.
Alunite, 23.
Alunogen, 23.
Amber, 23.
America, South, 5.
Ammonites, 183, 190.
Amphibolite, 104.
Amuri Bluff, 13.
Anabranch, 32, 44.
Analcite, 19, 118.
Anamorphism, 168.
Andesine, 16.
Andesite, 10, 17, 18, 102, 106, 139, 188, 199.
Animals, burrowing, 154.
Anita Bay, 19.
Anorthite, 16.
Anthracite, 158.
Anticline, 118.
Anticlinorium, 119.
Antimony, 11.
Aorere system, 173, 176, 209.
Aparima River, 32.
Apatite, 21.
Aplite, 99.
Aragonite, 15.
Archaean, 133, 173, 174.
Argentite, 12, 149.
Argillite, 167, 179.
Arkose, 83.
Arsenic, 10.
Arsenopyrite, 13, 149.
Arthur, Mount, 20.
Asbestos, 20.
Atiu Island, 207.
Atmosphere, 53.
Atolls, 155.
Aucella, 186, 187.
Auckland, 8, 18, 94, 106, 205.
Auckland Islands, 206.
Augite, 17, 45, 81, 162.
Aurum, Mount, 12.
Australia, 1.
Australia, Western, 5.
Avalanche, 61.
Avicula, 168.
Awaruite, 11, 146.
Azurite, 16.
Baculites, 190.
Baiera, 182.
Balfour Shoal, 3.
Banks Peninsula, 8, 17, 77.
Bar, river, 78.
Barewood, 151.
Barite, 21.
Barrier Islands, 8, 12, 16.
Barrier reefs, 155.
Basalt, 19, 103, 140.
Basanite, 103, 140.
Base-level, 27.
Basin, 119.
Batholite, 93, 96.
Baton River, 13.
Baton River system, 173, 177.
Baveno law, 16.
Bay of Islands, 8, 11, 15, 90, 161.
Beaches, 10, 76, 80, 142.
Beheaded river, 37.
Belemnites, 186.
Bell Rock, 35.
Bergschrund, 68.
Beryl, 19.
Biotite, 20.
Bismuth, 10.
Bituminous coal, 159.
Blatt plane, 125.
212
INDEX.
Blowhole, 73.
Blue Mountains, 28.
Bluespur, 146.
Bluff, 17.
Bombs, volcanic, 87.
Bonanza, 10, 147.
Bostonite, 101.
Bounty Islands, 185, 206.
Bowen Falls, 65.
Bowenite, 20.
Breccia, 140.
Brighton, 75.
Brown coal, 159.
Browning's Pass, 20, 55.
Brunner, 18.
Building-stones,. 136.
Bytownite, 16.
Cabbage Bay, 13.
Cainozoic, 4, 5, 7, 21, 94, 173.
Calcite, 15, 162.
Caledonian Mine, 149.
Cambrian system, 173, 177.
Camel, Mount, 7.
Campbell Island, 5, 13, 19, 49, 53, 206.
Camptonite, 103.
Canterbury Plain, 6, 7, 30, 31, 52, 55.
Caples River, 20.
Carboniferous, 159, 173, 179.
Carcharodon, 193.
Cardium, 193.
Carlsbad law, 16.
Cassiterite, 14, 146.
Caswell Sound, 15.
Catlin's, 73.
Cavalli Islands, 74.
Caversham, 19.
Caves, 48, 72, 91.
Cement, 141.
Cementation, 50.
Cements, natural, 50, 83.
Centra, earthquakes, 127.
Central Otago, 20.
Cerussite, 15.
Chabazite, 19.
Chalcedony, 13.
Chalcocite, 12.
Chalcopyrite, 12, 149.
Chatham Islands, 4, 5, 206.
Chemical erosion, 48.
Chiastolite, 164.
Chicken Islands, 74.
Chlorite, 20.
Chonetes, 178.
Chromite, 14, 153.
Chrysotile, 20.
Cinnabar, 12, 149.
Cirques, 68.
Clarendon, 21, 142.
Clay, 21, 45, 141.
Clay, red, 82.
Cleavage, 9.
Cleavage, rock, 165.
Clent Hills, 13.
Cliffs, 72.
Clinochlore, 20.
Clinton, 28.
Coal, 157, 190, 193.
Coast-line, 82.
Coastal plain, 7.
Collingwood, 11, 12.
Columns, basaltic, 94.
