Rhyolite.—A hard, compact rock, usually jointed into massive rectangular blocks. Through the action of weathering agents the rock generally presents a surface which is stained to a light-brown colour. When fresh it is either white or may vary in colour from green to grey or black. Megascopic crystals of quartz and garnet (almandine) are locally abundant.
Microscopic examination of specimens of this rock taken from various parts of the gorge showed that, while the texture of the groundmass varied considerably, both the mineral content and the structure of the rock was very uniform throughout, the chief mineralogical difference between the various specimens being the presence or absence of garnet. Moreover, apart from that shown in a single flow as described below, there seems to be no progressive change throughout the mass of the rock either in degree of crystallinity of the groundmass or in abundance of garnet. Almost holocrystalline flows are irregularly interstratified with flows whose groundmass may be almost completely glassy. Types rich in garnet also appear apparently irregularly arranged amongst types poor or lacking in these phenocrysts.
The minerals present as phenocrysts are quartz, orthoclase, plagioclase (andesine to oligoclase with occasional albite), garnet, and biotite. Minerals of the groundmass are quartz, feldspar, apatite, magnetite, and rarely zircon. The groundmass varies in texture in different slides, and in different parts of the same slide, from a dark-brown glass to a microfelsitic (Iddings, 1909) matrix. In a few slides indistinct micrographic intergrowths of quartz and feldspar are apparent in the more crystalline parts of the groundmass, together with patches showing microspherulitic structure. In general, however, spherulitic intergrowths are absent. Flow-structure is nearly always present to some extent, and may be very prominent. A feature of nearly all the rhyolites examined was the development of “flow-breccia” structure (Iddings, 1909, p. 331).
Pitchstone.—A brittle, easily-weathered rock, showing either (as in the outcrop at the upper end of the gorge) very perfect rectangular jointing, or (as at the lower end of the gorge) massive outcrops with little jointing. The rock is pitch-black in colour, with the characteristic vitreous lustre and conchoidal fracture of volcanic glasses. Phenocrysts of clear or slightly discoloured quartz are visible, and garnets are abundant in all of the outcrops in this district.
Under the microscope the pitchstones are markedly uniform in character and closely resemble the more glassy types of rhyolite; the occasional presence of small phenocrysts of hypersthene is the only difference in mineral content, and, except that no microspherulitic intergrowths were observed in the pitchstones, their micro-structure is similar. Perlitic structure, especially in the rock of the outcrops at the lower end of the gorge, is perhaps more marked than in the glassy rhyolites.
Andesite.—The rock of which the flows are composed is a hard dark-grey or black rock, which, however, is very susceptible to the action of weathering agents. Fresh specimens of the rock are difficult to obtain except where erosion has been so rapid that the products of decomposition have not been able to remain in place. In the field the rock generally appears as a reddish or greenish mass, presenting a somewhat incoherent and crumbling surface. The only crystals visible megascopically are of feldspar, and these occur as numerous, evenly-distributed, rectangular prisms. The presence of these phenocrysts is very characteristic of the rock, and they remain visible to the naked eye or under a pocket-lens even in most weathered specimens. A very general feature of this rock is the abundance of secondary silica present as quartz of different varieties—chalcedony, amethyst, agate, and opal. These minerals, together with chloritic decomposition-products derived from the ferromagnesian minerals of the rock, are found filling veins, druses, steam-vesicles, and other cavities throughout the whole mass. In places the rock becomes scoriaceous, and the vesicles are filled with bright-green amygdaloids composed chiefly of chlorite. The presence of these minerals has led to the popular supposition that copper-ores and gold may be found in these rocks, but prospecting has failed to detect any minerals of economic value in payable quantities. The amethysts, which have attracted some attention, are of a pale colour, usually occurring in druses as clusters of small hexagonal prisms terminated by pyramids. Specimens have been collected from the face opposite the Mount Hutt homestead up to 4 in. in length, but these are exceptionally large for this locality
Calcite is also common as a vein-mineral in the andesites. On the southern slopes of Round Top there are veins varying in size up to 2 ft. which have been partially filled with calcite, quartz being subsequently deposited as a coating on the calcite, and giving rise to negative pseudomorphs.
The breccias associated with the andesite-flows are typical volcanic breccias consisting of angular fragments of the andesite rock varying in diameter up to 2 in. or 3 in. with occasional larger blocks cemented together in a matrix of finer material. There is no apparent regularity in arrangement of the flows and fragmental deposits, but the former are in far greater abundance. The breccias occur merely as occasional strata, never more than a few feet in thickness, interbedded between the flows. The secondary minerals associated with the andesite-flows are also abundant in the breccias.
