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

Igneous Rocks

Igneous Rocks.

The most important zone on the eastern side of the central chain, where igneous rocks are largely developed, is situated in the Malvern Hills. In one locality at the Kowai corner, they form a small hill of a crateriform appearance, with a lava stream running in a southerly direction. The rocks on the summit of this little system are granitoid, whilst some dykelike projections, passing through them, have a fine grained texture. Here also the rule holds good that that portion which has cooled more slowly in the form of dykes is more page 301compact and fine-grained, than the streams which have a more porphyritic structure; besides the example just given, the dolerites in the Acheron show this difference very conspicuously. The felspar in some of these latter beds is most probably nepheline. Another series here to be considered is the youngest. It occurs all along the eastern side of the district, forming large sheets of doleritic rocks, which have flowed over the bottom of the sea, and afterwards, when being raised to form subaerial hills, have protected the beds of loose quartzose sands below them. There are several streams, mostly close together, which in the Harper's hills and the Dean's range show all their characteristic features. The first stream flowing over the sands has generally had a vitrifying influence; the sands have been melted for a few inches, or even one foot, and have been changed into a flint-like rock. In other localities doleritic tufas overlying the sands have protected them so effectually, that no change is visible. Between the lava-streams, having a thickness varying from a few feet to nearly 100 feet, sometimes, layers of pitchopal occur, often enclosing stems and leaves of plants; in some other localities palagonite tufas have been formed. The basic rocks under review have the peculiar character to which the name of anamesite has been given by Leonhard, some of them are scoriaceous, having the vesicular cavities lined with sphærosiderite. The latter variety would offer good material for mill stones, a similar rock being extensively used on the continent of Europe for that purpose.

It might perhaps be useful if I allude here to the various characteristic features and age of these different basic rocks, which have during that whole period made their appearance, and through which some important changes have taken place in that district. There are three important divisions in the basic or basaltic rocks appearing during this geological period in that part of New Zealand:—First.—Rocks coeval with or even prior to the formation of the principal Brown coal beds, the latter having been altered effectually, both vertically and horizontally, by the former. Second.—Rocks appearing only after the deposition of the calcareous fossiliferous beds overlying the Brown coal series, or even after the formation of the greensands. Third.— Rocks of which the extensive streams of anamesite consist, forming a well defined horizon, and closing an important geological epoch in the district. The rocks of the first division are very peculiar. They have a dark greenish colour, are granular, and resemble in hand specimens some eruptive (diabasic) rocks of older origin. Some of them have the peculiar greenish tint which an admixture of chlorite usually gives, although I do not think it exists in them.

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In Germany, rocks of this character are named hyperites, but I have no doubt - that the analysis of the New Zealand rocks will show that this resemblance is only apparent. No doubt the peculiar conditions under which these rocks made their appearance may account for their structure. They were, doubtless, deposited in shallow water, or even some of the beds might be of subaerial origin. Two forms of texture are easily identified, both occurring in the dykes and the coulées or streams. One of them more compact, although still having a doleritic and sometimes a porphyritic texture, with small grains or imperfect crystals of felspar interspersed. Another more granular and amygdaloidal, the pores and cavities either lined with sphærosiderite or filled with chalcedonic quartz. Some of the streams are of considerable dimensions, Mr. M. B. Hart having passed through one which has a thickness of about 75 feet. There are two localities where dykes belonging to another series occur, having been erupted during the formation of the greensands above the saurian beds. These dykes have quite a basaltic texture. Their being of deep-seated submarine origin may account for this difference in texture. In any case we have to classify them with the next series—the true doleritic rocks of which many have a porphyritic or even granitoid texture. They have been erupted after the formation of the coal-bearing beds, often dislocating and altering them in a most remarkable manner, and having preserved portions of these formerly more extensive strata from entire destruction.

The igneous rocks on the western side are different in texture, and resemble some of those varieties of basic rocks on the continent of Europe, to which the term melaphyre has been applied, but they are also different from the basic rocks underlying the quartziferous porphyries on the eastern flanks of the Alps. The basic rocks, north of the mouth of the Paringa, occur in different textures, of which a porphyritic melaphyre is the most conspicuous. It has a compact black matrix, containing a large number of hornblende crystals, and grains of magnetic iron. The hornblende crystals enclose sometimes very small grains of a yellowish green mineral, possibly olivine. The melaphyres at Arnott's point have also a compact black matrix, sometimes with a blueish tinge, enclosing small crystals of felspar (labradorite?), and hornblende; the joints are filled with carbonate of lime. The rock breaks in such small polyhedrie pieces, that it is almost impossible to secure, a properly sized specimen for the cabinet. It forms here coulées of 15 to 30 feet in thickness, alternating with page 303layers of brecciated wacke. The latter has also a compact black matrix and angular pieces of melaphyre, wacke or compact rocks of indistinct character, generally of reddish or brownish tints, often resembling porcelain jasper, are enclosed in it. The enclosed pieces are often surrounded by calcareous spar, also appearing in strings and veins, and filling up all the interstices. Small cavities in the calcareous spar are again filled with zeolites, of which crystals of apophyllite, sometimes with a highly lammellar structure, and of a beautiful brick-red colour, are most conspicuous. Other zeolites, as for instance, stilbite and analcime, are also present. In the coulées of melaphyre, near the contact with the brecciated wacke, pieces of the latter are usually enclosed. At the southern extremity of the melaphyres at Tauperikaka Point, the rocks have a compact black matrix, with greenish or bluish tinges. Besides very small crystals of hornblende, they contain also grains of magnetic iron and minute crystals of felspar, too small for recognition. They have a somewhat tabular, but sometimes an irregular columnar structure, and are here of very great thicknes; as, notwithstanding the coulées are standing at a very high angle, they not only form the coast-line, but reach far into the sea in the form of small islets and rocks, against which the surf breaks with vehemence. South of Jackson's Bay, both melaphyres and brecciated wacke occur, but my stay was so very short that I could devote no time to a closer examination, beyond observing that in lithological character they resembled the rocks previously described, and that they formed a considerable portion of the coast. Finally, I wish to say a word on the nomenclature, and the changes which are necessarily to be made in it, in order that all the beds belonging to the same formation may be brought together. Although Professor von Hochstetter gave the name to this formation in his synopsis of the New Zealand formations, he followed only Professor Owen, who basing his opinion upon the occurrence of Plesiosaurus Australis, thought that the beds in which this saurian occurred were probably Jurassic, an opinion which I shared till the other fossil contents became known to me. I then was forced to the conclusion, that as high up as the Weka Pass stone they could not be separated from the series containing not only the Grey and Shag point Coal Measures, but also the older Brown coal series found all over New Zealand. In fact, there is enough evidence before us to lead us to believe that the Buller and Pakawau Goal Measures, and also all the older Brown coalfields in the Provinces of Otago, Nelson, and Auckland, which latter were hitherto classed as older tertiary, have to be included in the Waipara formation.