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

Texture of Rocks and Position of Strata

page 274

Texture of Rocks and Position of Strata.

I shall now proceed to give a general outline of the beds of which this formation is composed, and of their general features and sequence.

Speaking more specially of their arrangement, we may describe it as a continuation of anticlinal arches and synclinal troughs; but instead of finding the mountains to be formed by the arches and the valleys by the troughs, careful examination shows convincingly that exactly the reverse has taken place. Thus to give only one instance, whilst the enormous mass of Mount Cook occupies a synclinal trough, the broad valley of the Godley river runs along an anticlinal arch. The occurrence of such enormous changes by which the arches or mountains have been converted into deep valleys, and the troughs into high serrated mountains, will give us a faint idea of the amount of time which has elapsed, and the enormous waste which has taken place, before the Southern Alps assumed their present form. The general strike of these sedimentary beds is south-west to north-east but varying considerably, sometimes even in short distances according to the numerous foldings and bends. The strata dip generally at high angles, and stand sometimes almost vertically, a south-east and northwest dip being the most usual. As the greater portion of the beds consist of sandstones of nearly the same character, we may describe the Southern Alps as sandstone chains. Alternating with the sandstones we find slates, shales, brecciated beds (graywacke), and conglomerates in almost endless succession, forming generally sharply defined strata. Here and there we observe inter-stratified with them diabasic beds, sometimes in the form of ashes. The sedimentary beds by contact with these ashes have undergone considerable changes, so as to assume all the characteristics of chertose rocks. Small beds of limestone, generally in the form of marble of whitish tints, are sometimes intimately associated with these eruptive rocks, whilst under all other conditions the absence of limestone is very striking, even rocks with a calcareous matrix being generally wanting.

Thus it is evident that we see before us the deposits of rivers falling into the palaeozoic sea, and judging from the position and succession of the boulders of which the conglomerate beds are composed, we may conclude that a large continent or island existed to the east or southeast of New Zealand, of which probably the old sedimentary and semi-metamorphic rocks of the Chatham Islands are still a small page 275remnant, whilst the main land has long since disappeared below the sea We perceive, moreover, that according to the state of the rivers or the changes of the currents, the character of the deposits also changed, the pebble and conglomerate beds representing the immediate neighbourhood of the months of the rivers, whilst the fine-grained Sandstones represent the littoral zone, and the clay-slates, and shales those regions of the bottom of the palæozoic sea where only fine particles of ooze could be deposited. The great scarcity of animal and vegetable remains in these rocks is very remarkable, and as we have no reason to believe that the sea was devoid of organic life, we either must assume that in many instances various agencies have been at work to destroy its record, or that from other physical causes animal life was very scarce. In order to explain more fully what I mean I may point out that there are markings or obscure exuviæ of an Annelid in many of the shales, which rocks consequently could have preserved to us those of Mollusks had they been abundant. I have found these identical fossils near the sources of the Rangitata and of the Rakaia, in the very heart of the Southern Alps, near the mouth of the Hurunui, on the East Coast, and in many localities in the Malvern Hills, whilst sandy shales with markings of fucoid plants, identical in character, are met with in many localities such as the Gorge of the Ashley, on the southern base of Mount Cook, and in the Tour Peak range.

If we apply the late deep sea researches and their inductions concerning the Mediterranean, as given by Dr. Carpenter, F.R.S., and Mr J Gwyn Jeffreys, F.R.S., in the Proceedings of the Royal Society, Volume XIX No. 125, to the physical conditions which might have prevailed in our palæozoic seas during the formation of the New Zealand strata under review, our difficulties under that head will be easily removed We know that in our palseozoic strata there are beds of conglomerate of enormous thickness, the boulders of which they are composed showing that they must have been derived from ranges of great lithological variety, and indicating that they were brought down by large rivers, whilst the general character of the whole strata associated with these conglomerates indicates clearly that they could only have been formed from the sediment of such rivers. Thus it is easily conceivable that one of the conditions might have existed which now prevails in the more central portion of the Mediterranean to the prejudice of the existence of animal life, namely, turbidity of the bttom water. Of course this might have been only one of the causes page 276of this remarkable scarcity of animal life, but I need scarcely point out of what value it is to the geologist now to be able to account for facts which, before the deep sea dredgings lately undertaken, could only hypothetically be explained. However, in one locality, there is ample evidence that animal life was not missing, by the occurrence of fossiliferous beds on the western side of Mount Potts, Upper Rangitata, for it contains many brachiopods and a few gasteropods, of which I have already given the principal genera on page 272.

