Art. LXXII.—On the Wanganui Tertiaries.
[Read before the Wellington Philosophical Society, 8th August, 1874.]
The Wanganui tertiary beds, or the “Wanganui Formation,” as they are called, have already been described by Mr. Buchanan and other competent observers, and there being, so far as I am aware, no great geological problem dependent upon their further examination, another paper on the subject may by some be deemed superfluous. Original observations of physical phenomena always possess, however, a certain value, and as I have lately been enabled to make a personal survey of the tertiaries in the neighbourhood of the town of Wanganui, I determined to put the results of my observations on record.
The characteristic stratum of the Wanganui formation is the blue clay, the development of which is best seen at Shakespeare Cliff, opposite the town of Wanganui, where a thickness of forty feet is displayed. It is composed of a fine greyish blue mud, shown by the fossils which it contains to have been deposited in water of a moderate depth, and probably at the mouth of a river or the outlet of a lake. It is important to notice that it was deposited in a tranquil sea, and which must have remained quiescent for a considerable period to allow of the rich accumulation of fossil shells of which the clay forms the storehouse. A fine oyster bed is conspicuous in the lower part of the stratum, and from top to bottom Ostrea, Pecten, Turritella, Murex, Terebratella, Ancillaria, and many other sea shells abound, all being inhabitants of a coast lying between low-water mark and a depth of 100 fathoms. Sharks' teeth and a species of Bryozoa are scattered here and there. The oyster bed is, however, so marked a feature of the stratum, while, moreover, beds of oysters and Pectens exist in other strata of the same age in the district, that it would be appropriate to call the blue clay and all the strata above it the “Wanganui Oyster Beds,” which name would be more descriptive than the somewhat unmeaning term “Wanganui Formation.” Shakespeare Cliff has been so thoroughly ransacked by collectors that its fossils are well known, and the only new shell I can offer is a species of Waldheimia which does not appear in the Catalogue of the Tertiary Mollusca, issued by the Geological Department, among the fossils of the Wanganui formation.
The blue clay rests upon a layer of pumice, which seems hardly to have been noticed by previous observers, but which is quite as worthy of attention as the blue clay itself. At Shakespeare Cliff the pumice bed is very thin indeed, but it extends along the bank of the river from the mouth for several miles up, varying from three to fifteen feet in thickness, and underlies the town. It becomes conspicuous on the east bank, a short distance north of Shakespeare Cliff, running parallel with the outcrop on the west bank, and gradually swells in thickness until, between three and four miles from the town, it becomes a component part of a cliff 100 feet high, and is filled with fossil shells. The section here discloses a layer of red volcanic mud, stratified, upon which lies from fifteen to twenty feet of pumice, reduced to lapilli, and forming a tuff, with coarse sand of nearly pure quartz, and an abundance of recent fossil shells of apparently the same age as those in Shakespeare Cliff, but quite different in character. Turritella and Pecten, so plentiful in the latter place, seem entirely absent, while Ostrea is scarce. Dosinia and Venus are prevalent, as on the modern sea beaches of the neighbourhood, with Buccium, and large species of Fusus and Voluta. A fine Crepidula and a small Salarium, are characteristic. These fossils are very fragile, crumbling to pieces in the hand, just as if they had been burnt in a kiln. Upon the pumice is superimposed sixty feet, or thereabouts, of mixed volcanic mud and lapilli, which, being weather-worn, presents a curious appearance.
It would seem that this spot was once the site of a submarine volcano, from which the lower bed of pumice was ejected. The pumice slopes at an angle of 45° up the river, i.e., northward, but it is clear this is not the result of movements from below, since the mud bed upon which the pumice rests is horizontal and undisturbed, which would not have been the case had it been upheaved by a movement which tilted the pumice into its present position. This mud stratum, and the stratum of mixed lapilli and mud above the pumice, are both destitute of fossils, although the tuff abounds with them, calcined as just described, and confusedly intermingled with the pumice, showing that they were hurled from their original bed into their present position with the pumice itself. The upper and non-fossiliferous stratum must, I apprehend, be referred to a later volcanic eruption.
