Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
Quartziferous Porphyry and pitchstone Series
Quartziferous Porphyry and pitchstone Series.
The basic rocks just described are, as previously stated, covered in many localities by a series of quartziferous porphyries and pitchstones, which in Mount Somers reach an altitude of 5223 feet. An examination of the junction of these two series of eruptive rocks in several localities, showed that the older rocks had undergone considerable denudation, or, at least, that their surface had been altered considerably by aqueous agency before the next beds were deposited upon them. Thus, for instance, among the Clent Hills, in Junction Creek, which joins the Stour, the uppermost beds of the melaphyres consist of a verv decomposed amygdaloid, gradually changing into a green tufa, of the quartziferous porphyries; upon them repose pitchstones, from a greenish-white colour gradually assuming a black tint, and becoming very vitreous. The porphyries which lie upon them, or more correctly speaking, of which they form the lowest portion, have a hyaline base, and follow each other in many streams, but generally with a layer of pitchstone betweem them. The principal localities where these porphyries are found are in the Malvern Hills, Hinds, Ashburton, and Banks' Peninsula districts. Although of a great variety of texture and colour they resemble each other in their principal characteristics, a porphyritic structure with grains of quartz and small crystals of garnets (almandine) being predominant. I have already alluded to the fact that these porphyries lie directly in some localities upon the melaphyres, but in other places they repose upon the palaeozoic sedimentary rocks, the streams of which they are composed following the outlines of the former sea-bottom, and.being at the same time underlaid by a succession of truly sedimentary deposits of page 286great variety. What gives these lowest beds forming the base of the porphyries a still greater interest is the occurrence of plant beds, and seams of brown coal, mostly of inconsiderable thickness, associated with them. The locality where the whole series can best be studied, is situated in the Cox Hills at the south-eastern base of Mount Somers. Upon the palæozoic sedimentary rocks, consisting principally of a coarse sandstone decomposing to a loose ferruginous gravel, repose thick beds of green sands, gradually altering to yellowish and white quartzose sands; thickness about 500 to 600 feet. They are capped by ferruginous shales three feet thick, overlaid by four feet of snow white sands. Twenty inches of shales, full of impressions of leaves, follow, but unfortunately with scarcely any pieces large enough for recognition. Some of these leaves, however, I believed at the time to be of a truly dicotyledonous character. Although, after repeated examination of a number of specimens received since my first survey was made, I am now inclined to refer them to the conifeæ. One of the most perfect of these fossil leaves has great resemblance with the Dammara of the overlying Waipara formation. These shaly beds are again overlaid by a great thickness of sands, mostly of a pure white colour, and with veins and irregular portions of yellow or salmon-coloured hues. The latter are covered by shales, with a dip of 46° to W.S.W., followed by three feet of a partially altered brown coal showing sometimes still the woody structure; this seam is covered by 16 inches of gritty sandstone.
Then follow, in ascending order:—Twelve inches of yellowish or bluish tufaceous clays, six to eight feet of shaly beds, with indistinct remains of plants, three feet of bluish or greenish tufaceous clays one to two feet of porcelain jasper, evidently altered and burnt by the next beds, which are of an eruptive character. These porcelain jaspers contain some lapilli, or even smaller-blocks of pitchstone enclosed in them. They gradually alter into a whitish tufaceous mass, consisting of lapilli and ashes cemented together, without doubt the result of the destruction of pitchstones during an eruption. These beds are full of angular or sub-angular pieces of black pitchstone, evidently much decomposed, and from one inch to 20 feet in diameter; the whole having a thickness of at least 100 feet. They are capped by a stratum of pitchstone about 10 feet thick, upon which quartziferous porphyries follow, first with a hyaline, afterwards with a more felsitic matrix. This first great stream forms the lowest of a great succession of similar ones rising to the summit of Mount Somers, that page 287is to say, the whole has a thickness of more than 3000 feet. Towards the Ashburton, the succession of the streams is not so well shown; but on the northern flanks of Mount Somers, where that fine mountain falls in some localities abruptly for more than 1000 feet in almost perpendicular cliffs, their position, nature, and succession, are well exhibited. Some of the coulées, occasionally of great thickness, have a more trachytic structure towards the centre, whilst the smaller ones are generally more hyaline, or at least felsitic, usually with selvages of pitchstone on both planes. In fact they may fairly be compared to lava streams of the present day, showing the same process of cooling, although, owing to different circumstances and mineralogical constituents, the magma out of which they originated naturally assumed different forms.
Examining the mineralogical structure of the acidic rocks in the Malvern Hills, we find that one of their principal characteristics, both in the porphyries as in the pitchstones belonging to them, is the occurrence of crystals of garnets of the red variety called almandine, as well as grains of bluish translucent quartz, both of which are seldom absent, being enclosed in a felsitic matrix. These series are most largely developed in several well-defined peaks and ranges, such as High Peak, Phillip's Range, Rocky Range, Rocky Peak, Mount Misery, etc.; also the lower portion of the Rakaia Gorge gives a capital insight into their characteristic features. They overlie not only the palaeozoic rocks, through which they have broken, but also the melaphyres on the partly decomposed surface of which they appear in many localities. They have a longitudinal extension from west to east, dipping mostly towards the south, but have in some localities an anticlinal arrangement where they appear on both sides of the melaphyric centre. They rise to an altitude of 3019 feet in High Peak, the highest summit of the whole system, and the principal centre of eruption, but they have suffered considerable denudation in that portion of the country now occupied by the broad valley of the Rakaia. In their eastern portion in Mount Misery and Mount Pleasant, where they assume sometimes tabular forms, and are of great thickness, they overlie exclusively the palaeozoic sedimentary rocks, but this contact is generally hidden by younger beds. In the western ranges, however, the said contact is clearly exposed in many beautiful sections, showing that they repose either on the melaphyres, or often directly upon the sedimentary palaeozoic rocks. Following the contour of the former, they are often only 30 to 40 feet thick, as page 288if they had been flowing in stream-like beds from fissures in the older sedimentary or basic rocks. They are accompanied by pitchstones, which mark the lines by which they were erupted, or where they caine in contact with other rocks, and thus it is evident that they were subject to the same rules as volcanic rocks of younger origin.
