Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
Chapter II. General Geological Structure
Chapter II. General Geological Structure.
Before examining the general geological structure of the two provinces under consideration, it will be necessary to devote a few moments to New Zealand considered as a whole, as it would be very difficult, without such a short review, to understand the conformation of this portion of the South Island alone. It has already been pointed out by Ferdinand von Hochstetter in the geological portion of the magnificent Novara work—(Geological portion, first part, first division, Geology of New Zealand)—that these Islands belong geologically together, and are portions of one and the same system which, with a south-west and north-east direction, forms a well-marked line of elevation in the Pacific Ocean. This longitudinal course is crossed by a second one nearly at right angles, that is to say, running in a nearly south-east and north-west direction. On this line, the northern continuation of the Northern Island is situated; also Cook's and Foveaux Straits strike in the same direction, conforming to a line N. 52° W., which, according to Dana, * can be considered the axis of great depression in the Pacific Ocean.
Not having at this moment, access to Dana's valuable work, I can do no better than translate from the above-mentioned publication of my friend, von Hochstetter, the passage having reference to it, and his conclusions based thereon, with which, moreover, I fully agree:—page 241
"Dana has drawn attention to the fact that a line drawn from the Pitcairn Island (in the Paumotau Group) in a westerly direction passing north of the Society, Samoa, and Solomon Islands, to the Palaos Islands (East of the Philippine Islands), will form pretty correctly the boundary between the Atoll—or Low islands to the north and the High islands to the south. He denotes the broad arm of the sea between this line and the nearest High islands in the north-east, the Sandwich Islands—a plateau nearly 2000 nautical miles broad, and 6000 nautical miles long, dotted all over with about 200 Atoll Islands—as a great area of depression. Nearly all groups of islands belonging to this zone, strike in a south-east and north-west direction and a line drawn from Pitcairn Island, in a N. 52° W. direction to the centre of the Japanese Archipelago, would be the central line of this area of depression, and the axis of its greatest depth. However, if we place this line on a map on Mercator's projection, it will be found that, beginning at Pitcairn Island, such a line drawn in a direction of N. 45° W. will reach the northern coast of the Japanese Island of Jesso, and will probably still more correctly form the middle line of the area of depression under consideration. Now if, according to the direction of the Southern Alps which form the highest chain of mountains in the islands of the Pacific Ocean, we take the mean longitudinal direction of the islands forming New Zealand to be N. 45° E., we observe then that two principal lines, the line of depression and the line of elevation, cross each other at a right angle. It is remarkable that the general geological importance of these two directions for the Pacific Ocean, is also strikingly observable in the confirmation of the eastern coast line of the Australian Continent. The east coast of Australia and the west coast of New Zealand form almost parallel lines, although separated from each other by nearly 1000 nautical miles. "The principal elevations of New Zealand, plutonic as well as volcanic, conform to the north-east direction of the line of elevation, whilst the great diagonal fissures by which Foveaux and Cook's Straits were formed lie in the direction of the great line of depression, to which also the third great area of dislocation which the northern portion of the Northern Island owes its origin to, also conforms."
Thus, looking at New Zealand as a whole, from a geological point of view, we observe that its principal characteristic feature consists of a high longitudinal chain, which runs from Windsor Point, the most westerly part of the South Island, to the East Cape, the most easterly page 242point of the North Island. This great mountain chain, broken through by Cook's Straits, falls rather abruptly towards the west, having from the divide, only a breadth of about eight to ten miles. Towards the east it slopes down more gradually, a breadth of 50 miles not being uncommon. This remarkable chain, of which the geological structure is generally uniform throughout, is only the eastern wing of a huge anticlinal arrangement, of which the western portion has either been destroyed, or submerged below the Pacific Ocean. It has thus the same one-sided arrangement, so conspicuous in almost every alpine chain of which the geological structure is known. The which still, in some localities in Westland, are open to our inspection. This granite area is mostly confined to the northern portion of Westland between Lake Brunner and the Waitaha, rising in isolated mountains from the West Coast plains, and being generally detached from the Southern Alps, so that it is exceedingly difficult to find any contact between the plutonic rocks and the lowest beds which form the western slopes of the Southern Alps; a low saddle, if not a valley or broader depression, separating them from each other.
