Art. XLIX.—On the Occurrence of Granite and Gneissic Rocks in the King-country.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 25, 1892, Page 353

Art. XLIX.—On the Occurrence of Granite and Gneissic Rocks in the King-country. By James Park, F.G.S., Lecturer, Thames School of Mines. [Read before the Auckland Institute, 27th June, 1892.] The whole of New Zealand belongs to one great orographical system which forms a line of elevation traversing the Pacific Ocean in a north-east and south-west direction, with a long northern prolongation stretching away to the north-west. But, while parts of one system, the geology of the greater islands is specialised by many important features, among which stands conspicuously the absence in the North Island of the ancient Palæozoic crystalline rocks so widely and typically developed in the Provincial Districts of Otago and Nelson. Their absence has been the subject of many interesting discussions among geologists, and many curious and sometimes ingenious hypotheses have been suggested as to the probable conditions existing in this area in older primary times. The late Dr. Von Hochstetter, in his lecture on the geology of the Province of Auckland, delivered to the members of the

Auckland Mechanics' Institute on the 24th June, 1859, said, “The first striking characteristic of the geology of this province—and probably of the whole of the Northern Island of New Zealand—is the absence of the primitive plutonic and metamorphic formations, as granite, gneiss, mica-slate, and the like.” Sir James Hector, in his “Outlines of New Zealand Geology,” observes that schists and metamorphic rocks are unknown in the North Island. The oldest rocks hitherto known to exist in this Island were certain hard splintery greywackes and slaty shales which form the greater portion of the Tararua, Ruahine, and Kaimanawa Ranges, in the Province of Wellington. The same rocks also occupy smaller isolated areas at Tuhua, Taupiri, and Hunua Ranges, and form the basement rock of the Hauraki Goldfields. In the absence of fossil remains, their age has always been a matter of much uncertainty, but they have generally been placed by geologists in the Carboniferous period, mainly, it would appear, from a lithological likeness to some rocks ascribed to that period on the south side of Cook Strait. The circumstances which led to the recent interesting discovery of granites and other crystalline rocks in this Island are briefly as follow: Last March Mr. Max von Bernewitz, the well-known assayer to the Bank of New Zealand at the Thames, showed me, in his melting-room, some fragments of rocks which had been forwarded from the King-country by Mr. G. T. Wilkinson, J.P., Government Native Agent, for the estimation of the gold which they were believed to contain. Among the rocks I at once identified examples of true granite and a granitic gneiss, and at the same time expressed some doubt as to their belonging to any part of this Island. Mr. T. L. Murray, the general manager of the bank, who always has shown the keenest appreciation of geological facts and problems, at once placed me in communication with Mr. Wilkinson, with the result that at Easter I visited and examined the locality where the specimens were found, acting upon instructions received from the Hon. the Minister of Mines, by whose permission I am now enabled to place the facts of this important discovery before the society. On my arrival at Otorohanga I found that the rocks referred to were accidentally discovered by Mr. Charles King while mustering his sheep in the Mangaone Valley. Judging from their great weight and colour, he thought them likely to contain gold; and, aided by Mr. Arthur Ormsby and Mr. Griffiths, of Alexandra—the latter an old alluvial miner—the locality was diligently prospected, and one or two “colours” are said to have been found in the wash in the bed of the Mangaone Stream, while some fragments of granite, which

