Art. XVI.—Some Rocks of Mount Cargill, Dunedin.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 44, 1911, Page 163

Art. XVI.—Some Rocks of Mount Cargill, Dunedin. By J. A. Bartrum, M.Sc. Communicated by Dr. Marshall. [Read before the Otago Institute, 3rd October, 1911.] In these notes an attempt will be made to describe a series of trachydolerites and phonolites outcropping near the summit of Mount Cargill, and, with them, such adjacent rocks as may be helpful in throwing light on their origin. A difficulty in arriving at the exact relationships of the rocks in this district lies in the fact that bush and debris obscure a large portion of country where outcrops are probable. Several references to Mount Cargill rocks have been made by different writers, notably Professors Ulrich and Park and Mr. C. A. Cotton, but Professor Marshall's paper on the “Trachydolerites”* Trans. Aust. Ass. Adv. Sci., vol. 10, 1904, p. 186; Dunedin. and that on the “Geology of Dunedin”† Quart. Journ. Geol. Soc., vol. 62, 1906, p. 381. gave the first comprehensive account of these rocks. The standard types arrived at by Professor Marshall in the latter of these two papers have been made full use of, and very much personal advice and help has been given by Professor Marshall to the writer during the preparation of this paper. It is hoped that some of the information brought forward may help to supplement previous knowledge of the rocks described. General Geology. The Mount Cargill rocks form part of the volcanic complex of the Dunedin district. In several exposures the volcanic rocks overlie unconformably the Caversham sandstone, a member of the Oamaru series, which is generally referred to a Miocene age.‡ Park, “Geology of New Zealand,” p. 25. From the fact that volcanic rocks apparently have been unaffected by the earth-movements that caused disturbance of the Oamaru series, these former probably first were emitted at a period later than that of the disturbance of the Oamaru beds. The age of the earliest volcanic outburst must thus be at earliest post-Miocene. That there are two main periods of volcanic activity in the Dunedin area is evidenced by the occurrence of a conglomerate of volcanic material overlying plant-beds that are unconformable to the Caversham sandstone.§ Marshall, “Geology of Dunedin,” Quart. Journ. Geol. Soc., vol. 62, 1906, p. 390. Professor Park considers the trachydolerites to belong to the first period.∥ “Geology of New Zealand,” p. 148. To this period he assigns a Pliocene age.¶ “On Geology of North Head, Waikouaiti,” Trans. N.Z. Inst., vol. 36, 1904, p. 418. They were extruded probably through trachytoid phonolites that have been described by Professor Marshall** Loc. cit. and Cotton,† † Trans. N.Z. Inst., vol. 41, 1909, p. 111. and that occupy a large area on Signal Hill, about two miles south-east of Mount Cargill.

In describing a section exposed at the North Head, Otago Harbour, Professor Marshall, in his paper “Geology of Dunedin,” shows that the phonolite was one of the earliest volcanics of the district. No definite statement as to the period during which the trachydolerite eruption took place can be made from the field evidence afforded by the outcrops of the Mount Cargill area, beyond the fact that the trachydolerites are nowhere overlaid by other lava-flows. Professor Marshall, remarking on this, and on the additional fact that no pebbles of trachydolerite are found in conglomerates formed in the interval between the two peroids of eruption, considers that the trachydolerites are amongst the latest of the Dunedin rocks. Physiography. Mount Cargill lies about five miles north-east of the Town of Dunedin, and forms a moderately well-rounded spur, rising to a series of sharp knobs in the Main Peak (2,232 ft.), Butter's Peaks (2,040 ft.), and Mount Holmes. There are several other less-prominent peaks at a lower elevation than these. The main spur or ridge runs south-west from the Main Peak towards Pine Hill, and on the south-east there is a gradual drop to the saddle between Mount Cargill and Signal Hill. On the north there is a steep bush-clad face leading down into the watershed of a stream draining towards Waitati. On the west flattish slopes lead out to the Leith Valley. From the south face of the Main Peak the North-east Valley Stream drains, and has cut a well-rounded valley between the Mount Cargill and Signal Hill ridges. Occurrence of Rock Types. From the Main Peak, looking north-east and east, three knobs are prominent. The nearest one—an abrupt rocky knoll called Butter's Peaks—is composed of a basic variety of trachydolerite and of a probable nephelinitoid phonolite dyke. The Main Peak itself, a steeply cleft ridge, running for about 10 chains in a north-east by east and south-west by west direction, is composed chiefly of the general lava trachydolerite. A far rocky peak to the north-east—Mount Holmes, or Remarkable Rocks, by name—shows a splendid example of columnar structure in the basalt of which it is formed. A good illustration of this outcrop is given in Park's “Geology of New Zealand,” p. 150. A quarter of a mile to the south-east of this basalt peak is a knob called Mount Zion, with a lofty precipitous face edging the Main North Road, and composed of a type of trachytoid phonolite called by the type-name “Logan's Point.” This phonolite outcrops in a series of knolls for about a quarter to half a mile in a south-west direction from Mount Zion. In a south-west and west direction from the Main Peak the rounded and flattish slopes stretching towards Pine Hill and the Leith Valley show, in scattered outcrops, a comparatively unvaried type of trachydolerite. On these slopes occasional rough hexagonal jointing is seen, and the disposition seems to add strength to the view of Professor Marshall that the flow of the trachydolerite was from Mount Cargill towards Mount Flagstaff.* “Geology of Dunedin,” Quart. Journ. Geol. Soc., vol. 62, 1906, p. 407. About a mile and a half south-west by south from the Main Peak, on the upper portion of the Pine Hill slopes, is a profusion of large blocks of rubble showing abundant large feldspathic and ferro-magnesian minerals