What I have called volcanic mud is generally spoken of as ordinary clay, but a cursory examination will show it to be a volcanic product; the mere fact that beds of it perfectly destitute of fossils lie between beds of other composition richly endowed with remains of animal life is convincing. We thus gain an idea of the immense volcanic discharges, extending over a vast period of time, which must formerly have taken place in this region, and which, indeed, can only be said to be pretermitted at the present moment, seeing that the district lies within the range of the yet smouldering volcano of
Tongariro. The lowest stratum visible here is the red clay—a volcanic product. Then comes the pumice bed itself, extending miles and miles in length and breadth, a terrible witness of the vigour of a huge submarine volcano, whose effects were augmented here and there by lesser outlets, as in the cliff mentioned. A long interval of calm must have supervened to allow of the deposition of the blue clay, and the birth and death of the inhabitants of the many generations of shells which there lie buried. Another mud torrent from a distant point followed, rolling over the blue clay, entombing its molluscs, and covering it at Shakespeare Cliff with a layer ten or fifteen feet thick, and elsewhere of a much greater thickness. Again, volcanic activity ceased for a time, and for no brief period, since it was long enough to allow of the accumulation of a shelly stratum twelve to eighteen inches thick, with a large Pectunculus as its characteristic fossil. But it was only the prelude to a more terrible fiery storm. A perfect deluge of mud came down, forming a layer fifty feet thick, and covering perhaps as spacious an area as the pumice. Upon this lies three feet of marine clay, with recent shells.
Superimposed is a remarkable bed of dark cemented gravel, ten to fifteen feet thick, covered by twenty feet of loose grit, both without fossils. This bed of cemented gravel is exposed at Shakespeare Cliff, at the cliffs below Putiki called the Landguard Bluff, at the end of Victoria Avenue, and in several other places situated at a distance from each other, showing that it likewise spreads over a large tract. Was it the bed of a lake occupying the present valley of the Wanganui? It bears the aspect of a lacustrine deposit, and the overlying beds in the sections made by the roadmen in the cliffs at the west end of the town, all of which are non-fossiliferous, certainly seem to be of that character.
The gravel is capped by one hundred feet and more of volcanic mud, which in many places has suffered much from denudation. An indication is given of how denudation has lessened the thickness of this bed, and perhaps of the lower mud beds also, at the Landguard Bluff. There is a fault here, the bluff at the sea end having sunk about fifty feet, bringing the gravel bed down to the water's edge, yet the top surface of the cliff is level, proving that at least the depth of the fault has been denuded.