In the chapter on the Physical Geography of Canterbury, I have referred to Mr E. Dobson's interesting observations that all the principal valleys from the Hurunui in the north to the Makaroa in the south, radiate as it were from a common centre situated about 50 miles to the north of Mount Darwin, or about 40 miles west of Hokitika. It is a remarkable coincidence that the granitic zone, stretching from the south of the Waitaha round Lake Brunner to the west coast, 20 miles north of the River Grey, forms a segment of a circle round that point. A similar segment of a circle is formed by the eruptive and volcanic zone on the eastern slopes of the Southern Alps round Banks' Peninsula. An examination of the Geological Map of the two provinces, attached to this Report, will show that the different principal formations, according to their age, and the metamorphie action they have been subjected to, follow each other from west to east, while the general sections across the Island show this still more convincingly. Thus, beginning at the western slopes of the Alps, we find invariably, where open to our inspection (the deep gorges of the rivers, generally offering us clear sections) that the lowest beds consist of gneiss granites in many varieties of composition and texture. Sometimes they appear in hand specimens, and even in small exposed sections as true granites, but invariably when examining these sections over a larger space, we observe that the rock always page 243shows a rough stratification; above and below, the rock has the usual texture of gneiss. These lowest beds stand at a very high angle, invariably dipping to the east. Veins of granite, sometimes of a very large grained texture, so that it even may be called giant granite, pass through them.
As before observed, the granitic axis is only exposed in Westland, the whole western wing of the anticlinal having disappeared. Exception must, however, be taken of a few localities, as, for instance, near Lake Hall, south of the Paringa river, where gneiss granite and gneissic beds dip at a high angle to the west of a granitic zone, so that they may possibly be a remnant of the western wing. As we advance towards the east, we find the eastern gneissic beds of great thickness, overlaid by mica, chlorite, and other metamorphic schists of similar origin, having, like the former, a high easterly dip. After four or five miles, these rocks usually alter to clay slates, semi-crystalline sandstones, and felstones, which, although mostly confined to the western slopes of the Southern Alps, in several instances form the divide, and even reach several miles across on to the eastern slopes of the central chain They generally contain auriferous quartz veins. Upon them now reposes the great sandstone, conglomerate, clay-slate, and shale formation of which by far the greatest portion of the Province of Canterbury is formed, and which, in many instances, can be followed without intermission for more than seventy miles to the east. I have named this extensive series of rocks, the Mount Torlesse formation. It forms, on the eastern side of the great anticlinal under consideration, a succession of huge folds, dipping throughout at high angles, but these folds are so much destroyed during numberless ages, first by marine action, and afterwards by sub-aerial causes of which the Great Glacier period is one of the most important, that at present the synclinals or troughs generally form the summits of the mountains, whilst the deep and broad valleys run often along the anticlinals or saddles. These beds, of a thickness of at least 25,000 feet, have been formed during a gradual depression of the sea bottom. A large island continent lying to the east of New Zealand, of which a portion of the Chatham Islands is a small remnant, being the country whence numerous rivers brought the material in the form of boulders, pebbles, sand, and ooze, into the ocean, and from which these beds were built up. During their formation, a number of sub-marine eruptions took place by which diabasic ashes, and, in a few instances, diabasic streams, melaphyres, and amygdaloids were interstratified with them. These diabasic ashes, and altered beds in connection with them, offer ua a page 244splendid horizon for sub-division, which owing to the want of fossils in most localities for the examination of beds of such enormous thickness, is of great importance. These beds reach to the east coast, and it is evident that only after they had been deposited and consolidated, the great movement of the earth's crust took place, through which they were uplifted and folded.