on the weathering surfaces. This is hypabyssal trachydolerite. Near it is also found abundant rubble of a basaltic or basanitic nature. In a quarry on the road running from lower Pine Hill north-eastwards along the southern slopes of Mount Cargill is a type of trachytoid phonolite similar to that occupying a large area on Signal Hill, and known as the Signal Hill type. This phonolite runs south-west along the hillside from the uppermost forks of the North-east Valley Stream, just below the steep southern face of the Main Peak. Near these forks begins a winding road towards the Junction School. Along this road hypabyssal trachydolerite in large rubbly blocks is first met; a space covered solely by basaltic debris intervenes; and then there is an outcrop of basaltic scoria. Fragments of the so-called “Junction basalt” are found plentifully all around, and shortly the solid rock is exposed in a shallow quarry near this road. On a knoll about 1,200 ft. high, about a quarter of a mile north-east of the scoria outcrop, a fresh basanitic rock is found in plentiful rubble. No outcrop was discovered. Basaltic and basanitic rocks, and also a nephelinitoid phonolite, outcrop in the valley of the North-east Valley Stream, below the forks mentioned above. The last-mentioned rock is of a peculiar type, and seems to be the same nephelinitoid phonolite that occurs in the North-east Valley quarry, about a mile and a quarter down-stream from the upper outcrop. Basaltic rubble is extensive on the hillside north-west of the North-east Valley tannery. No recognizable outcrops were found, and its relation to the Signal Hill phonolite cannot definitely be determined. Petrology. Description of Rock Types. A. Trachydolerites. (a.) General Lava Trachydolerite. In hand-specimen this is a heavy greyish-black fine-grained rock showing fairly prominent feldspar and, in places, pyroxene crystals. It breaks with a rough fracture. Feldspar and pyroxene show up prominently on weathering surfaces. This rock is described by Professor Marshall in his paper on the Dunedin trachydolerites.* Trans. Aust. Assn. Adv. Sci., vol. 10, 1904, p. 183. The microscope shows a base of irregular feldspar laths, with enmeshed aegirine-augite and other crystals, enclosing moderately abundant phenocrysts. A porphyritic character is shown by the feldspar, and to a less extent by the nepheline and pyroxene. The phenocrysts are sanidine, augite (chiefly of various brownish tints), resorbed hornblende, sodalite, nepheline, olivine, and occasionally aegirine-augite and anorthoclase. The commonest phenocrysts are those of augite. The sanidine shows marked corrosion, and its margins are usually dentate with aegirine-augite. Its twinning is by the Carlsbad law. The extinction in many cases is parallel to well-marked cleavage, and in some idiomorphic sections to the edge between the faces 100 and 010. The crystals are usually small, but are found up to 5 mm. by 4 mm. in size. Clear glassy crystals are characterisitic, but both liquid and aegirine-augite inclusions are common.

A few irregular biaxial interference figures were obtained, but no sections were found suitable for definite optical tests. Anorthoclase occurs in occasional phenocrysts, especially in the rocks from the upper Pine Hill slopes. The fine indefinite pericline and albite cross-twinning is characteristic. Oligoclase occurs in a few crystals. It is recognized by its albite twinning, with a low extinction-angle on either side of the twinning-plane. In one case sodalite is included in a crystal of sanidine. The nepheline phenocrysts are large and fairly plentiful. Characteristic hexagonal cross-sections are not uncommon, but corrosion has been active as a general rule. In some sections no nepheline of the first generation appears; in others, especially in those from rocks towards Pine Hill, the mineral is comparatively coarse and plentiful. Sodalite is plentiful. A few large sharply idiomorphic forms, simulating hexagonal cross-sections of nepheline, are present, but the characteristic sodalite crystal is irregular and flaky. Olivine generally is a most plentiful phenocryst. The crystals are large, fresh, and rounded. A “celyphitic” structure, in which pale pinkish-brown augite and iron-ore form a “corona” around the olivine, is marked. Where the olivine, has not this corona it shows typically a corrosion border of magnetite dust and a deep fringe of aegirine-augite granules. Resorbed amphibole is a constant and characteristic phenocryst. All stages of resorption are exhibited. The cross-sections of even the wholly resorbed mineral show characteristic shape and prismatic angles. The unresorbed mineral shows intense pleochroism, in colours varying from deep brown to golden-yellow. In some sections, particularly those from rocks near the outcrop of hypabyssal trachydolerite, amphibole, next to augite, is the most abundant of the minerals of the first generation. This amphibole has been classed tentatively as barkevicite. The occurrence of pyroxene is most commonly in glomeroporphyritic phenocrysts of a pinkish-brown variety of augite. The cleavage is characteristic. Both simple and polysynthetic twins are common. Zonal structure is noticeable. This pyroxene was more or less unstable in the original magma, and, though of idiomorphic outline, is almost always edged by a border of aegirine-augite. A purplish-violet pleochroic augite is found rarely in well-shaped crystals. The pleochroism is—parallel to c deep purplish-violet, and parallel to a and b greyish-violet. A rare deep-green to golden-yellow pleochroic pyroxene is ascribed to aegirine or aegirine-augite. It shows good augitic cleavage on basal sections. Two or three crystals of this mineral are of large size (2 mm. by 3 mm.), and include abundant prisms of apatite and squares of magnetite. They indicate by their irregular boundary that either they themselves have been resolved, or that the aegirine-augite and magnetite are the resorption-products of an earlier mineral. The constant association of aegirine-augite and magnetite with resorbed amphibole strengthens the supposition that these two minerals are the resorption-products of the amphibole. In one case a pale-green augite crystal includes one of olivine. Magnetite is infrequent otherwise than as a resorption-product.