Most of the higher ranges such as Rocky Peak, High Peak, Mount Misery, consist of such rocks, which having, doubtless, flowed from several centres, all forming rounded or conical hills, have cooled in that form. Many of these rocks have a trachytic appearance, without doubt from partial decomposition, to which they have been subjected for numberless ages, and which has affected them to a great depth. However, in some favourable localities, namely, in deep gorges and landslips, I was enabled to observe how this earthy appearance of the base gradually changes into a true felsitic structure. This gradation can be more easily seen in the different dykes of the same rock, which radiate from the small outlier east of Rocky Peak, and which have filled up fissures in the palaeozoic rocks. The selvage of these dykes assumes sometimes the structure of an imperfect pitchstone, but the sedimentary beds close to them have been very little affected, the only difference being that generally they break into smaller polyhedral fragments than the same rocks a few yards further distant.
The principal centre of eruption appears to have been at the head of Rockwood Creek, at the base of High Peak, which is here surrounded by vertical cliffs very difficult of access. Here a synenitic granite porphyry occurs, which, however, I was not able to find in situ; it is identical with the Porphyre Granitoide of French authors. Dufrenoy's description of this rock from Signon and Aix applies fully to our rock specimen, No. 348. It has been traced by him and [unclear: Eliede] Beaumont in its gradual change into porphyry similar to that of High Peak, but never into granite. (See Elie de Beaumont and Dufrenoy explication de la carte geologique de France, I. 30). Similar rocks occur in the Thuringian Forest, also together with "Felsitporphyr," (see Lehrbuch der Petrographie Zirkel, I. 528), they contain quartz, felspar, and hornblende, the latter found sometimes in well-defined crystals, but mostly in scaly aggregations; the felspar is white, and, without doubt, Oligoclase. Besides this beautiful rock there appear in these precipices some brecciated porphyries, and others, more or less laminated; also, in the so-called Rocky Ridge east of High Peak some of the porphyries have a page 289similar brecciated structure, through, which other portions, of a more felsitic character in the form of dykes appear. Although this, in many cases, may be only an appearance, and the effect of a different state of cooling, or re-arrangement of the particles during the cooling, some of those dyke-like masses may nevertheless be real dykes which at various periods were ejected, and filled up fissures, in the previously erupted rocks of the same description. Where the porphyries repose directly upon the paleozoic sedimentary beds, or upon the amygdaloids, at the junction they appear either as pitchstones or they have a more or less tufaceous character, just in the same manner as a lava stream of younger origin which is stony in the centre, is always scoriaceous or tufaceous at the bottom, where it reposes upon other rocks. There is, however, one locality in the Gorge of the Rakaia where true tufas, and even shales appear immediately below the porphyries. Besides the crystals which have, doubtless, been formed during the cooling of the rock, these porphyries enclose sometimes small pieces of slate and sandstones either angular or having their edges rounded. These enclosed pieces are generally quite unaltered, but I observed one piece of sandstone which was remarkably fritted.
The quartziferous porphyries in Banks' Peninsula occur near and along the opening leading from the head of Governor's Bay to Lake Ellesmere, by Gebbie's Pass, and stretch as Manson's Peninsula far into Lyttelton harbour. They have broken through, and cover sedimentary rocks, probably belonging to the Waihao formation. At the base we also find tufaeeous deposits, brecciated agglomerates, and shaly beds, with obscure markings of plant remains which might possibly belong to them. In texture and composition these porphyries, sometimes containing garnets, agree with those of the more western zone, whilst in other instances their compact, or sometimes felsitic matrix, is full of crystals and grains of smoky quartz. When treating of Banks' Peninsula, I shall return to this subject.
Even now when the coulées, although they have become greatly destroyed, possess in some localities a total thickness of 3000 feet, it is evident that before the next, or Waipara formation was deposited, these eruptive rocks had already been exposed for a considerable time to disintegrating agencies; consequently a long period must have intervened between the eruption of the acidic rocks and the deposition of the Waipara beds. Numerous specimens of fossil wood have been, collected from the strata at the base of these eruptive rocks. Generally page 290it occurs in flattened stems, some of large dimensions, the interior being filled with flint, woodopal or woodstone, and the bark alone preserving the woody structure, thus suggesting that these trees either belonged to the Acrogens or Endogens division. A few stems of trees have also been obtained exhibiting the woody structure throughout, but they have not as yet been microscopically examined, to settle whether they are the remnants of coniferæ or of dicotyledonous trees. Several of the most characteristic sections, both from the Malvern Hills and the Mount Somers district, illustrating the relation of the melaphyres and quartziferous porphyries to each other, and to the palasozoic rocks underlying them, have been added on plate No. 3.
The following analyses were made in the Laboratory of the Imperial Geological State Institute of Vienna by Carl Ritter von Hauer:—
Silica | 73.31 |
Alumina | 11.00 |
Protoxyde of iron | 1.55 |
Lime | 5.32 |
Potash | 4.70 |
Soda | 2.44 |
Water | 0.30 |
98.62 |
With traces of protoxyde of Manganese.
Silica | 76.00 |
Alumina | 7.16 |
Protoxyde of iron | 5.00 |
Magnesia | 3.76 |
Potash | 5.30 |
Soda | 2.22 |
Water | 0.66 |
100.10 |