The foregoing remarks will give an idea of the force and long duration of volcanic eruptions in this region; but only an idea, because there is no section of country laid bare which reveals a full development of the series of eruptions and intermissions. That, of course, could hardly be expected. We find, for example, that the second shelly layer above the blue clay, which at Shakespeare Cliff is only from twelve to eighteen inches thick, expands at the Landguard Bluff to a stratum twenty feet thick, composed almost entirely of shells, a large Pecten (P. jacobœus) here playing the part which Ostrea does at
Shakespeare Cliff. At the latter spot the gravel bed lies immediately upon the shelly stratum, but at Putiki a soft red sandstone, non-fossiliferous, intervenes. In fact, the district through which the Wanganui flows in the lower part of its course contains a variety of local beds of limited area, and beds of, fossils completely different in character occur within a short distance of each other, which, taken alone, might lead to the belief that the beds were of different ages. Their synchronism is, however, indubitably proved by their relative positions to the pumice, blue clay, and cemented gravel, which are constant members of the series throughout the region, and, moreover, the character of these fossils itself determines their age. It must be remembered that molluscs, although possessing an organisation extremely simple compared with that of many of the classes in the animal kingdom above them in rank, and a great tenacity of life, are remarkably susceptible to surrounding influences. An animal like the horse, with a highly complicated structure, can range over many degrees of latitude and longitude and country of varied nature without showing signs of being affected thereby, but to the humble Patella it makes a vast difference whether its home be in a tranquil cave or on the ocean beach, exposed to the constant buffetings of the polyphlœsbian sea. Every bay, every harbour, every strand has its peculiar molluscan fauna, and should the embryos of a new species be accidentally conveyed into a new habitat sufficiently hospitable to bring them to maturity, the form and appearance of that species are speedily modified by an unseen and silent working, but omnipotent, power into that fashion which best fits it for its new residence. You can verify these statements for yourselves by diligently examining and comparing the molluscs inhabiting Evans Bay with those of Lyall Bay, or of the inner harbour of Port Nicholson. When, therefore, we find the fossils of Shakespeare Cliff and of the Landguard Bluff differing considerably in species, though not in genera, while in the one genera are present which are absent from the other, we must not conclude their age to be different on these grounds alone; but the lithological character of the strata must be taken into account, and, above all, the type of the fauna. If that be similar in both cases, we may conclude that the age of the strata is the same. For while, from the Cambrian to the pleiocene eras, the multifarious forms of molluscan life so blend with one another that it is impossible to say where one ends and the other begins, still the naturalist who has made himself acquainted with the entire fauna feels that certain periods are distinguished from each other by the general type of their fauna, although, in the present imperfect state of human knowledge, he would be unable to describe in words the nature of that type. It is like the colours of the sky. Looking first at the zenith, and then at the horizon, we perceive clearly the difference of colour, but the intervening tints pass imperceptibly into one another, and no imaginary zones
of tint could be drawn, although were the human vision sufficiently acute the lines where each tint attains it maximum, and which give the tone to the whole sky, could doubtless be discerned.
I have collected from the Landguard Bluff a large assortment of shells, including Pecten jacobœus, Tellina (large and small species), Cardita, Rotella zealandicus, Myadora, a small and very fragile Venus (which I think is not catalogued), Nucula, Fusus (two species), Turritella, a small Scalaria, Ostreœ, Lucina, Natica, Crepidulœ, a large Voluta, Rissoa, Venericardia, and Donax, all, it will be observed, betokening a climate similar to the present. I refrain from attempting to give specific names, for the reason that I have not proper materials at command to determine them, and nothing has tended more to complicate the intensely perplexing nomenclature of shells than the hasty efforts of collectors to attach names to specimens which are not quite familiar to them.
Considering the abundance of cephalopods in the New Zealand waters of the present day, it is surprising that I have been, unable to discover a single fossil of a cephalopod in the district, nor is one marked in the Museum catalogue. What does this indicate? The fossils collected show the climate to have been suitable for cephalopods, which, however, have a wide range, and some are very hardy.
From this sketch it would appear that the oldest fossiliferous stratum within a radius of four or five miles from the town of Wanganui is the tuff in the cliffs mentioned on the east bank of the river; the next oldest, the blue clay of Shakespeare Cliff; and the youngest, the beds overlying the blue clay and those at the Landguard Bluff—the strata thus growing older as they ascend the river. To ascertain the comparative ages of these formations would require the fossils of each to be examined separately, with the view of determining the proportions of extinct to recent shells. Captain Hutton, in his Catalogue, has lumped the three formations together, and thus makes them out to contain seventy-six per cent. of recent shells, which would make the beds of about the same age as the Sicilian volcanic tuffs (newer pliocene). It will probably be found that the lower beds are of the same age as these tuffs, perhaps a little older, but there is such a marked difference, palæontologically speaking, between the tuff bed on the left bank of the Wanganui river and the upper beds of Shakespeare Cliff, which are of very recent origin, that their fossils ought not to be mingled together in order to strike an average.