The Groundmass. A network of predominant feldspar wraps around plentiful pale-green aegirine—augite granules, a little fine nepheline, and a little iron—ore. The feldspar, as a general rule, is in poorly shaped untwinned laths. Fluxional arrangement is rare. There is a little polysynthetically twinned plagioclase feldspar also present. The iron-ore is chiefly magnetite in small squares, but ilmenite is also present. The nepheline is only distinguished by staining the sections. It is in minute hexagonal forms. No cossyrite was observed in the many sections prepared of this rock, but there is an abundance of the mineral in a similar trachydolerite from Mount Flagstaff. The granules of aegirine-augite are always irregular, and at times simulate a mossy structure. Apatite forms stout though never plentiful prims. Order of Crystallization. Some of the relations are uncertain, but the probable order is (1) olivine; (2) apatite; (3) amphibole; (4) augite; (5) sodalite; (6) sanidine, nepheline, with possibly anorthoclase and oligoclase, and then, in the ground-mass, iron-ore, aegirine-augite, nepheline, and feldspar, in the order named. Sections of a transition type of trachydolerite come from a little east of the Main Peak. Olivine, in coarse aggregates of fresh rounded crystals, with a corrosion border of magnetite dust and aegirine-augite granules, is very common. Pinkish augite has been corroded, and is edged by aegirine-augite. Large crystals of resorbed amphibole are rare, but the mineral may be represented by numerous small groups of secondary magnetic material. Feldspar sometimes encloses this magnetite. Nepheline is rare. There is a little very opaque cossyrite. The groundmass is very dense and fine-grained; it exhibits occasional flow structure. Feldspar continues to be more important than the aegirine-augite. Chemical Characters. Two analyses of the trachydolerite from two different localities are appended, and with them, for purposes of comparison, two other analyses. A. B. C. D. SiO2 50.43 49.02 51.86 50.06 Al2O3 18.00 19.50 19.87 17.00 Fe2O3 3.78 4.37 6.30 2.96 FeO 5.65 6.60 3.11 5.42 MgO 2.91 2.14 2.33 3.61 CuO 5.76 6.78 3.77 8.14 K2O 4.79 1.70 6.20 3.40 Na2O 5.76 7.35 4.88 3.53 H2O 1.37 1.18 1.48 4.85 Cl 0.38 Not det. 0.51 MuO 0.14 P2O5 Not det. Not det. 0.36 0.66 T1O2 Not det. Not det. Not det. 0.51 98.83 98.64 100.67 100.28 A. Trachydolerite, Main Peak, Mount Cargill. (Analysis, J. Bartrum.) B. Trachydolerite, near Pine Hill. (Analysis, J. Bartrum.) C. Trachydolerite, Dr. P. Marshall.* “Geology of Dunedin,” Quart. Journ. Geol. Soc, vol. 62, 1906, p. 407.D. Shoshonite, Yellowstone National Park.† Rosenbusch, “Elemente der Gesteinslehre,” p. 355, No. 13, 1901 ed.

There is seen to exist a certain similarity in chemical composition between the trachydolerites and the alkali basalts. This is not borne out by the petrological and mineralogical characters, in which the trachydolerites approach closely to the phonolites of the adjoining area. The analyses, to be given later, of these phonolites show also how closely they merge into the trachydolerites in chemical characters. (b.) Hypabyssal Type of Trachydolerite. Two areas show extensive rubble of this rock—the one on Pine Hill, and the other near the headwaters of the North-east Valley Stream. It was not found actually in situ, but so great a heap of angular blocks as there is on Pine Hill is not likely to have travelled far. The differences from the lava type are mainly textural, and are indeed, few. In hand-specimen large pyroxene, amphibole, and feldspar crystals are conspicuous. The feldspar and soda-pyroxene are porphyritic. Olivine, nepheline, and sodalite are less plentiful than in the lava type, but feldspar is more so. The corrosion of the feldspar is a noticeable feature. The pyroxene is chiefly a pleochroic green soda-bearing variety of augite, or aegirine-augite. A little pink faintly pleochroic augite is also present. The aegirine-augite is usually idiomorphic. Resorbed hornblende in places includes a little feldspar. In the groundmass the aegirine-augite is less important than the feldspar, and is less mossy than in the general lava rock. The plagioclase feldspars—varying between oligoclase and acid andesine—also much increase their importance. The main features of the type are the increased size of the phenocrysts in general and the more open nature of the groundmass. (c.) Dense Basic Type of Trachydolerite. In hand-specimen this rock is indistinguishable from the dense green nephelinitoid phonolites and tinguaites common in the Dunedin district. It has a leek-green very fine-grained matrix, in which are a few prominent crystals of feldspar and pyroxene. Many variations of a minor nature are exhibited by the rocks included in this class. Under the microscope the distinguishing features are—(1) scarcity or lack of nepheline in the groundmass; (2) abundance of aegirine-augite and small amount of feldspar in the groundmass; (3) the dense nature of the groundmass; (4) typical scarcity of phenocrysts. The relative importance of the different phenocryst minerals varies from section to section. An interesting feature is the occurrence of small rounded leucite crystals with characteristic radial inclusions of aegirine-augite. Another peculiarity lies in the alteration (or, may be, corrosion) of the olivine phenocrysts. These have been more or less wholly replaced by a clear colourless secondary mineral and magnetite dust. The fibrous nature and other characters of this secondary mineral seem most characteristic of serpentine. A bluish-green chloritic mineral is sometimes connected with this alteration of the olivine. Sharply idiomorphic, fresh olivine crystals are, however, not uncommon. There is an occasional corona of augite and magnetite to the olivine. Phenocrysts of feldspar are less common than those of the ferromagnesian minerals; of them, sanidine is the commonest, but anorthoclase