Colville, Cape, 7.
Comb structure, 147.
Composite cones, 92.
Concretions, 49.
Cone, 89.
Conglomerate, 83.
Consequent streams, 40.
Contact action, 163.
Contours, ocean, 6.
Cook Islands, 94, 155.
Cook, Mount, 133.
Cook Strait, 81.
Copland Springs, 51.
Copper, 12.
Corals, 155.
Coromandel, 10, 13.
Corrosion, river, 26.
Corundum, 14.
Cossyrite, 18.
Crater, 85.
Cretaceous system, 173.
Crevasses, 61.
Crystallography, 9.
Cucullaea, 193.
Cuprite, 14.
Current bedding, 37, 170.
Current, river, 35.
Cycads, 186.
Cycle of erosion, 28.
Dacite, 102.
Darran Mountains, 17, 18, 131
Dart River, 80.
Darwin's theory, 156.
Deep, oceanic, 2.
Degradation, 30.
Delta, 80.
Dentalium, 193.
Deposition of sediment, 29, 35, 44, 75.
Devonian system, 159, 173, 178.
Diabase, 102.
Diallage, 17.
Diatoms, 82, 84.
Differentiation of magma, 108.
213
INDEX.
Diorite, 18, 101, 139, 175.
Dip, 116.
Divide, 6.
Dolerite, 103, 139, 199.
Dolomite, 157.
Dome Mountains, 38.
Dome Pass, 41.
Dome, structural, 119.
Downs, 7.
Downthrow, 123.
Drift, coastal, 78.
Druses, 11, 147.
Dunedin, 12, 15, 17, 18, 19.
Dunes, 8, 53, 82.
Dunite, 105.
Dun Mountain, 12, 14, 16, 18, 19.
D'Urville Island, 131.
Dusky Sound, 9, 11, 12, 18, 19, 175. Dust, 55, 86.
Dykes, 72, 92, 193.
Dynamo-metamorphism, 168.
Earth pillar, 25.
Earthquake, 75, 126.
East Cape, 1, 7.
Eden, Mount, 87, 89.
Edgecumbe, Mount, 129.
Egmont, Mount, 9, 17, 18, 30, 37, 89, 94.
Elevation, land, 29.
Endeavour Inlet, 11.
Enstatite, 17.
Eocene system, 173.
Eosaurus, 182.
Epidote, 19.
Epsomite, 23.
Era, 172.
Erosion, chemical, 48.
Erosion, glacial, 33.
Erosion, stream, 27.
Erratics, 71.
Escarpment, 41,
Fagus, 194.
False bedding, 37.
Fan, river, 29.
Farewell, Cape, 8.
Farewell Spit, 78.
Fault, 123.
Fault, reversed, 125, 135.
Fault, trough, 68.
Feilding, 8.
Feldspar, 16, 21, 45, 162.
Festoon, Australasian, 4.
Fiji, 4.
Fiords, 74, 174.
Flint, 13, 49.
Flora and fauna of New Zealand, 4, 5.
Fluorite, 13.
Fold, recumbent, 125.
Fold, structural, 4, 118.
Folds, isoclinal, 122.
Foliation, 166.
Foraminifera, 82.
Frost, 57.
Fuchsia, 210.
Fuchsite, 20.
Fuel, 141.
Fumaroles, 10, 111.
Funafuti Island, 157.
Gabbro, 5, 17, 102.
Galena, 11, 15, 149.
Gangue, 149.
Garnet, 18, 80.
Gash veins, 151.
Geodes, 21.
George Sound, 20, 175.
Geyser, 112.
Geyserite, 13.
Glacial advance, 202.
Glacial erosion, 33, 63, 64.
Glaciers, 43, 59, 61, 197.
Glass, volcanic, 96.
Glauconite, 21.
Glenomaru, 41.
Glenorchy, 23.
Globigerania ooze, 82.
Glossopteris, 182.
Gneiss, 9, 15, 164, 167.
Gold, 10, 144, 149.
Golden Bay, 16.
Golden Ridge, 151.
Gondwana Land, 210.
Goodletite, 14.
Gorge River, 11.
Gossan, 147.
Guland Downs, 139.
Graben, 44.
Granite, 3, 5, 19, 20, 21, 99, 139, 151 188.
Granitite, 99.
Ganophyre, 99.
Ganulite, 167.
Graphite, 9, 158.
Gaptolites, 176.