were rough-crushed and washed by Mr. Griffiths, are also believed to have yielded a speck or two of the precious metal. The exact locality of the discovery is situated on the left bank of the Mangaone Stream, about a quarter of a mile above its junction with the Turitea River, which rises among the wooded Tapuaea Hills, lying some two miles east of the Hauturu Range, and falls into the Waipa River four miles south of Alexandra. From Otorohanga southward to the Upper Mokau, and westward to the sources of the Waitomo and Moakurarua Streams, the country is occupied by long, low, undulating or flat-topped ridges and spurs, in most places covered with a dense growth of fern and tutu, with here and there, in sheltered places, isolated clumps of forest vegetation which have survived the general destruction of the great forest which in comparatively recent times covered the whole of the Waipa and Mokau Valleys. This land, I am informed, still belongs to the natives. It comprises an area probably not far short of 200,000 acres, almost all of which are most desirable for European occupation. It is readily accessible to a constructed railway, and when thrown open to settlement will become one of the richest and most prosperous pastoral districts in the North Island. These fern-covered hills and ridges are principally composed of soft green and yellowish-coloured calcareous sandstones, interstratified with blue marly crumbling clays, called “papa” by the natives. Both the sandstones and clays contain an abundance of marine shells and corals; and towards their base, where there is an excess of calcareous matter, they pass into a hard limestone, in many places of great purity and of much value for burning into lime for agricultural and building purposes. They are of Cretaceo-tertiary age, and form an important and well-marked series of the coal-measures of New Zealand. In this district, as well as in other places in this central region of this Island, they are distinguished by the presence of valuable seams of superior brown coal, occurring at the base of the formation at the point where it rests on the old denuded floor of the country. It is a noticeable circumstance here, as at Whangarei and Kawakawa, that where the hard semi-crystalline limestone rests on, or approaches, the old rocky floor the coal is usually absent, or represented only by a streak of impure carbonaceous shale. This would tend to show that the limestone and coal were, in part at least, the result of contemporaneous developments of growths, the former on a sea-bottom, the latter on low-lying swampy areas contiguous to the sea.

From the sources of the Waipa the limestone, with the underlying coal - measures, extend southward almost uninterruptedly to Totaro, in the middle Mokau Valley, and westward to Kawhia Harbour and the west coast. On the slopes of Pirongia and the Hauturu Range they are much obscured by a covering of trachyte tuffs and lavas, which occupy all the higher parts of this watershed, and descend in some places to the sea. In the months of March and April, 1859, Dr. Von Hochstetter explored the valleys of the Waipa, Mangapu, and Upper Mokau. In his narrative of the journey he describes the pumice terraces in the Lower and Upper Waipa Valley, and makes a number of interesting observations relating to the physical and geological features of the country he passed through. The time at his disposal did not permit him to make a close examination of the geological structure of this district, and his route along the course of the Waipa and Mangapu Rivers carried him to the eastward of the rich limestone and papa country already described. In the month of January, 1885, under the direction of Sir James Hector, I geologically examined the whole of the country lying between Alexandra and the slopes of Pirongia, including the district around Hikurangi and the north end of the Hauturu Range, which was the most southern point I reached on that occasion, on account of the obstruction of the natives. I defined the boundaries of the limestone and associated marly greensands, and examined the coal-outcrops in the Okoka and Moakurarua Valleys. In my report I also described the occurrence of hard argillaceous and siliceous sandstones in the bed of the Ngutunui Stream, at the point where the road to Kawhia begins the ascent of Pirongia. Proceeding up the banks of the Mangaone Stream from the point where it joins the Turitea River, the rocks first met with are tough, grey, vesicular trachytes, which were found by a subsequent examination to occupy the narrow ridge dividing this stream from the Maokurarua River, presenting on both sides a long line of steep escarpment varying from 20ft. to 80ft. in height. At a point some 20 chains higher up the trachytes are cut off by the underlying calcareous sandstones, which appear first in the bed and then on the banks of the stream. Still proceeding up the valley, the calcareous sandstones are found tilted to the north-west at a low angle, and are seen to pass downwards into an impure limestone, which in its turn becomes first gritty, then pebbly, and immediately passes into a coarse conglomerate composed principally of large rounded or subangular boulders of granite and hard somewhat flaggy argillaceous sandstone. The granitic element reaches its greatest development a few chains above the impure limestone