also occurs in a few large crystals The feldspars exhibit the same rounding as in the general trachydolerite, but the edges are sharply defined. Pyroxene and brown amphibole also form phenocrysts. The pyroxene is generally idiomorphic pale-pinkish to pale-greenish-pink augite. It is commonly fringed by dust-like aegirine-augite. Aggregates of pinkish augite are common. In some sections nepheline forms important large well-shaped crystals. Sodalite in small flaky forms is moderately abundant. The augite includes a few apatite prisms. The impenetrable nature of the groundmass is given it by the felted dust-like granules of aegirine-augite. Typically, no cossyrite is present; but in a few sections, where the density of the groundmass is not so marked as in typical sections, a few opaque-brown dendritic growths may be of this mineral. A few minute feldspar needles are scattered throughout. Staining detects nepheline in the mesh of aegirine-augite dust in minute rare hexagonal and square forms. Magnetite is very scarce, unless it occurs with aegirine-augite as a resorption-product of amphibole. Chemical Characters, An analysis was made of this type, and comparison with the two other analyses appended shows how closely it agrees chemically with both the trachydolerites and the trachytoid phonolites. A. B. C. SiO2 54.24 56.19 55.10 Al2O3 18.08 20.25 19.25 Fe2O3 2.18 2.76 2.77 FeO 3.53 2.32 1.66 MgO 0.88 1.12 0.83 CaO 5.01 4.30 5.14 K2O 5.01 4.19 4.68 Na2O 7.29 6.33 7.41 H2O 1.79 0.65 2.19 Cl 0.63 MnO 0.32 TiO2 0.57 0.48 SO3 0.09 P2O5 0.54 0.41 98.64 99.47 100.46 A. Basic type of trachydolerite, Butter's Peaks, Mount Cargill. (Analysis, J. Bartrum.) B. Trachydolerite from Columbretes, Spain.* Rosenbusch, “Elemente der Gesteinslehre,” p. 355, No. 4, 1901 ed. C. Tracytoid phonolite.† Rosenbusch, loc. cit., p. 292, No. 4. (d.) Nephelinitoid Type of Trachydolerite. In hand-specimens this rock is indistinguishable from the preceding basic type. Within a few yards in the field this type merges, in successive variations, from the general trachydolerite to true nephelinitoid phonolite.

Thus field relations give no help in drawing distinctions between different petrological types, but indicate that the origin of all is differentiation of the one magma. There is, however, a possible exception to this, for an outcrop of nephelinitoid phonolite on Butter's Peaks may be a dyke. Under the microscope the chief feature of the type is the nephelinitoid, or cellular, structure of the groundmass, due to the numerous minute hexagons of nepheline seen, under moderate magnification. The phenocrysts are typically allotriomorphic; the most common are pinkish-brown augite, sodalite, sanidine, nepheline, and olivine. The nepheline is large and well-shaped, but is crowded with minute liquid inclusions. The crystals of sodalite are very large, and are usually crowded with minute gaseous inclusions; they show good dodecahedral cleavage. Sanidine is clear and glassy, but exhibits shadow extinction. A few characteristic anorthoclase phenocrysts occur. Olivine has either a dense corona of aegirine-augite with magnetite, or else a corrosion fringe of aegirine-augite and magnetite dust. Pleochroic aegirine-augite is shown in a few well-shaped crystals that have suffered slight resorption, and have been edged by the common pinkish augite. This latter variety sometimes encloses resorbed amphibole, showing that it did not separate out till after, or was connected with, the resorption of the amphibole. The groundmass is holocrystalline but fine-grained, and generally similar in minerals to that of the other types of trachydolerite. Aegirine-augite in mossy granules is dominant; if often assumes a lath shape, and then shows more or less parallel alignment. In sections of those rocks that, both petrographically and in field occurrence, approach the nephelinitoid phonolites cossyrite occurs plentifully, but is absent in other sections, unless some minute opaque dentritic growths can be referred to this mineral. Feldspar is moderately important, and enwrapping laths show up amongst the numerous minute hexagonal forms of nepheline. Very little magnetite is present; there are a few scattered flakes of sodalite. No analysis of this rock was made. B. Nephelinitoid Phonolite. As would be expected, in certain places this rock merges closely into the type of trachydolerite just described. In several sections segregations or inclusions of the basic trachydolerite previously described are very typical. They average about 7 mm. in diameter, and are most probably of the nature of segregations. Leucite again appears as a subsidiary mineral. It is difficult to distinguish it from numerous other rounded isotropic forms that are judged from their ready gelatinization with dilute acid, and from the high percentage of chlorine in the rock, to be sodalite. The leucite is in small rounded or idiomorphic shapes, and commonly shows characteristic radial arrangement of included aegirine-augite granules. In hand-specimen this rock is very similar to the dense basic variety of trachydolerite, but has a somewhat lighter colour and greasier appearance. It weathers very readily. Under the microscope true phenocrysts are rarely seen, unless in the proximity of the basic inclusions, where pink augite and olivine especially are common.