Grassmere, 41.
Gravel drift, 78.
Gravels, 76, 83.
Gravels, metalliferous, 144.
Greenstone, 179.
Greenstone (nephrite), 18.
Greisen, 99.
Grey Gorge, 33.
Grejnnouth, 141.
214
INDEX.
Greywacke, 10, 47, 50, 80, 83, 137, 164, 179.
Gossularite, 18.
Goundmass of rocks, 96.
Gyphaea, 181.
Gypsum, 23.
Hade, 123.
Haematite, 14.
Hlimeda, 155.
Halloysite, 21.
Hlobia, 181.
Hanging valley, 35, 67.
Hanmer Springs, 115.
Harbours, 74.
Hardness (minerals), 9.
Harpagornis, 204.
Harris, Lake, 43.
Hastings, 52.
Harzburgite, 105.
Hastings, 52.
Hauraki, 10.
Hawke's Bay, 7, 195.
Heave (fault), 124.
Henley, 25.
Hicks Bay, 195.
Hogben, 127.
Hohoura, 8.
Hokonui Hills, 6, 186.
Homalonotus, 178.
Hornblende, 18, 45.
Horohoro, 129.
Huka Falls, 34.
Hutt Valley, 52.
Hyalite, 13.
Hydro-metamorphism, 168.
Hypersthene, 17.
Lanthe Lake, 44.
Ice, 61.
Ichthyosaurus, 182.
Igneous rocks, 97.
IImenite, 14.
Impregnations, 153.
Inoceramus, 186.
Intrusive rocks, 96, 99.
Iron, native, 11.
Iron-ores, 15.
Jasper, 13.
Jasp-opal, 13.
Joints, 94, 122.
Jurassic system, 7, 173.
Kaihiku, 182.
Kaikoura Mountains, 16, 131, 134.
Kaimanawa Mountains, 129.
Kaitangata, 193.
Kakanui River, 80.
Kaolin, 21, 45.
Karamea River, 12.
Karangahape, 129.
Karapiti, 111.
Karioi, Mount, 94.
Katamorphism, 168.
Kauri-gum, 23, 142.
Kauri-tree, 210.
Kawau Island, 12, 16.
Kawhia, 7, 8, 123.
Kekenodon, 193.
Kermadec Islands, 1, 3, 4, 94, 206.
Kurow, 182.
Labradorite, 16.
Laccolite, 93.
Lagoons, coastal, 42.
Lagoons, coral, 156.
Lakes, 41.
Lake-shores, 80.
Lamna, 193.
Landslip Hill, 50.
Landslips, 27, 42, 51, 72.
Lapilli, 86.
Lateral secretion, 150.
Lava-flows, 8, 42, 86, 89.
Lherzolite, 105.
Lightning, 57.
Lignites, 23, 158.
Lima, 193.
Limestone, 7, 15, 47, 138.
Limestone Island, 15.
Limonite, 15, 50, 153.
Lithology, 172.
Lodes, 146.
Lodes, origin of, 150.
Loess, 55, 141, 203.
Lyall Bay, 54.
Lyell, 151.
Lyttelton, 17.
Macquarie Island, 6.
Magma, 105.
Magmatic water, 150.
Magnetite, 14, 80, 81, 146. 153.
Mahinapua Lake, 42.
Maitai, 127.
Maitai system, 173, 178.
Malachite, 16.
Malay Archipelago, 4.
Manapouri Lake, 42, 66.
Manapouri system, 173, 174.
INDEX
215
Manawatu Gorge, 27, 129.
Manawatu River, 38.
Manebacher law, 16.
Mangaia Island, 207.
Manganese minerals, 15.
Manukau, 72, 73, 195.
Maori, 204.
Mapourika Lake, 44.
Marble, 15, 138, 165, 177.
Marcasite, 12.
Marine plain, 38.
Marl, 84.
Marlborough, 6, 10, 23.
Mataura Falls, 34.
Mataura River, 38.
Mauisaurus, 190.
Mauke Island, 207.
Meander curves, 31, 44.
Melanesian traces, 204.
Melanterite, 23.
Melaphyre, 104, 179.
Mercury, 12.
Mesozoic, 173, 209.
Metamorphic rocks, 7, 162.
Metamorphism, 162.
Metasomatism, 147.
Meteorites, 11.
Metrosideros, 210.
Mica, 20.
Mica-schist, 10, 139, 163.