The microscopic character of the andesites is very uniform in all specimens collected from this district. The chief variations observed were in the proportion of ferromagnesian minerals present and in the texture of the groundmass. Plagioclase (acid-labradorite to andesine), hypersthene, and augite occur as phenocrysts in a groundmass which is typically composed of minute feldspar laths, some pyroxene in granular masses, magnetite, and a brown glass. The texture of this matrix varies in different slides: in some there is little glass and the feldspar laths attain a larger size, when they may be recognized as andesine or labradorite; in other slides the glassy material predominates, and the feldspars appear as scattered microlites. The amount of pyroxene present in the groundmass also varies, being in some slides apparently absent. In general the structure is hyalopilitic as defined by Rosenbusch, the “felted” character being prominent in the more crystalline
varieties. Throughout the rock there is much secondary quartz and opal, filling small veins and cavities. Small round vesicles frequently show a border of radiating, fibrous, chalcedonic quartz, with the centre filled in with opal.
The rock forming a dyke in the greywacke at the upper end of the gorge is much weathered, and presents a brown sandstone-like appearance. Plagioclase (acid-labradorite), augite, ilmenite, apatite, and a large amount of secondary quartz and calcite are recognizable under the microscope. The texture is of a fine, even grain, the bulk of the rock being composed of feldspar laths with occasional patches of glassy residuum. It would seem more probable that the intrusion of this dyke was associated with the Cretaceous volcanic activity than with the Tertiary basic intrusions, solely on account of the andesitic character of the rock.
The following are the results of analyses made in the Dominion Laboratory of specimens of each of the effusive rocks of this group, with their classification according to the C.I.P.W. system :—
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|Water lost above 105°||C.||0.66||4.98||1.23|
|Water lost below 105°||C.||0.58||2.16||2.60|
Rhyolite: the island, lower end of Rakaia Gorge.
Pitchstone: lower end of Rakaia Gorge, north side.
Andesite: lower end of Rakaia Gorge, south side.
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In (1) and (2) the analyst reports that the carbon dioxide is not present as calcite, as it is liberated only on heating to about 100° C with dilute hydrochloric acid.
I 3(4) 1″ 2″ (Magdeburgose).
I 3(4) (1)2 4 (Alsbachose).
I (II) 4 3 (3)4 (Yellowstonose).
The chief points of interest regarding the composition of these rocks arise out of a comparison of the rhyolite and pitchstone. As is suggested below, the pitchstone may represent merely certain of the acidic flows of these eruptions, which, either owing to rapid cooling or other causes by which free molecular movement was hindered, have consolidated in a more vitreous form. If this is the case the rocks should show similar chemical compositions. The difference in relative proportion of the alkalis is, however, marked: the potassic nature of the rhyolite shows itself mineralogically in the abundance of orthoclase relative to the soda-bearing feldspars, while in the pitchstone the reverse occurs. The chemical similarity of the two rocks, however, seems sufficient to admit of their being successive flows of the same eruption, a conclusion to which the field evidence points.
Comparison of these analyses with those given by Speight (1922, p. 79) of Banks Peninsula rhyolites and pitchstones shows a general similarity.
Owing to the abundance of secondary silica contained in the andesite, the C.I.P.W. quantitative classification, which the authors definitely limit to fresh specimens, does not illustrate the character of the rock. Classified as “yellowstonose,” it is grouped with dacites and rocks corresponding in composition with quartz-diorites, whereas its microscopic character clearly shows that it belongs to a more basic group.
The Origin of the Pitchstones.—Haast (1871 and 1879) considered the pitchstone to be merely a facies of the rhyolite—the first flows to be erupted on to a cold land-surface. Cox (1884, p. 40), however, considered the pitchstone to be dykes belonging to the same system as the dolerite and basalt intrusions found penetrating the coal-measures and older rocks. The following evidence seems to confirm Haast's view that in this and neighbouring districts they are merely glassy facies of the rhyolite :—
1. No instances have been reported of the pitchstones associated with the volcanic rocks here considered penetrating any rocks other than the rhyolites.
2. In every outcrop observed the directions of the pitchstone masses conform to those of the apparent flows of the rhyolite. The presence of jointing frequently makes it difficult to determine the orientation of the flows of the rhyolite, but where determination is possible the pitchstone is found to be in parallel arrangement. This is especially true of the occurrence at the lower end of the Rakaia Gorge, where the direction the rhyolite-flows is clear.
3.There are certain undoubted rhyolite-flows which may be considered as intermediate in character between the normal pitchstone and rhyolite types. They are black in colour, show a somewhat vitreous lustre, and microscopically appear as very glassy rhyolites. Other flows which are chiefly composed of normal rhyolite at the margin grade into a pitchstone type as shown above.