The phenocrysts are of sanidine, of brownish-pink augite, of almost completely resorbed amphibole, of sodalite, and occasionally of nepheline. Sanidine is the most common; it is usually markedly corroded, but occasional good idiomorphs show up. Carlsbad twinning is common. When nepheline occurs it is in very large crystals; sodalite is in numerous rounded and flaky forms. Pinkish-brown augite not infrequently forms an outgrowth to resorbed hornblende. One or two deep-green pleochroic and idiomorphic aegirine-augite phenocrysts are present. Under moderate magnification the groundmass exhibits a prominent nephelinitoid, or cellular, structure. The nepheline of these clear cellular portions is in minute hexagonal cross-sections. Highly pleochroic aegirine-augite and cossyrite aggregates are scattered regularly and fairly plentifully in the nepheline base. All branching portions of these aggregates are in crystalline continuity, and extinguish together. The pleochroism of the cossyrite is from bright reddish-brown to brownish-black, and of the aegirine-augite from deep grass-green to greenish-yellow. The identification of the cossyrite is based on its description in this and similar rocks of the district by Professor Marshall. No forms approaching idiomorphism were found on which to apply optical tests. In portions only of certain sections feldspar shows up well in minute needles that have rough parallelism, but elsewhere it is relatively scarce. There are a few scattered granules of magnetite. From the east end of Butter's Peaks one section made was found to differ from the others, and to present an undoubted nephelinitoid phonolite. It probably represents an unimportant local variation of the general basic trachydolerite. Cossyrite is very scarce in this section; it is in minute dense growths. The aegirine also is very dense, and is of much less importance than in the typical nephelinitoid phonolite. Nepheline forms almost the whole of the predominant clear base of the groundmass. Chemical Characters. The analysis made of this Mount Cargill nephelinitoid phonolite shows a close agreement with that of the nephelinitoid phonolite represented by analysis B. A. B. SiO2 54.88 55.01 Al2O3 22.80 21.67 Fe2O3 3.66 1.95 FeO 3.6 1.86 MgO 0.38 0.13 CaO 2.24 2.12 K2O 3.65 3.54 Na2O 7.53 9.78 H2O 0.91 2.17 Cl 0.63 0.08 MnO2 0.22 P2O5 0.4 99.94 99.41 A. Nephelinitoid phonolite from Butter's Peaks, Mount Cargill. (Analysis, J. Bartrum.) B. Nephelinitoid phonolite from Hohentwiel, Hegau.* Rosenbusch, “Elemente der Gesteinslehre,” No. 6, p. 292, 1900 ed.

A nephelinitoid phonolite that has probably intruded earlier basanites is found in a small quarry alongside a branch track that leaves the North-east Valley to Junction School Road, and follows up the North-east Valley Stream. The phenocrysts, which are almost entirely sanidine in Carlsbad twins and a little bright emerald-green to yellowish-green aegirine-augite, are sharply idiomorphic. The groundmass is chiefly nepheline in small hexagonal formsA. Deep-green mossy aegirine-augite aggregates and flakes of sodalite are also very plentiful. This rock is similar to, and possibly the same as, the nephelinitoid phonolite that is quarried lower down-stream in the North-east Valley quarry. C. Trachytoid Phonolites. No hard-and-fast line can be drawn between the nephelinitoid and some of the trachytoid phonolites of Mount Cargill. These latter phonolites present in the Mount Cargill area fall under two types, named respectively “Logan's Point” and “Signal Hill” by Professor Marshall in his paper “Geology of Dunedin,” referred to previously. The more important on Mount Cargill is the Logan's Point type, which forms Mount Zion and other knolls, and through which the Mount Holmes basalt and the trachydolerites have probably been forced. The Logan's Point is probably earlier than the Signal Hill type of trachytoid phonolite. Cotton, in a paper, “Geology of Signal Hill, Dunedin,”* Trans. N.Z. Inst., vol. 41, 1909, p. 111. brings forward evidence that supports this view. The apparent succession of types in the Mount Cargill area will be dealt with later. (a.) Logan's Point Type of Trachytoid Phonolite. In the hand-specimen this is a dull leek-green fine-grained rock, showing a few sanidine phenocrysts. Its field outcrop shows a platy structure. Under the microscope practically no phenocrysts appear beyond a few poorly shaped corroded ones of sanidine, and a few of pleochroic aegirine-augite and resorbed amphibole. Cossyrite and aegirine-augite, both in the allotriomorphic mossy growths common in the allied Mount Cargill rocks, are evenly and plentifully distributed in the groundmass in a clear base of nepheline and feldspar. The pleochroism of both minerals is the usual pleochroism noted already. The feldspar of the groundmass is typically allotriomorphic, and, as well as enwrapping the aegirine-augite and cossyrite, encloses in its most intimate meshes minute nepheline crystals that often are only distinguished by staining-tests. In other sections an abundance of nepheline causes a cellular structure of the groundmass. Occasionally the feldspar of the base shows good fluxional arrangement; the laths then prominent are polysynthetically twinned, and are referred, on their extinction-angles, to oligoclase. When the greater part of the base shows this structure the rock merges into the Signal Hill type. Similarly, where the base is highly nephelinitoid the rock verges on the nephelinitoid phonolites. This is particularly the case in sections from a small conical knob near Butter's Peaks. In the Logan's Point rock magnetite is absent; a few prisms of apatite are included by aegirine-augite. In a section made from an outcrop of this type near Butter's Peaks large phenocrysts of olivine and pinkish augite were found. Partial resorption

