Art. XLI.—On the Geological Structure of the Southern Alps of New Zealand, in the Provincial Districts of Canterbury and Westland.
[Read before the Philosophical Institute of Canterbury, 27th November, 1884.]
The publication of a new geological map of New Zealand accompanied by sections, issued by the Geological Survey Department, induces me to offer the following remarks on the geological structure of the Southern Alps, which I consider in some of its most essential features to have been altogether misunderstood by the officers of that survey.
In my former publications I stated that the Southern Alps are only the eastern wing of a huge anticlinal arrangement, of which the western portion has been either destroyed or submerged below the Pacific Ocean. It thus exhibits the same one-sided features so conspicuous in almost every alpine chain of which the geological structure is known.
The lowest beds on the western slope are gneiss-granites, overlaid by mica, chlorite and other metamorphic schists of similar origin. These rocks are followed by clay-slates, semi-crystalline sandstones and felstones, which in some instances form not only the summits of the central chain, but even reach several miles across to its eastern slopes. They generally contain quartz veins. Upon them reposes the great sandstone, conglomerate, clay-slate and shale formation, of which the greatest portion of the Provincial District of Canterbury is composed, and which in many instances can be followed for nearly seventy miles, to the east. I have named this extensive series of rocks the Mount Torlesse formation. On the eastern side of the great anticlinal it forms a succession of huge folds, dipping throughout at high angles, but these folds have been so much destroyed during numberless ages, that at present their synclinals generally form the summits of the mountains, while the deep broad valleys often run along their anticlinals. Besides this folding a great deal of crumpling has taken place, so that, although the general character of the arrangement has been preserved, over a short space of ground the strata often strike and dip in all directions of the compass. During my first journey to the head-waters of the river Rangitata, in 1861, I discovered in the Clent Hills a series of beds containing numerous impressions of plants, and some twelve miles distant in the Rangitata Valley at Mount Potts other beds containing fossil shells and saurian bones. Professor F. McCoy, in Melbourne, to whom I sent the collections made, for identification and description, informed me that the plants were of Jurassic and the molluscs mostly brachiopods of Upper Devonian or Lower Carboniferous age, both being identical with exuviæ found in the coal fields of New South Wales. However, judging from the position and sequence of the strata in both localities, agreeing with each other in a remarkable manner, though the Mount Potts beds are of much greater thickness, I could not accept this conclusion, being convinced that they were of the same age. Since that time it has been proved by a number of experienced geologists, that the beds in New South Wales, to which Professor McCoy alluded, are interstratified, and that consequently they must be of the same age.
Both palæontologists and geologists have agreed that if there exist in any geological horizon beds containing a marine fauna of an older together with a terrestrial flora of a younger aspect, the former will more correctly indicate the age of the beds. Thus, if the fossil shells of any given formation have a palæozoic and the plants a mesozoic character, the beds in which both occur have to be classified as palæozoic.
It would be foreign to the object of this paper were I to enter more fully into this important question, but I may observe that both in India and
South Africa a similar mixing of an apparently older marine fauna with a younger terrestrial flora has been observed, and that universally it has been admitted that the same explanation as that given for the coal fields in New South Wales has also to be applied to both those countries.
Considering the same character and sequence of the rocks overlying both the shell and plant beds, I have always held that the facts observed in New South Wales should guide us in New Zealand, and consequently that the beds under review ought to be classified as young palæozoic. In the geological maps, issued by the director of the New Zealand Geological Survey in 1869 and 1873, this view was accepted, and consequently the whole of the eastern portion of the Southern Alps was so coloured. However, since then the director himself, as well as the officers of the Geological Survey, have visited and revisited that portion of the colony with the result that the experience gained in New South Wales has been put aside or ignored, and that beds not only having the same lithological character, but situated in the same horizon, have now been divided solely according to their fossil contents, thus creating such an utter confusion that it will take years of hard work to put matters right again.
Thus instead of the eastern sides of the Southern Alps forming one wing of the great anticlinal, a synclinal arrangement has been given to the series of beds, a broad zone of mesozoic rocks forming the central portion lying between Mount Torlesse and Mount Hutt on the one side and the higher portion of the central chain on the other side which have been put down as palæozoic, thus reversing the facts, which examination in the field and experience elsewhere have taught us to be correct.
It would be quite impossible to pass in review the whole of the publications of the Geological Survey upon which these conclusions have been based; but a few observations in further explanation of the points at issue may not be amiss. Since the Clent Hills plant beds were discovered by me, further localities have been found at Mount Harper, in the Malvern Hills on both sides of the Selwyn, in the Taylor Stream, in the Mount Hutt Range, in the high Mount Somers Range, near the foot of the high ranges east of Lake Coleridge, near the Coleridge Pass, and some few other localities. In fact they generally appear where great denudation has taken place, and thus the lowest strata of the formation under review have become exposed; but now for a number of years this unmistakable position has invariably been explained by the officers of the Geological Survey, by their assertion that these plant beds were lying either above or against the strata containing the marine shell beds, or they simply denied the facts, against all evidence brought forward.**
[Footnote] * See amongst other instances Geological Survey Reports, 1879–80, p. 106.