The three sections of the Chromo-lithographed General Section Plate will offer to the reader a clear insight into the general features of the remarkable geological structure under review. Although faults and are not wanting, I have not hitherto found eruptive rocks any-where amongst them, appearing to have been injected during the formation of the folds, or afterwards when further dislocations and faults were formed. The most careful search along the east coast has also not revealed any plutonic or eruptive rocks to which a part in the folding of these sedimentary strata might be assigned, and the vera causa of such enormous changes in the earth's crust has therefore to be attributed to single or repeated movements of a more general character, and to which I have alluded, when quoting fron von Hochstetter's classical work. The folding must have taken place, as before observed, shortly after the deposition and consolidation of the Mount Torlesse formation, which I consider to be of young palæozoic age. After assuming their present form—I mean to say, after the western wing of the anticlinal had almost disappeared—new sedimentary rocks were deposited along the western flanks of the central chain, being of deep water origin, and mostly in the form of slates, of which a great portion, in course of time, has undergone such considerable changes that some of the rocks have quite a metamorphic structure. This has partly been caused by the intrusion of granites, syenites, and hornblende rocks. They now generally stand at a high angle, and are, moreover, full of quartz and other mineral veins, containing gold and other valuable metals. Not having been able to distinguish between the western older crystalline metamorphic zone, of which I believe, as I have already stated, a remnant is still to be found near Lake Hall, and the metamorphic beds and the newer slates from which these latter have been derived, I thought it expedient to colour, in the Geological Map, the whole as old palæozoic, leaving it to a more detailed examination in future years to unravel the complicated relations in which they stand to each other. Although carefully searching for fossils whenever I had an opportunity, I was never able to discover any in these newer slates, nor am I aware that other geologists visiting the West Coast after me have been more fortunate.page 245
On the eastern side of the central chain, no rocks of the same age have been recognised by me, although some of the beds forming the upper portion of the synclinals might be of younger age than the rest, but in the absence of fossils and all the rocks possessing the same lithological character, it has hitherto been impossible to trace any difference in age, the whole, without exception, appearing to belong to the same series. I may here add that the large area of crystalline metamorphic schists covering a considerable portion of Otago, and in which its principal goldfields are situated crosses near Lake Wanaka into the Province of Canterbury. It has been coloured, in the Geological Map attached to this report, the same as the gneiss granite formation in Westland.
It is evident that for a long time, the eastern ranges have undergone great denudations before any newer beds were formed. In fact, the general orographical conformation of the country in its main features and the lines of great valleys, and direction and form of the great mountain chains between them had already been fixed, before newer beds were deposited over them. We may thus conclude, from the evidence before us, that the Southern Alps after rising from the ocean had been forming high land above its level for a long period. I stated previously that on the eastern side of the eastern wing of the great anticlinal, no eruptive rocks of contemporaneous origin were existing; however, it is evident that such a remarkable folding could not have taken place without weakening the crust of the earth considerably in the direction of the strike of the beds. The folding of the strata having taken place in young palæozoic times, but before the auriferous slate formation at the West Coast was deposited, an enormous period appears to have intervened before new beds were formed, because we have to reach the latest mesozoic times before we can trace such beds with certainty. Eruptions on a large scale then began on the eastern side of the anticlinal, stretching from the Orari south to the Oxford hills north. The eruptions commenced with melaphyres, either in the form of crystalline rocks or as tufas and amygdaloids, and they were doubtless all deposited on the sea bottom, although no traces of marine fossils have been discovered in them. These basic eruptive rocks underwent in their turn, considerable denudations, before new out-bursts, this time of an acidic composition, appeared in the same districts. They consist of quartziferous porphyries of great variety with their pitchstones and tufas, and generally follow the same line, partly surrounding Banks' Peninsula, as a segment of a circle. They either page 246repose on the decomposed surface of the melaphyres or on the palæozoic rocks, generally appearing below the former, altered and impregnated with siliceous matter. In the tufas at their base we find impressions of leaves, but too indistinct for description, and trunks of monocotyledonous trees, often of enormous size, of which the bark has been silicified, showing its original structure, whilst the hollow interior appears as a dense mass of semi-opal or flint.