has effected a rounding of these crystals, and an edging of aegirine-augite. Unless these minerals have been caught up from elsewhere, their occurrence shows that a close relationship to the trachydolerites exists. Comparison with other Logan's Point trachytoid phonolites shows that the rock from Logan's Point itself is much denser and has less mossy cossy-rite and less idiomorphism of the feldspar phenocrysts. There are no feldspar phenocrysts, but abundant intensely green almost unpleochroic pyroxenes, in a trachytoid phonolite from the foot of the North-east Valley to Junction School Road. In the same type of rock from the North Head there are conspicuous phenocrysts of sanidine, but none of aegirine-augite. The nepheline of the groundmass can only be detected by staining. A knob on Signal Hill, above Burke's, is composed of a rock almost identical with that of Mount Cargill; few or no feldspars show up in the first generation. Chemical Characters. An analysis of the Logan's Point trachytoid phonolite is compared below with analyses of similar rocks. Reference to the analyses of nephelinitoid phonolites on page 171 will show how closely the Mount Cargill Logan's Point phonolite resembles in chemical composition the nephelinitoid types. A. B. C. SiO2 56.12 56.8 57.00 Al2O3 21.32 19.7 18.56 Fe2O3 2.59 2.2 4.58 FeO 3.29 3.7 2.76 MgO 0.56 0.4 0.41 CaO 2.30 2.2 1.05 K2O 4.81 7.1 6.13 Na2O 5.79 4.3 6.34 H2O 1.54 2.5 2.96 Cl 0.34 98.66 98.9 99.79 A. Logan's Point trachytoid phonolite, Mount Zion, Mount Cargill, Dunedin. (Analysis by J. Bartrum.) B. Trachytoid phonolite from East Lothian, Scotland.* Rosenbusch, “Elemente der Gesteinslehre,” p. 292, 1901 ed. C. Trachytoid phonolite from Logan's Point.† Marshall, “Geology of Dunedin,” Quart. Journ. Geol. Soc., vol. 62. Aug., 1906, p. 402. (b.) Signal Hill Trachytoid Phonolite. This rock does not occupy any important area on Mount Cargill itself, but is extensive across the North-east Valley, on Signal Hill, and also covers a large portion of Pine Hill. Occasional sections cut from the Mount Zion phonolite show examples of this type, but these seem to be far from typical. Basalts apparently underlie this rock towards the headwaters of the North-east Valley Stream. This agrees with the succession described by Professor Marshall at the North Otago Head.‡ “Geology of Dunedin,” Quart. Journ. Geol. Soc., vol. 61, 1906, p. 418.

Macroscopically the Signal Hill phonolite is a greasy green fine-grained rock of platy nature, showing occasional feldspar crystals. Under the microscope there is a noticeable scarcity of phenocrysts as compared with the typical rock from Signal Hill. In the rock outcropping in the North-east Valley Stream, however, there are plentiful conspicuous amphiboles up to 1¼ in. by ¼ in. in size. Resorption of this amphibole is noticeable, and its pleochroism is marked—a, pale golden-yellow; b, rich brown; c, dark opaque-brown. Though the tests made were scarcely satisfactory, apparently c ∧ ć = 6°, A = à, B = b, and mineral is positive. This amphibole is probably barkevicite, and it is considered that it is the same as that in the various other allied rocks of Mount Cargill. Sanidine, in much corroded crystals of small size, is persistent, but never plentiful. There are occasional corroded crystals of anorthoclase, of oligoclase, and of a more basic feldspar that is apparently andesine. The only other phenocrysts are small scarce ragged crystals of greenish and pink augite. The groundmass is the most characteristic feature of the type. A dense web of small feldspar laths, showing remarkable flow structure, constitutes the greatest part of the groundmass, and entangles fairly plentiful augite granules, very minute nepheline prisms, and a little scattered magnetite. Most of the augite is the greenish soda-bearing variety, but in many sections pink augite also is common. The chief feldspar of the groundmass is sanidine. Cossyrite is absent. The typical rock from Signal Hill shows in comparison with the above an abundance of resorbed amphibole and of coarse feldspars, amongst which oligoclase and anorthoclase are prominent. A little serpentinized olivine also is present. Chemical Characters. No chemical analysis of this rock was made. D. Basalts. Of three main basaltic areas to be described, the most important is the old neck of Mount Holmes. There a good example of columnar jointing is shown. The disposition of the columns is irregular, but indicates that the vent from which the basalt flowed was of the nature of a fissure. This Mount Holmes basalt has apparently burst its way through the Logan's Point phonolite outcropping on Mount Zion. A basalt covers a considerable area near the Junction School, and is the same as that described from there by C. A. Cotton.* “Geology of Signal Hill,” Trans. N.Z. Inst., vol. 41, 1909, p. 121. Mineralogically it agrees with the Mount Holmes basalt, but, as one would expect, textural differences are marked. It is very probable that this lava flowed from the former vent of Mount Holmes. A basaltic-scoria bed of an amygdaloidal nature is found on a branch road leading north-west from the Junction School, and apparently is part or the surrounding Junction basalt. On a knoll close to the west side of the Main North Road, about half a mile north-west of its junction with the Port Chalmers Road, is a different type of basaltic rock. It is similar in general characters to basanites found to the north-east of the district. Professor Park, in his paper on the geology of North Head, Waikouaiti,† Trans. N.Z. Inst., vol. 36, 1903, pp. 423, 424. mentions Mount Cargill as the