in a rounded spur which terminates abruptly at the stream. Beyond this point the granite boulders become fewer, being rapidly replaced by those of the argillaceous sandstone, and in a short distance they disappear altogether. The section for the next few chains is somewhat obscured by the trachytes which descend from the higher part of the range at this point; but a little higher up, at the falls, the argillaceous sandstones are seen in sitû, being well exposed in the bed of the stream for a distance of 4 or 5 chains. At the falls the sandstones alternate with thin beds of hard crumbling clays or mud-stones, and often exhibit a tendency to weather in rounded concretions consisting of thin radiating concentric layers enclosing a hard nucleus. At a place about 4 chains above the falls they contain a thin irregular streak of fine bituminous coal, and I noticed that here the sandstones themselves were finely micaceous. They strike N.–S. (magnetic), and dip west at an angle of 45deg. Higher up the valley the coal-measures wrap over the outcrop of the older sandstones and close the section in that direction. [A sketch was here included, showing the position and probable relation of the rocks met with in the line of section just described, extending from the mouth of the Mangaone to the falls.] Returning to the conglomerate, a close examination showed that the fragments of granite and sandstone were mostly well water-worn, rounded or subangular, but large angular blocks of granite, sometimes over 1ft. in diameter, were found in the bed of the stream below the outcrop of the conglcmerate, and pointed to the proximity of the granite in sitû. A collection of rocks composing the granite comprised, besides the hard flaggy sandstones, granites of all degrees of texture, granitic and hornblendic gneiss, greisen, and quartzite. Among the granites, the typical fine-grained grey variety was well represented, but the predominating crystalline rock was a coarse-grained, dark-grey-coloured granite, in which, the muscovite was replaced by biotite. The felspar was in most cases well developed, and in some examples possessed a light flesh-colour. In the granitic gneiss the laminated structure of the constituent minerals was often well exhibited, even in hand-specimens, and when the mica was biotite, as was frequently the case, the rock possessed the characteristic schistose structure of a true gneiss. The hornblende gneiss is composed principally of hornblende and orthoclase, and might perhaps be more accurately defined as a syenitic gneiss. It possesses a most remarkable likeness to the hornblende-gneiss rocks associated with the Lower Silurian rocks of the Mount Arthur and Pikikiruna Ranges in Collingwood. They are not

common in the conglomerate, but, like the granites, they are found in varieties of all degrees of texture. The colour of the hornblende is dark bottle-green, and of the felspar white or grey. In the finer-grained varieties these minerals exist in about equal proportions, giving the appearance of a black rock speckled with white. In the coarser varieties the hornblende predominates, and occurs in large irregular masses, enclosing comparatively large confused crystals of orthoclase. No fossils were found in the conglomerate, but the pebbly limestone immediately overlying it contains numerous marine forms, chiefly corals, mixed with small angular fragments of sandstone, granite, and quartzite. The sandstone, which was found in situ at the falls, has all the characteristics of the Jurassic sandstones at Kawhia. Its presence as large and small blocks in the granite conglomerate proves conclusively that the conglomerate belongs to a later formation, while the small angular pieces of granite and sandstone contained in the pebbly beds overlying the conglomerate also prove that these beds are younger than the conglomerate. From these facts it may be inferred that the materials forming the conglomerate accumulated on a broken rocky shore-line, being the detritus derived by a Cretaceous torrent from the erosion and destruction of a wide land-area, of which no trace now remains. The former existence of a large land-surface in this region, composed principally of granites and other crystalline rocks, fringed by a mantle of Middle and Lower Secondary rocks, and subject to the erosion of rivers with great trans-porting-power, is conclusively established by the presence and composition of the conglomerate. The probable direction of the extension of this old submerged continent is not so easy to determine. It is perhaps permissible to suppose that a floor of granite and granitoid rocks extends eastwards throughout the central portion of this Island and westward towards Kawhia. In his report on Kawhia district, Mr. A. McKay mentions the occurrence of a syenite conglomerate on the south side of the harbour. Whether the syenites possess a trachytic or granitic character is not stated, and, as the rock called for little comment, it may be judged to present nothing uncommon or remarkable. In view of the present discovery of granites in the Waipa Valley, it would be interesting to know if there were any indications of these rocks in the Kawhia conglomerates. Granites are often of an eruptive character, and in Scotland are said to have been found of Tertiary age penetrating Jurassic strata. In New Zealand no granites younger than Silurian are known to exist, and, as those in the Mangaone conglomerate, as well as the associated gneiss and quartzite