While it thus seems more likely that the pitchstones do not form dykes, yet Haast's inference that they represent those flows resting directly upon the older rocks, and therefore cooled more rapidly than the succeeding flows, requires some modification. The presence of normal rhyolite between the pitchstone and the older (Maitai) rocks shows that, whatever may have been the cause of the glassy facies, it was not due to conditions attendant only upon the first flow to be put out on to the land-surface. Furthermore, the pitchstones occurring near, but here also not next to, the junction with the andesites, may only be due to this rapid cooling of the first flow if the andesites represent the older of the two volcanic series. As shown below,
however, the weight of evidence seems to point to the andesite being of younger origin than the rhyolite. Thus while it is possible that the first flows of the rhyolitic series in places may have assumed a glassy character, yet the pitchstone facies is by no means confined to that horizon.
In the Rakaia Gorge it must be considered as accidental that each of the pitchstone outcrops occurs close to, though, as careful examination shows, not in actual contact with, the junction of the rhyolites with other rocks.
Order of Eruption of the Rhyolites and Andesites.—Haast, in treating of this and the adjacent Malvern Hills and Mount Somers districts, always considered the andesitic eruptions to have preceded the rhyolitic. Cox, who was the last to discuss at all fully the mutual relations of these two series of rocks, agrees with Haast that the andesites were the first erupted rocks, but in view of inconsistencies arising from an adoption of this theory in the Malvern Hills and Rakaia Gorge he postulates a special type of eruption for the rhyolites. These rocks he considered to have been erupted in a very viscous condition, the accumulation taking place by a process of endogenous growth, as described by Judd (1881, p. 134). Of recent years, however, there has been some doubt cast on this hypothesis, and a close examination of the Rakaia Gorge district, supplemented by more rapid observations in the Malvern Hills and Mount Somers districts, would seem to show that there is a balance of evidence in favour of the rhyolite being the first erupted rock.
Haast does not state definite reasons for considering the andesites to be older, but Cox bases his endogenous-growth theory upon the following observations :—
Micro-structure of the rhyolite indicates a viscous lava.
On the flanks of Mount Somers the andesites dip inwards towards the main rhyolite mass.
“The fact … that the melaphyres are lying on the liparites [in the Rakaia Gorge] at the angle of dip which they assume is in itself a proof of greater age, when we consider that beds of tufa are interstratified with the solid floes.” (Cox, 1884, p. 39.)
Of these lines of evidence, (1) is of merely accessory value; (2) and (3) may both be criticized in the light of later advances in the science of New Zealand geology. At the time of Cox's report the conception of a Pliocene period of crustal movements had not been developed. The work of McKay, Cotton, and others, however, has since shown that the present topography of New Zealand is independent of that of pre-Notocene times. Thus the fact that the andesites flanking Mount Somers dip towards the centre of that mass must be considered as possibly due to the presence of faults. The third argument is also open to criticism. The coal-measures overlie the andesites in this place with a dip of not less than 35°; so that, taking Cox's angle of dip of the andesites (45°), and allowing that no movements have disturbed the igneous rocks relative to the coal-measures, the angle of slope at which the tuffs and breccias were laid down could not have been greater than 10°. This occurrence therefore seems insufficient as “a proof of greater age” of the andesites.
A further point which doubtless influenced both Haast and Cox in considering the relative age of these rocks is that they supposed that there were beds of rhyolitic tuffs interstratified with the coal-measures at Mount Somers and the Rakaia Gorge. As shown below, however, the coal-measures in the Rakaia Gorge are probably entirely of cataclastic origin.
With regard to the possible objection that if the andesites overlie the rhyolites the solutions which deposited the silica, calcium carbonate, &c., in them would be expected to have penetrated the underlying rhyolites, two suggestions may be made. Such solutions were probably closely connected with the magma itself, and deposition of the secondary minerals would follow closely upon the consolidation of the rock, and be to a large extent confined to it. Secondly, the rhyolite, being of a compact nature, would not easily be penetrated by these solutions. The fact that small amethysts, with some veins of chalcedony, occur in the rhyolite on the right bank at the lower end of the gorge shows that these solutions did to some extent affect this rock.
The evidence in favour of the rhyolites having preceded the andesites is—
The andesites in each outcrop of the Rakaia Gorge occur between the outcrops of rhyolite and coal-measures, and in Round Top rest upon the rhyolite. The possibility of faulting having caused an inversion of the older rocks over the younger is unlikely.
The nature of the contact of the andesites with the rhyolites at the lower end of the gorge. The rhyolite which lies between the pitchstone and the andesite is much weathered and decomposed to an easily eroded white mass.
Where the rhyolite rests upon the older Maitai rocks there is little evidence of alteration of the erupted rock. This seems to indicate that the weathered rhyolite represents an old land surface upon which the andesites were erupted.
The supposition that either the andesites or the rhyolites of the Malvern Hills and Rakaia Gorge were not contemporaneous with those of the Mount Somers district would clear away the chief difficulties involved in this problem. The rhyolites of both districts, however, in megascopic and microscopic character are almost identical, and both have similarly associated pitchstones. The andesites of both districts are also of a closely similar nature, and since both series are definitely of Cretaceous age it must be considered very improbable that the eruptions of each type were not contemporaneous in both districts.