probable point of origin of basanitic pebbles found in gravels at Mount Cronin. It is uncertain whether this supposed basanite is that recorded by Professor Ulrich from the Mount Cargill area. A similar rock is found in the valley of the North-east Valley Stream above where it strikes in a northerly direction away from its previous course alongside the North-east Valley Road. After some difficulty, staining-tests made on these rocks showed a few small crystals that may be nepheline. As, however, some undoubted olivine had gelatinized and absorbed the stain, there is doubt as to this identification. For this reason these rocks are only tentatively classed as basanites. (a.) Mount Holmes Basalt. This is macroscopically an open-grained basaltic-looking greyish-black rock, showing plentiful small crystals of olivine and augite, and weathering out to a greyish-fawn colour. Microscopically it is holocrystalline, and of porphyritic, hypidiomorphic structure. The chief phenocrysts are faint greenish-pink augite and olivine; they are of large size. The olivine is very fresh and sharply edged. Augite is frequently of a dirty-green colour; it occasionally enwraps the olivine. The augite also commonly includes magnetite, and in a few instances a little feldspar. Feldspar and augite are both porphyritic. The feldspar typically is much corroded, and has numerous inclusions of groundmass. Twinning by the Carlsbad and albite laws is prevalent. The varieties vary from andesine-labradorite to labradorite. The open fine-grained base which encloses the phenocrysts consists of a plexus of well-shaped feldspar laths which enwrap plentiful microlitic almost colourless augite granules, fairly abundant magnetite, a little coarser olivine, and a little ilmenite. Crystallites of indistinct nature occupy the finest interspaces between the augite granules. Chemical Characters. An analysis of this rock shows that it is a fairly typical basalt. The percentage of ferrous iron is particularly high, and is probably due largely to the greenish augite, as well as to the magnetite and ilmenite. An analysis of a basalt, quoted from Rosenbusch's “Elemente der Gesteinslehre,” is also appended. A. B. C. SiO2 4.5.89 4.2.75 Al2O3 17.17 17.24 Fe2O3 2.60 8.01 FeO 11.77 5.88 CaO 10.05 11.14 K2O 1.54 2.48 Na2O 3.60 4.21 H2O 1.20 1.06 TiO2 2.13 99.63 100.46 A. Basalt, Mount Holmes, Mount Cargill, Dunedin. (Analysis, J. Bartrum.) B. Basalt.* Rosenbusch, “Elemente der Gesteinslehre,” p. 323, No. 15, 1901 ed.

(b.) Junction Basalt. The occurrence of this rock over an area around the Junction School has already been noted. In hand-specimen the Junction basalt is similar to the Mount Holmes rock, but breaks with a much less regular fracture than the latter. Under the microscope the chief difference is seen to be in the structure. The groundmass is dense and microcrystalline; it consists of predominant magnetite in small squares, plentiful colourless augite granules, and interstitial microlitic grains of feldspar. Large laths of feldspar showing albite twinning are fairly plentiful, and, with olivine and augite, comprise the phenocrysts. The variety of feldspar is chiefly labradorite. Olivine and augite are in large very plentiful crystals. Celyphitic arrangement of the augite about the olivine is not infrequent. The augite is a pink variety, and the olivine often shows alteration to serpentine and to carbonates. A few large crystals of ilmenite and magnetite are present. A curious feature of the rock is the occurrence of occasional large crystals of nepheline that have suffered considerable resorption; there is a wide fringing zone of small feldspars and a central remnant of the nepheline. The nepheline has probably been caught up from contiguous rocks. It is comparatively plentiful in a rock found on the hillside northwest of the North-east Valley tannery—a basalt characterized by abundant feldspar and sharply idiomorphic violet augite phenocrysts, and by a very dense groundmass—and has there the same peculiarities. The order of crystallization in the Junction basalt seems to be: Phenocrysts (in order), iron-ore, olivine, augite, feldspar, and then (in order) the magnetite, augite, and feldspar of the groundmass. Chemical Characters. An analysis of the Junction basalt is appended, with, for purposes of comparison, analyses of two other basalts. The analysis shows high percentages of silica, magnesia, and ferrous iron, which correspond well with the abundance of augite in the rock. A. B. C. SiO2 4.5.80 4.7.68 4.8.97 Al2O3 17.91 17.90 16.37 Fe2O3 6.14 4.48 1.33 FeO 8.69 9.05 8.56 MgO 3.92 8.71 6.22 CaO 8.10 5.65 7.49 K2O 1.77 2.68 1.72 Na2O 4.71 2.35 4.09 H2O 2.10 1.16 0.38 Cl 0.11 0.20 TiO2 0.35 3.95 P2O5 1.04 NiO 0.08 BaO 0.06 99.50 99.86 100.26 A. Basalt No. 1.* Cotton, “Geology of Signal Hill,” Trans. N.Z. Inst., vol. 41, 1909, p. 122. B. Junction basalt, Mount Cargill. (Analysis, J. Bartrum.) C. Scoriaceous basaltic lava from recent eruptions at Pantellaria.† H. S. Washington, “Titaniferous Basalts of the Western Mediterranean,” Quart. Journ. Geol. Soc., vol. 63, Feb., 1907, p. 75.