Micro cline, 16.
Milford Sound, 10, 17, 20.
Minerals, description, 8.
Miocene, 90, 141, 173, 197.
Mississippi, 29.
Mitre Peak, 68.
Moa, 203.
Moeraki boulders, 49.
Mokau River, 49.
Moke Creek, 12, 16.
Mollusca, 5, 154.
Molybdenite, 11.
Monchiquite, 103.
Monotis, 4, 182.
Monowai Lake, 66.
Moraine, 42, 44, 61, 63, 202.
Mosasaur, 190.
Motueka River, 6.
Mountain, 6.
Mountains, 7, 116.
Mountains, block, 134.
Mountains, Otago, 6.
Mountains, residual, 121, 134.
Moutere Hills, 198.
Movement of coast-line, 35.
Movement, rock, 37.
Mud, 81.
Mud volcano, 111.
Mudstones, 83.
Murray's theory, 156.
Muscovite, 19, 21, 45.
Mytilus, 183.
Natrolite, 19.
Natural bridge, 73.
Nelson, 6, 12, 18.
Nepheline, 18.
Nepheline syenite, 101.
Nephrite, 18.
New Caledonia, 3.
New Guinea, 4.
New Hebrides, 4.
Ngaruawahia, 8.
Ngauruhoe, 90, 93, 131, 205.
Ngongotaha, 129.
Niagara Falls, 67.
Nickel, 3, 11.
Niue Island, 207.
Nokomai River, 38.
Norfolk Island, 3.
Norite, 102.
North Cape, 8.
Notornis, 204.
Nugget Point, 41, 78.
Nuggets, 10, 11. 146.
Oamaru, 15, 21, 84.
Oamaru system, 173, 188.
Obsequent streams, 40.
Obsidian, 100.
Ohau Lake, 44.
Oligocene system, 173.
Oligoclase, 16.
Olivine, 18, 45, 162.
Omapere Lake, 42.
Ooze, 82, 84.
Opal, 13.
Orakei Korako, 23.
Ordovician age, 49, 133, 173, 176.
Orepuki, 10, 11.
Orthis, 178.
Orthoclase, 16, 21.
Osmium, 11.
Ostraea, 183, 193.
Otago, 10, 11, 13.
Otago Peninsula, 8, 77.
Otaki, 8.
Outcrop, 116.
Ouvarovite, 18.
Overlap, 171.
Owen River, 10.
Pacific Ocean, 6.
Pacific, South, 1.
Pakawau, 9.
216
INDEX.
Palaeontology, 171.
Palaeozoic age, 173.
Palmerston Island, 155.
Panmure Basin, 88.
Papa rock, 49, 84.
Paparoa Mountains, 131.
Parapara, 12, 15.
Patea, 8.
Patea River, 32.
Pearson Lake, 41.
Peat, 159.
Pecten, 193.
Pectunculus (Glycimeris), 193.
Peneplain, 28, 134.
Perched valleys, 67.
Percolating water, 45.
Peridotite, 188.
Period, geological, 172.
Permian system, 173, 182.
Petroleum, 24, 141.
Phonolite, 18, 46, 101, 139.
Phosphate rock, 142, 153.
Phosphorite, 21.
Phyllite, 167.
Phyllograptus, 176.
Phyllopteris, 182.
Piako River, 32.
Pihanga, 92.
Pikikiruna Mountains, 49, 138.
Pipeclay, 46.
Piracy, river, 38.
Pirongia, Mount, 94.
Pitchstone, 100.
Placers, 10.
Plagioclase, 17.
Plain, Canterbury, see Canterbury Plain.
Plains, alluvial, 41.
Plains, gravel, 6, 7, 8.
Plains, marine, 75.
Plants, 154.
Plasma, 13.
Plateau, volcanic, 8.
Platinum, 11, 144.
Pleistocene period, 68, 173, 200.
Plesiosaurus, 190.
Pliocene period, 37, 173, 198.
Plutonic rooks, 7, 93, 96.
Pneumatolysis, 151.
Podocarpus, 186.
Porosity of rocks, 137.
Porphyrite, 102.
Porphyritic structure, 96.
Porphyry, 19, 96, 99, 101.
Porridge-pots, 111.
Port Pegasus, 14, 16, 20.
Potholes, 30, 35.
Poverty Bay, 11, 24.
Prase, 13.
Primary era, 173.
