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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

Chapter VIII

page 281

Chapter VIII.

The Melaphyre and Quartziferous Porphyry Eruptions.

Before proceeding to the next, or Waipara formation, it will be useful to treat separately of the intervening period when a series of eruptions, consisting of basic and acidic igneous rocks took place on a large scale along both sides of the Southern Alps, those on the eastern slopes being the most extensive. These eruptions had long ceased before sedimentary beds belonging to the newer era were deposited on their flanks, the material for them being generally derived from their disintegration and destruction. It is impossible to say, at least at the present stage of our knowledge, when the palæozoic rocks described in the previous pages were folded, crushed and denuded to such a remarkable extent, or when they appeared above the sea, and how long they remained above it under powerful atmospheric influences, being subjected at the same time to erosion by rivers or by the sea along the coast, but we can affirm with safety that this period, during which they were denuded to a remarkable extent, was a long one. Moreover, there is sufficient evidence to show that even the forms of several of the present main valleys were already indicated. Thus, when the next period of disturbance occurred, the main configuration of this Island was already so far sketched out that the addition of new rocks made no considerable difference in its outlines. These rocks, as previously observed, consist of two main groups, both of igneous origin, of which the older belongs to the basic and the younger to the acidic sub-division.

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We have no data from which we can judge when those rocks appeared, but there is sufficient evidence to show that they broke through the palæozoic beds upon which they rest unconformably, after the latter had generally assumed their present positions, and their surface had already been denuded and partly decomposed; in fact, a very long period of time must have elapsed before the first eruption took place.

Extent.

In this group I include the melaphyres with their tufas and amygdaloids, the felsitic and quartziferous porphyries with their pitchstones and tufas of the East, and the melaphyres of the West Coasts. The first zone begins in this province on the southern declivities of Oxford Hill, where a small outlier of melaphyres and amygdaloids occurs. It then becomes largely developed in the southern portion of the Malvern Hills, and on both banks of the Rakaia, near the Upper Ferry. In this district, the melaphyres rise to an altitude of 2900 feet, and the quartziferous porphyries to 3019 feet (in High Peak).

Another small outlier of the latter is situated five miles from the beginning of the Canterbury plains, on the right bank of the northern Ashburton. On both banks of the southern Ashburton and Hinds rivers, both divisions of these eruptive rocks cover a large area, forming the summits of mountains of considerable dimensions, the basic rocks rising in the Clent Hills to 4100 feet, and the acidic rocks in Mount Somers to 5223 feet. The latter also crop up in one locality on the southern banks of the Orari. Another zone, possessing very interesting features, is found on Banks' Peninsula, consisting of quartziferous porphyry, pitchstones, and some tufaceous beds derived from them; whilst we meet, on the northern declivities of Mount Harper, with a small zone of melaphyre; on the right or southern banks of the Rangitata, between Forest and Coal Creeks, a large portion of the McLeod range consists of felsitic porphyry, amygdaloids and melaphyres, stretching across into the Upper Orari. In the more southern portions of the province, no eruptive rocks were observed by me in situ, but I found repeatedly boulders of similar porphyries in the lower Waihao, without however being able to discover the locality whence they became detached. I believe that the porphyries in the River Mandamus, north of the Hurunui, and in the Leslie Hills, north page 283of the Waiau river, described in my Report on the Geology of the Amuri district (Reports of Geological Explorations during 1870-71, Geological Survey of New Zealand) form the northern continuation of this zone. On the West Coast, only basic rocks appear to have been erupted, by which, south of Bruce Bay, a considerable portion of the coast line is now formed. I have already stated that they appear to be of a somewhat younger age than the eruptive rocks on the eastern side, because if the sedimentary beds, consisting of conglomerate, grits, shale, and coal, with which the melaphyres are associated, and which are doubtless of the same age as the Grey Coal Measures, are contemporaneous with the cretaceo-tertiary formation on the eastern side, which is very probable, they ought to be classed with the basic rocks, making their appearance during that period. However, my reason for placing them with the former is, that they possess all the characteristics of true melaphyres, having, moreover, extensive beds of tufas and amygdaloids associated with them.