In Banks' Peninsula, where we meet with a partly altered nucleus of palæozoic rocks (schists), a small zone of quartziferous porphyry and tufa appears near it, which may be of the same age as the belt of acidic eruptive rocks west and south-west of it. Whilst the highest points of the older basic eruptions rise in the Clent Hills to 4212 feet, the acidic rocks reach a still higher level in Mount Somers, namely, 5240 feet, both being situated due west of Banks' Peninsula, and due east of Mount Cook.
It is a remarkable fact that here, where the Southern Alps attain their highest elevation, the mountains are not only much narrowed in lateral extent, but the island itself is also much compressed, notwith-standing the great extension of the eruptive rocks, if we do not take the Canterbury plains into consideration. It thus appears that the abyssological forces, by forming here the highest foldings, did so at the expense of the breadth; moreover, it is clear that the highest mountains, being the greatest condensers of humidity, could offer an enormous supply from their nevès to the gigantic glaciers at their base, and thus aid effectually in their own destruction.
The country continued to sink gradually for a considerable time, during which sands, more or less glauconitic or calcareous, were deposited, the uppermost bed being in many localities a highly calcareous glauconitic sandstone (known as the Weka Pass stone). On the western side of the central chain, the Grey Coal Measures and their accompanying shales, sandstones and limestones, form, without doubt, the equivalent formation, stretching all along the West Coast, where favourable circumstances for its preservation have existed. On both sides of the Southern Alps, during the deposition of this extensive formation, as well as at its termination, large eruptions of igneous rocks took place, by which the coalseams on the western side have undergone such remarkable changes that they have assumed all the characteristics of a true black coal, those on the eastern side being only affected locally. The eruptive rocks on the western side have generally more the character of melaphyres, whilst on the eastern, they are either doleritic, anamesitic, or basaltic.
During the whole tertiary epoch, oscillations of the ground have continued, the beds deposited in one period being partly, if not almost entirely, destroyed in another. The change of level in Canterbury and Westland was evidently greatest during the middle division of our tertiary epoch (the Oamaru formation), when some portions of Canterbury were at least submerged for 5000 feet, as shown by beds of that age being preserved to that altitude amongst the eastern divergent chains. New Zealand appeared then above the sea level, only as a number of high rocky islands, generally striking in a south west and north-east direction, with narrow straits between them, and flanked on both sides by a number of smaller rocky islets, just rising above the sea level. It is evident, from the complicated structure of page 248Banks' Peninsula, that a long period of time was necessary before that volcanic system, as it now appears, could have been built up, there being at least three well-defined series erupted one after the other; in each case, the former craters having been partly destroyed before the new ones were formed. We have no fossiliferous beds in the neighbourhood to guide us as to the time when these volcanic eruptions took place, but I think that they occurred at the close of the Pareora formation, when the country rose again to, or above, its present level. Towards the end of the pliocene period, the country had nearly assumed its present form, but the mountains were higher and more plateau-like; the valleys not so deep and wide, and moreover, extensive cretaceo-tertiary and tertiary strata were lying on both sides of the high alpine ranges, often blocking up the valleys, or covering the lower regions with their deposits. The Great Glacier formation of New Zealand now made its appearance, the effects of which upon the configuration of the country were of great importance. The whole plateau-like central range was covered with enormous masses of perpetual snow, from which gigantic glaciers descended by every opening to lower regions, and which have left ample traces, of striking appearance, in all directions, on the West Coast for some hundreds of miles along the coast; on the East Coast descending as low as 700 feet above the sea level. We may say that even at present we are not yet at the termination of that period, the great glaciers, which have now retreated so far back amongst the ranges, after repeatedly advancing again, still continuing their work of destruction, the rivers which issue from them extending the plaids along the sea shores of both coasts, whilst inland they excavate or raise their valleys and fill up lakes through which they flow. Along the sea coast sand dunes are still being formed, by which the land gains upon the ocean, whilst in other localities it is destroyed and the sea takes its place. And now, man, by his engineering works along the coasts, and at or near the mouths of rivers, by destruction of forests and virgin vegetation, by draining swamps, by planting trees, and other works necessary for the colonization of a country, also brings about material changes which, although at present scarcely visible, will be of the utmost importance in centuries to come, altering, or at least modifying, some striking physical conditions of the country.