Psilomelane, 15.
Puhipuhi, 13.
Pukaki Lake, 42.
Puketoi Hills, 38, 135.
Pumice, 8, 87, 198.
Pyrargyrite, 13
Pyrite, 10, 12, 49.
Pyroxenite, 104.
Pyrrhotite, 12.
Quartz, 13, 45, 149.
Quariz porphyry, 99.
Quartz veins, 10.
Quartzite, 50.
Quaternary era, 173.
Queenstown, 14.
Quercus, 194.
Radiant, Mount, 11, 12.
Raglan, 8.
Rainfall, 45.
Rangitoto, 74, 89, 107.
Rapids, 29, 35.
Rarotonga Island, 18, 207.
Recent period, 204.
Recrystallization, 167.
Red Hill, Nelson, 14, 20.
Red Hill, Otago, 4, 11, 14, 19, 20.
Reef, fringing, 155.
Reef, metalliferous, 146.
Reefton, 10, 151.
Rhyolite, 100, 187, 199.
Richmond Hill, 13, 19.
Richmondite, 13.
Rimu, 14, 20.
Rimutaka Mountains, 129.
Rivers, action of, 26.
River-valleys, 30.
Roches moutonnees, 67, 100.
Rock-paintings, 204.
Rodingite, 105.
Romohapa, 41.
Rotoaira Lake, 42.
Rotokawa Lake, 10.
Rotomahana Lake, 42, 43, 108.
Rotorua Lake, 13, 23, 42.
Routeburn Valley, 66.
Ruahine Mountains, 129.
Ruamahanga River, 32.
Ruapehu, 3, 17, 30, 87, 93, 131.
Ruapuke Island, 139.
Ruby, 13.
Rutile, 14.
Saddle lodes, 151.
Samoa, 3, 6, 90.
Sand, 53, 77, 80, 144.
Sand-drift, 78.
217
INDEX.
Sandstones, 7, 83, 137.
Sandymoimt, 54.
Saunders, Cape, 13.
Scalaria, 193.
Scheelite, 23, 149.
Schist, 7, 14, 20, 80, 133, 139, 166, 178. 179.
Scoria, 86.
Scree, 57.
Sea-coast, 72.
Secondary era, 173.
Selenite, 23.
Selwyn River, 77.
Separation Point, 16, 21.
Septarian nodules, 49.
Sericite, 21.
Serpentine, 3, 11, 20, 45.
Shales, 7, 10, 15, 83.
Shotover, 12, 15. Sill, 93.
Silurian system, 173, 177.
Silver, 10, 146.
Sinbad Gully, 68.
Sinclair Head, 74.
Slate, 165.
Snares Island, 206.
Snow-line, 59.
Solfatara, 110.
Somers, Mount, 13.
Sophora, 5, 210.
Soundings, ocean, 2, 6.
Southern Alps, 6, 134, 163, 169, 177.
South Island, 6, 7.
Southland plains, 30, 31.
Specific gravity of minerals, 9.
Spenser Mountains, 131.
Sphalerite, 12, 149.
Sphene (titanite), 21.
Sphenodon, 204.
Sphenopteris, 184.
Spheroidal weathering, 46, 47.
Spirifera, 178.
Spiriferina, 182.
Spirigera, 184.
Springs, 50.
Springs, artesian, 51.
Springs, hot, 51, 110, 114.
Squalodon, 193.
St. Mary, Mount, 182.
Stacks, 74.
Stalactites, 48.
Stalagmites, 48.
Steam, volcanic, 86.
Stewart Island, 16, 19.
Stibnite, 11, 149.
Stirling Falls, 65.
Stockwerks, 151.
Strata, 36, 116.
Stratification, 116.
Stratigraphy, 170.
Streak, mineral, 9.
Stream tin, 14.
Strike, 116.
Structural axis, 133.
Structure, North Island, 7.
Structure, South Island, 6.
Subsequent valleys, 33, 40.
Suess, Professor, 5, 129.
Sulphides, 147.
Sulphur, 10, 142.
Suspended matter (streams), 28.
Sutherland Falls, 65.
Syenite, 100.
Syncline, 118.
System, 172.
Taeniopteris, 183, 186.
Taiaroa Heads, 54.
Taieri Gorge, 27, 35.
Taieri Moraine, 203.
Taieri Mouth, 15.
Takaka, 15.
Takaka system, 177.
Takitimu Mountains, 180.