rocks, bear so striking a resemblance to the crystalline rocks of the Pikikiruna Range and many parts of Collingwood, there can be little hesitation in placing them as Silurian. Hitherto no granites or ancient crystalline rocks of any kind have been known to exist in the North Island, and this may be regarded as one of the most important geological discoveries of the last thirty years. The discovery is not only unique and interesting from a geological and scientific view, but it also possesses an important bearing of an economic kind. Granite and gneiss are well known as the bearers of metalliferous deposits, more especially of those of tin, silver, and copper, and it is possible that the exploration of the broken country lying west of the Hauturu and south of Kawhia Harbour may disclose the presence of valuable ores of these metals. The central volcanic region of this Island is occupied by a vast development of rhyolites and trachytes, the most highly acidic of recent volcanic rocks. Their ancient prototypes, granite and syenite, are the most acidic of all ancient crystalline eruptive rocks, and it would be interesting to discover if the occurrence of these recent and ancient acidic rocks in the same area was a mere coincidence or the result of the operation of natural agencies. At the present time there is a great diversity of opinion among geologists as to the probable source or origin of the solid products erupted from volcanic rents and fissures. According to one hypothesis the interior of the earth is occupied by a fiery liquid mass of molten matter surging against the outer shell or crust. Some believe that this molten magma consists of two distinct portions—an upper layer, composed of the lighter and more acidic materials, and a central portion, composed of the more dense and basic materials. This hypothesis was invented to satisfy and explain the supposed discovery of a regular succession in the character of volcanic products, according to which the acidic lavas came first and the basic lavas last. However this hypothesis may suit the countries where it originated, it certainly will not be possible to reconcile it with the facts relating to the order of the eruption of igneous rocks in New Zealand, as I will presently show in the following statement of the character of the contemporaneous eruptive rocks from the beginning of the geological record up to the present time. It will be impossible to push our inquiries beyond Silurian times for facts based on actual observations. Nothing whatever is known as to the composition of the first original crust which formed on this incandescent sphere, before geological time began, but its basic character may be inferred from the

great difference between the mean density of the materials forming the accessible portion of the crust of the earth and the mean density of the globe as a celestial body. The oldest sedimentary rocks in New Zealand are certain flagstones, slates, and quartzites forming the core of the Pikikiruna Range, lying between the Motueka and Takaka Valleys, in Nelson. These underlie a vast development of Lower Silurian rocks, and in my report on “The Geology of the County of Collingwood” (Geological Reports, 1888–89) I classified them as Cambrian. They do not appear to have been intruded by contemporaneous igneous outbursts, and therefore supply no information relating to the present inquiry. In the Silurian period were erupted acidic lavas, forming granites and syenites; in the Devonian period, basic lavas, forming diabases, basalts, and ultrabasic serpentines and peridots; and in the Carboniferous period, acidic lavas, forming syenites. From the Permian period, through the whole of the Secondary formations up to Eocene times, there appears to have been a general cessation of volcanic activity in the New Zealand area. After this long period of quiescence, volcanic energy of a most violent, devastating, and widespread nature manifested itself towards the close of the Eocene formation. There is abundant evidence to show that this sudden display of pent-up energy was accompanied by severe and prolonged earthquakes, causing the formation of deep fissures and great faults. The faults which originated at this time were numerous and deep-seated, and caused many permanent modifications in the physical features of the country. At the Port Hills, Nelson; at Shakspeare Bay, Picton; along the whole course of the Inland Kaikoura Mountains; at Lake Wakatipu; at Martin's Bay and Big Bay, on the west coast of Otago; and at many other places in New Zealand, hundreds of feet of strata have been involved among old Palæozoic rocks by great faults, some of which have established themselves along lines of weakness along which displacements have taken place at intervals up to the present time. It was probably at this period of eruption that the great continental area which was supposed by Hochstetter to have existed off the west coast, and of whose existence proofs are not wanting, became finally submerged in the Pacific Ocean. At this later Eocene period were doubtless erupted the great series of auriferous pyroclastic tuffs and andesites of the Hauraki Goldfields, and also the basic and semibasic lavas and tuffs at Banks Peninsula, Oamaru, and Port Chalmers. The next grand outburst of volcanic activity took place in Pliocene times, and converted the central portion of this Island into one vast theatre of volcanoes, discharging showers