Returning to the Mount Torlesse formation, which Dr. Hector in his annual Geological Survey Report classifies as Carboniferous (or Maitai series), the only fossil upon which reliance is placed, is a peculiar annelid; this annelid, however, of which well-authenticated specimens are in the Canterbury Museum, has been found in numerous other localities now marked mesozoic on Dr. Hector's map.
Any one knowing the character of the rocks in our eastern chains, close to the Canterbury Plains, will be rather astonished to see that a few miles north of the Waimakariri, in the northern continuation of Mount Torlesse, the Puketeraki Range, the Mount Torlesse formation comes abruptly to a close, and only mesozoic (Permian, however, included) continue towards the north. I know this range well, having crossed it in various directions repeatedly, and I can simply affirm that no change of formation takes place, but that the character of the rocks up to the Hurunui is exactly the same. Of course I have to repeat what I stated in my Report on the Geology of Canterbury and Westland, on page 279, that it is even more than probable that this huge assemblage of beds may belong to several distinct periods, ranging from the palæozoic to the lower mesozoic; but hitherto it has been impossible to divide this (Mount Torlesse) formation, for the present at least, into smaller groups, owing to the want of fossils. Since this was written, beds with triassic fossils have been found in the Okuku Range, lying east of the Puketeraki Range, so that there is evidence of younger rocks existing near or amongst the older Mount Torlesse formation. It would have been far more suitable to have marked clearly those localities in the map, than to have coloured all their surroundings of the same age, without being able to bring forth the necessary proofs.
Dr. Hector now classifies the Mount Potts beds as Permian. It is not my object to defend the opinions of such an excellent palæontologist as Professor McCoy, who classifies them as Lower Carboniferous or Upper Devonian, but it is greatly to be regretted that we have still, I fear, to wait for an indefinite period for a reliable description of the older New Zealand fossils, which ought to have been published years ago, and without which we are still groping in the dark.
In the volume of Geological Reports, containing the geological map of New Zealand, I observe that Mr. Cox does not attempt to subdivide the Permian to Jurassic series in this Provincial District, but places them together for the present. He, however, separates them from the Lower Carboniferous or Maitai beds.
Mr. Cox states that to the latter belong the Mount Hutt, Mount Somers and Palmer Ranges, but he fails to explain how in deep gullies near the very centre of the two former ranges the plant beds could occur.
Another curious statement is that of the Director,** that probably the same rocks (Maitai or Mount Torlesse) continue to the west coast watershed, judging from the shingle in the Rakaia River. Thus the information conveyed to us in the geological map in the very same volume, which shows a broad belt of mesozoic rocks on both sides of that river, is now put aside simply on the cásual observation of river shingle by one of the geological surveyors. Thus there is no doubt that the director of the Geological Survey himself is now inclined to abandon, at least as far as the country near the Rakaia is concerned, his present mapping of a large belt of mesozoic rocks between palæozoic beds to the east and west of it. All I wish to contend is that the attempted separation of our younger palæozoic rocks into two divisions according to their fossil contents is incorrect and not according to the evidence in the field, as far at least as I was able to understand it. Moreover, it would be a most remarkable and unique fact in palæontology, that we possess in New Zealand the same fossil fauna and flora that occur interstratified with each other in the neighbouring continent of Australia, to which either a Permian or Carboniferous age has been assigned, but which with us, owing to the circumstances that hitherto they have not been found together, are said by the Geological Survey of New Zealand to belong to two distinct periods, the molluscs and saurian beds to the Permian and the plant beds to the Upper Oolite period.††
This, in the face of it, is evidently a hypothesis which can never be sustained, the more so as the geological evidence in the field is against it, notwithstanding all that has been written in its support by the director and officers of the New Zealand Geological Survey. I might also show how fallacious it is to support any sub-division of our young palæozoic or old mesozoic rocks, by asserting that there exist two series of reddish and purple slates of different age, or by the presence or absence of cherts, which are found from the east coast to the western watershed, but the object of this paper is not to refute in all their bearings the statements contained in the Reports of the New Zealand Geological Survey, for which I have not the time at my command and of which many seem apparently to have been written only to find fault, year after year, with the work of other geologists, who are no longer connected with that Survey. It is simply a protest against the deductions arrived at by the New Zealand Geological Survey on some of the most important points of our stratigraphical geology. If I were to continue silent, it might be construed into my agreeing with the conclusions published by that department.
[Footnote] * Progress Report, p. xx., in that volume.
[Footnote] † Report of Geological Explorations during 1879–80, p. xxii.
Fortunately the evidence before us will remain there for all time to come (not geologically speaking), and our successors will be best able to judge what is really the truth, which we all ought earnestly to strive to discover.