The lowest beds visible consist of shales often very carbonaceous; of micaceous gritty, generally tabular, sandstones, mostly of light colours, some of them being full of fucoidal markings. The shales are often slaty, and have a peculiar serpentinous look; probably they consist of ashes or are altered beds. Associated with them are conglomerates, the matrix of which, mostly a quartzose ferruginous sand, is sometimes so little binding that it is easily decomposed when exposed to atmospheric action, but some of them are so hard that they form buttresses standing prominently out from the mountain sides, and then generally form the summits of the ranges. These conglomerates, like the succeeding beds, are often grouped in several horizons separated from each other by thousands of feet. They are overlaid by a series of compact diabases, diabasic tufas and cherts, with occasionally beds of more or less altered limestones between them. Where these latter occur, they very often constitute the summits of the ranges; thus, to give only one instance, they form in the Malvern Hills, the summits of the Flagpole Hill on the one side, and the Four Peak range on the other; while between them the saddle is an anticlinal, consisting of loosely cemented conglomerates of the lower beds. As before observed, the diabases of the Malvern Hills have a compact structure, and I may here add that the more granular varieties of the same rock occurring in the Nelson province, have without doubt been erupted in the same geological period, and are of a similar origin.

It is very striking that these rocks have such a large extension in our palæozoic beds, both vertical and horizontal, and that in this respect they closely resemble the diabases of the Hartz Mountains in Germany, where they are also interstratified in beds of considerable thickness, ranging from the lower devonian to the upper carboniferous period. Professor E. Kayser, of Berlin, has given an excellent monograph of these Hartz beds, in the Journal of the German Geological page 277Society for 1869, which is so applicable to our own Malvern Hill beds, that in most instances, if we would substitute for the Grerman names such as Alrode, Lupbode, &c., our New Zealand names Selwyn, flagpole Hill, they would describe and figure perfectly well our own sections. This shows once more convincingly, that however far the distance may be which separate two localities on the globe, the same abysso-dynamic causes were at work to build up the solid crust of the earth, tinder the same conditions, everywhere in like manner. Like the conglomerates, these diabasic beds occur in many localities all over this Province, but they are generally not so conspicuous as the chertose rocks, altered by contact with them, which often form turreted crests on the summit of the hills, or run up like walls on the mountain sides.

They are succeeded by sandstones, shales, and slates of various characters, of which a peculiar fine-grained sandstone, with a white decomposing matrix, is sometimes largely developed. In some localities, the latter is interbedded between the conglomerates and the chocolate-coloured slates or their equivalents, the diabasic ashes. The whole series is always of great thickness, folded in a remarkable manner, so that the beds stand at a very high angle, and sometimes even vertical. In some districts, rocks of the same facies forming beds of great thickness prevail. In others, beds of an arenaceous nature preponderate; whilst less frequently, shales or argillaceous beds of a slaty texture take their place. However, in many instances, the beds are very thin, and alternate with each other in a most remarkable manner., the divisions being sharply defined. I shall here give only one illustration to show this. On the left bank of the Grodley river, near the glacier of the same name, where a high cliff rises from the water's edge, the ribboned appearance of the beds was so conspicuous that I measured a small portion of the section consisting of greyish sandstones and dark bluish clay-slates, the latter in a few instances gradually getting thinner in their higher portion, and disappearing altogether, a bed of sandstone taking their place. The whole beds dip at an angle of 46 degrees to the south-south-west, and, follow each other in ascending order:—

  • 8 feet 0 inches, greyish fine-grained sandstone
  • 8 feet 3 inches dark clay-slate
  • 8 feet 4 inches greyish sandstone
  • 8 feet 3 inches dark clay-slatepage 278
  • 8 feet 2 inches greenish sandstone
  • 8 feet 3 inches dark clay-slate, running out 12 feet above river
  • 8 feet 4 inches greyish sandstone
  • 8 feet 2 inches dark clay-slate
  • 8 feet 3 inches greyish sandstone
  • 8 feet 4 inches dark clay-slate
  • 1 feet 10 inches greenish sandstone
  • 1 feet 0 inches dark clay-slate
  • 3 feet 0 inches greyish sandstone
  • 3 feet 4 inches dark clay-slate running gradually out
  • 6 feet 2 inches greyish sandstone
  • 6 feet 9 inches dark clay-slate
  • 4 feet 3 inches greyish sandstone
  • 4 feet 4 inches dark clay-slate

Above the latter bed, repose bluish greywacke sandstones full of small quartz veins, about 470 feet thick, after which another alternation of sandstones and slates, similar to the one given above, follows, succeeded by a series of shales of great thickness, overlaid again by bluish graywacke sandstone.