of frothy pumice and dust, and streams of liquid rhyolitic lava. This display culminated in later Pliocene ages in the formation of the massive mountains Ruapehu and Tongariro, the graceful Egmont, the table-topped Horohoro, and the ridge-shaped Tarawera. The evidences of volcanic activity in more recent times abound on every hand. We have the numerous basic cones of the Auckland isthmus, the semi-basic Ngauruhoe, and the acidic tuffs and muds at Tarawera and Rotomahana. This record of the igneous eruptions in New Zealand is merely a wide generalisation, but it serves to show that there has been no succession whatever of acidic and basic volcanic products in this area. It could easily be shown that even in recent times both classes have been erupted almost, if not quite, contemporaneously in regions not far separated from each other. It is therefore evident that the theory of the two magmas cannot be applied to New Zealand. Another and perhaps more admissible theory is that the liquid lavas and solid products which issue from volcanic vents are erupted from large cavities in the solid crust of the earth filled with molten rocks. These huge cavities are generally supposed to exist at no great distance from the surface, and they are believed to have been formed by the contraction of the crust causing irregular crumpling and folding, and consequent entanglement, of portions of the original magma. This hypothesis does not deny the existence of a central liquid magma of a homogeneous character, and it may help us to explain the diversified character of our igneous eruptive rocks. The elaborate experiments of Fouque, Delesse, Daubree, and others have shown that all the different kinds of igneous rocks with which we are acquainted could be produced by the fusion of existing aqueous or crystalline rocks. If these cavities do exist, whatever the cause of their origin, it is evident that the highly-heated liquid lavas or rocks with which they are filled would fuse the solid rocks in contact with them, and thus largely determine the character of the fiery mass. Where a number of independent cavities existed the nature of the solid materials erupted from them would vary according to the character of the rocks adjacent to them. Where the deep-seated rocks of a district consisted of slates and limestones the fusion of these would form a basic lava, and where sandstones or granites a highly-acid lava. It is well known that some volcanoes have erupted both acidic and basic lavas at different periods of their history, and this might be explained by the internal molten rock being at one time in contact with acidic rocks; at another with basic rocks. It would be interesting to discover that the existence of a great formation of granite,

gneiss, and other crystalline rocks had exercised an influence on the composition of the lavas and other solid products erupted by the volcanoes of the central volcanic region of this Island. In conclusion, I have to acknowledge my great indebtedness to Mr. G. T. Wilkinson, the Government Agent at Otorohanga, who, although unable himself to accompany me, with much forethought made all necessary arrangements for my guidance and conveyance to the desired locality. To his kindly action in sending the rocks to the Thames on behalf of the native owners of the land are due the circumstances which led to my identification of the granitic rocks; and for this also I have to thank him. I have also to gratefully thank my friends Mr. Charles King and his wife Hana Taare, Mr. Arthur Ormsby, Rawiri te Hauparoa, Horopapera, and Te Kapuranga (the daughter of Hoponi), for their escort to the Turitea, and ready assistance in helping me to investigate the problem surrounding the rare occurrence of granitoid rocks in that locality.