Character and Sequence of the Eruptive Rocks.—Melaphyre Series.

Although there are characteristic tufa beds interstratified with the melaphyres, no fossils of any kind have been found in them at present, and thus it is impossible to state when those eruptions, which were on a gigantic scale, took place, by which the melaphyres and their accompanying tufas were deposited upon the sea bottom. I can only repeat what I previously stated concerning them, namely, that they repose on the sedimentary strata, which, previously to the eruption of these basic rocks, had undergone great changes and denudations, and that since then no alteration of importance, except the rising of the land, has taken place. After these basic rocks were deposited, before the next, or acidic beds were formed, they, in their turn, underwent enormous denudations. I may here add, that generally in other countries the acidic formations (quartziferous or felsitic porphyries) constitute the lowest beds, whilst the melaphyres, or basic rocks, repose upon them—our beds thus differing in this respect. Of this denudation the isolated system of the Gawler Downs forms a notable instance, as it is only a remnant of a once continuous series of beds which all around have been removed, often for many miles.

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Sometimes the melaphyres, principally where they occur in higher positions, lie directly upon the edges of the palæozoic rocks, but in most instances, they, are preceded by beds of tufa, which often are So intermixed and blend so thoroughly with the decomposed sedimentary rocks below them, that it is impossible to fix the exact line of contact between them. These lowest beds are mostly wacke, more or less amygdaloidal, of brown and reddish tints, in some localities they are traversed by dykes of melaphyre of a compact texture. They are overlaid by regular coulées or streams of melaphyre. We may consider that some of the wacke and some of the amygdaloids are either ashes, deposited during submarine eruptions on the sea bottom, or if, at least, some of them were formed by regular coulées they have afterwards been so much exposed to hydrothermal action, that they have undergone great alterations and changes in their structure, whilst the melaphyres proper have been ejected during larger submarine eruptions, following each other in quicker succession than the former, by which the whole series became more protected. Some of the streams are of great thickness and extent, as I was enabled to trace them for from five to six miles continuously. Their number is also very great, into which fact the Gawler Downs, amongst other localities, give us a splendid insight. There, streams of melaphyres, many of them standing nearly vertical—a dip of 79° to 82°, generally to the E. S. E., being common-alternate for several miles in an almost endless succession with tufas of various character. Some of them, full of concretions of green earth, are true amygdaloids, or are full of geodes of agate, chalcedony, and all the numerous varieties of quartz usually found in these old basic rocks, vividly reminding me of Oberstein, in Germany.

The lithological character of the rocks under review is very varied. We find some of which the component parts are so distinctly crystallized that they could be mistaken for dolerites, others are more porphyritic, others so compact that they appear like basalts, whilst a few of them have a pitchstone-like appearance. When in an undecomposed state, they are generally black, with greenish, bluish or reddish hues. They contain numerous minerals as accessories, such as agate, chalcedony, amethyst, jasper, calcareous spar, sphærosiderite, delessite, and epidote, but I was unable to find any traces of ores, and the different localities which were pointed out to me as containing copper ores proved unfortunately to consist only of masses of green earth, as accessories to the rocks, or colouring them intensely. Concerning their position, they are invariably found to be below the page 285quartziferous porphyries. In the Malvern Hills they crop up along the sides as well as towards the centre of the acidic zone where the rocks belonging to the latter have been much denuded. In the gorge of the Rakaia, where fine sections are exposed, they occur in the centre, being flanked on both sides by the porphyries. The relations of these rocks to each other are still more clearly exhibited in the Ashburton-Hinds zone, where the melaphyres occupy the outer margin, the quartziferous porphyries forming the centre and covering the former. On Section-plate No. 3 I have copied from my field-books some of the most interesting sections, illustrating the relations in which these eruptive rocks stand to each other.

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:—

White Quartziferous Porphyry with Garnets.—Mount Misery, Malvern Hills.
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.

Green Quartziferous Porphyry.—Snowy Peak, Malvern Hills.
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