Basanites. Mention has been made already of the so-called basanites. In the hand-specimen they are fine-grained dense blackish rocks, showing few conspicuous phenocrysts. Besides the occurrences already noted, a basanite is found as extensive rubble near the hypabyssal trachydolerite on Pine Hill. Under the microscope these rocks show a typically dense base and a paucity of phenocrysts; augite is the commonest of the few phenocrysts that are seen. It is most often of a pale-pinkish colour, but a green augite with pink border and a purplish-violet variety are also present. In the basanite found north of the Junction School a strongly pleochroic mica is prominent. It occurs, along with a little serpentine, as an alteration-product of the olivine; its pleochroism varies from deep brown to bright golden-brown. It is thought to be anomite. Feldspar seldom is a phenocryst in the Mount Cargill basanites; a few very large feldspars show albite twinning, and seem to be oligoclase. They are, however, so crowded by augite granules and other inclusions that an exact determination cannot be made. The groundmass is very dense, and is composed mainly of small grains and squares of magnetite. In the anomite-bearing rock the magnetite is less important, and an interstitial feldspar is the chief constituent, along with grains of colourless augite. The augite is usually in fair amount in these rocks, but feldspar typically occurs only in a comparatively few needle laths. No chemical analysis of any of the basanites was made. Relationships of the Trachydolerite and Phonolite Series. The analyses given in the subjoined table show how gradual a passage there is chemically from the more basic trachydolerites to the phonolites. Petrological characters also indicate that such a gradation is not a matter of chance, but represents a differentiation of many types from the one magma. In certain cases this is due to differences in the rate of cooling. All evidence from the Mount Cargill area would show that the Logan's Point trachytoid phonolite is a portion of the main alkaline magma, and, in fact, a modification of the trachydolerites and the nephelinitoid phonolites. Evidence from other parts of the district dispels any idea of its contemporaneity with these latter rocks. A. B. C. D. E. SiO2 50.43 4.9.02 54.24 54.88 56.12 Al2O3 18.00 19.50 18.08 22.80 21.32 Fe2O3 3.78 4.37 2.18 3.66 2.59 FeO 5.65 6.60 3.53 3.26 3.29 MgO 2.91 2.14 0.88 0.38 0.56 CaO 5.76 6.76 5.01 2.24 2.30 K2O 4.79 1.70 5.09 3.65 4.81 Na2O 5.76 7.35 7.29 7.53 5.79 H2O 1.37 1.18 1.79 0.91 1.54 Cl 0.38 0.63 0.63 0.34 98.83 98.64 98.72 99.94 98.66 A. Trachydolerite, Main Peak, Mount Cargill. B. Trachydolerite, near Pine Hill, Mount Cargill. C. Basic type of trachydolerite. Butter's Peaks, Mount Cargill. D. Nephelinitoid phonolite, Butter's Peaks, Mount Cargill. E. Logan's Point trachytoid phonolite, Mount Zion, Mount Cargill.

Source and Sequence of the Mount Cargill Rocks. In his paper on the “Geology of Signal Hill,”* Trans. N.Z. Inst., vol. 41, 1969, p. 113. Cotton deduces that the Logan's Point phonolite is earlier than the Signal Hill type. His statement is based on evidence brought forward by Professor Marshall showing the relative sequence of the two rocks at the North Head. It is probable that the Mount Cargill and Signal Hill occurrences of the two phonolites are portions of the same flows, and, if this is the case, field evidence at Mount Cargill makes it certain that the flow of the later of the two trachytoid phonolites must have been south-west, down a steep slope of the earlier. Basaltic rocks in the North-east Valley Stream, near its headwaters, may represent basic outpourings intermediate between the two phonolites—a supposition in accordance with the sequence noted by Professor Marshall at the North Head.† “Geology of Dunedin,” Quart. Journ. Geol. Soc., vol. 62, 1906, p. 418. The trachydolerite seems to overlie a surface of Signal Hill phonolite that slopes gently south-west. This, together with the fact that the Mount Holmes basalt is the north-east boundary of the trachydolerite, strongly supports the contention that the flow of this latter rock was in a south-west direction from Mount Cargill. A series of rough joints, very steeply inclined, and running approximately east and west along the strike of the prominent rock ridge of the Main Peak and of Butter's Peaks, together with petrological evidence showing differences in the rates of cooling, tends to indicate that the eruption of trachydolerite was from a fissure occupying the site of the present ridge. The North-east Valley seems to have begun its existence after the extrusion of the trachydolerite and before the ejection of the Mount Holmes basalt, for basalt remnants are found on both Mount Cargill and Signal Hill slopes, and a simple explanation of this is that the basalt from Mount Holmes flowed down the already formed valley. The origin of the basanites is uncertain; possibly many of them are more of the nature of intrusions than flows, but it is probable that they are in some way connected with the basanitic outpourings that were frequent in the district north-east of the Mount Cargill area. The nephelinitoid phonolite of Butter's Peaks may be a dyke. The other types outcropping near it are simply modifications of the main trachydolerite flow. All probably originate from the one magma. Quantative Classification. The quantative classification of this series of Mount Cargill rocks has been worked out by the method of Cross, Iddings, Pirsson, and Washington.‡ “Qualitative Classification of Igneous Rocks.” The following is the result:— 1. General lava trachydolerite— Class II. Dosalane. Order 6. Norgare. Rang 3. Salemase. Subrang 4 Salemose.

2. Pine Hill lava trachydolerite— Class II. Dosalane. Order 4. Austrare. Rang 5. Andase. Subrang 4. Andose. Dense basic type of trachydolerite— Class II. Dosalane. Order 6. Norgare. Rang 4. (Not named.) Subrang 4. (Not named.) 4. Nephelinitoid phonolite— Class II. Dosalane. Order 5. Germanare. Rang 2. Monzonase. Subrang 4. Akerose. 5. Logan's Point trachytoid phonolite— Class I. Persalane. Order 5. Canadare. Rang 2. Pulaskase. Subrang 4. Laurvikose. 6. Mount Holmes basalt— Class III. Salfemane. Order 6. Portugare. Rang 4. (Not named.) Subrang 3. (Not named.) 7. Junction basalt— Class.—Between II (Dosalane) and III (Salemane). Order 5. Germanare. (Gallare.) Rang 3. Andase. (Camptonase.) Subrang 3. Shoshonose. (Kentallenose.)