Talc, 20.
Talus slopes, 74, 122.
Tangiwai, 20.
Taniwhasaurus, 190.
Taramakau, 18.
Taranaki, 7.
Tararua Mountains, 52, 129.
Tarawera, Mount, 87, 91, 107, 205.
Tarn, avalanche, 44.
Tasman Bay, 6.
Tasman Mountains, 131.
Tasman Sea, 6, 82.
Tasman Valley, 44.
Tauhara, Mount, 129.
Taupiri Range, 38.
Taupo, 8, 10, 13, 17, 43.
Te Anau Lake, 42, 43, 66.
Te Aroha, 12, 15.
Tekapo Lake, 42.
Temperature, 57.
Terawhiti, 7.
Terebratula, 182.
Terminal face (glacier), 61.
Terraces, river, 30.
Terraces, Rotomahana, 108, 112.
Tertiary era, 173
Thames, 10, 12, 15, 17, 21, 23, 29, 32, 148.
Thermo-met amor phism, 168.
Three Kings Islands, 8.
Throw, fault, 123.
Thrust plane, 125, 135.
Tin, stream, 14.
INDEX.
218
Tinguaite, 101.
Tikitapu, 114.
Tikitere, 10, 111.
Timaru, 78.
Titanite, 21.
Tonga Islands, 1.
Tonga Ridge, 3, 4.
Tongariro, Mount, 42, 93.
Tongariro River, 80.
Topographical divisions, 7.
Torlessia Mckayi, 180.
Totara Lagoon, 78.
Tourmaline, 19.
Trachyte, 101, 140.
Trias-Jura, 178, 180.
Triassic rocks, 4.
Triassic system, 173, 182,
Trigonia, 181, 183.
Tuff, 86.
Turritella, 193.
Unconformity, 133, 170.
Valleys, river, 27.
Veins, metalliferous, 146.
Veronica, 5, 210.
Vesicular structure, 84.
Victoria Mountains, 131.
Virginia Lake, 42.
Vivianite, 21.
Volcanic activity, 106.
Volcanic ash, 86, 108.
Volcanic rocks, 96.
Volcano, 85, 129.
Voluta (Scaphella), 193.
Vug, 147.
Waiheke Island, 7, 15.
Waihi, 12, 148, '195.
Waihohonu Springs, 90.
Waihola River, 32.
Waikaia, 11.
Waikanae, 8.
Waikaremoana, 43.
Waikato, 8, 38, 46.
Waikawa, 138.
Waimangu, 111, 113.
Waimea (Nelson), 30.
Waiotahi Mine, 10.
Waiotapu, 10, 23.
Waipara, 190.
Wairakei, 113.
Wairarapa, 7, 11.
Wairarapa Plain, 30.
Wairea Gorge, 4.
Wairoa River, 31.
Waitahuna, 12.
Waitakerei, 8".
Waitangi Falls, 33, 67.
Waitomo caves, 49.
Waiwera Falls, 33.
Wakatipu Lake, 12, 15, 20, 43, 66.
Waldheimia (Magellania), 193.
Wanganui, 7, 27, 32, 52.
Wanganui River, 29, 34.
Wanganui system, 173, 198
Waterfalls, 33.
Water-gaps, 40.
Waves, marine, 76.
Weathering, spheroidal, 46, 47
Wellington, 126.
Westland, 18, 19, 20, 51.
Westmere, 42.
Westport, 10.
Whakarewarewa, 151.
Whangape Lake, 42.
Whangarei, 8, 15, 33, 94.
Whangaroa, 8, 195.
White Island, 142, 205.
Wilberforce, 10.
Wilsonite, 100.
Wind, action of, 53.
Zeolites, 19.
Zinc, 12.
Zircon, 19.
By Authority: John Mackay, Government Printer, Wellington.
1,500/1/10—3953
Permanent link to this item
https://paperspast.natlib.govt.nz/books/ALMA1912-9915982793502836-Geology-of-New-Zealand
Bibliographic details
APA: Marshall, P. (1912). Geology of New Zealand. Govt Printer.
Chicago: Marshall, P. Geology of New Zealand. Wellington [N.Z.]: Govt Printer, 1912.
MLA: Marshall, P. Geology of New Zealand. Govt Printer, 1912.
Word Count
62,922
Geology of New Zealand Marshall, P., Govt Printer, Wellington [N.Z.], 1912
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