Art. LIII.—On the Igneous Rocks of the Province of Wellington.
[Read before the Wellington Philosophical Society, 21st August, 1875, and 29th January, 1876.]
The volcanic group of Tongariro and Ruapehu, within which are situated the greatest volcanic mountains of the North Island, lies within the Province of Wellington, but it is not so much my intention to describe it as to call attention to the few scattered indications of trap dykes which are found in other parts of the Province.
I am in hopes that, by calling the attention of observers to the direction in which to look, further discoveries may be made, for, as far as I know, not a single additional igneous rock has been found since those that I discovered as far back as the year 1861.
Those which I found were not numerous. After several days' canoe voyage up the Rangitikei River, I came to a bar composed of large igneous boulders over which the river ran in rapids.
I came to the conclusion at the time that these were carried boulders, as I could not trace the rock into or under the tertiary cliffs forming the river boundary, but in thinking over the matter afterwards, I do not see very clearly how these large boulders could have been carried there. I would therefore suggest a further investigation, to see whether an igneous dyke does or does not run across the country in that locality. The boulders, if I remember right, are composed of a very hard doleritic rock.
In the valleys of the Upper Hutt; of the Waiohine, and of the Ruamahunga, I have found boulders of a vesicular trap, the small vesicles filled with what I took to be carbonate of lime.
As I found these boulders inside the gorges of the Hutt, of the Waiohine, and the Ruamahunga, and therefore in a position where it would be
difficult to suppose that they could be carried from without, or from a lower level, I think it probable that trap-dykes may be found towards the central parts of the Tararua Ranges. As I found no trap boulders in the western streams, the Otaki and the Waikanae, I should expect to find the trap dykes more readily on the eastern and southern sides of the ranges than on the western side.
I found, at Waikekeno, on the East Coast, not far from Flat Point, reefs of diallage, or bronzite, standing up on the beach. My impression was that these formed parts of an east and west dyke, and that this may perhaps be traced in the direction of Tararua.
I found what we may call suspicion of igneous rocks at the Muka-muka Rocks; but of such an undeterminate character that it was difficult to decide whether they were igneous or not.
The above are all the igneous rocks as yet discovered within this Province; I think it is high time that we should find a few more. Mr. John Buchanan lately found an igneous boulder in the clay between Hill and Sydney streets; but there does not seem to be much evidence to show how it got there. It may have found its way in former ages from the sources of the Hutt River, or it may have been carried by man.
Along the West Coast we find plenty of igneous boulders brought down from Tongariro and Ruapehu by the rivers Whanganui and Wangaehu. These boulders are eventually washed out to sea and distributed along the coast as far as the Rangitikei, or even farther south. The pumice-stone, which is continually floating down the Wanganui and other rivers, is distributed over much larger areas. Indeed I suppose there is nothing to prevent it making long sea voyages, even to Australia or South America.
The object of this paper, however, is not to call attention to the volcanic products which come from Tongariro, but to point out how to get more information as to the trap dykes of the Province. In particular I would recommend person in the Wairarapa, of enquiring minds, to look well to the valleys of the Ruamahunga, the Waingawa, the Waiohine, and the Tauherinikau, examining the banks of these rivers towards their sources. An endeavour should also be made to trace the diallage at Waikekino inland, and persons living towards the sources of the Hutt River might be on the look out for igneous rocks. Any persons ascending the Rangitikei River should carefully examine the bars of igneous boulders and endeavour to get more information about their locality in situ.
Since the above paper on this subject was read before the Wellington Philosophical Society, I have perused in Vol. VII., of the “Trans. N.Z. Inst.,” page 453, a paper by Mr. C. W. Purnell, on The Whanganui Tertiaries.
In this paper Mr. Purnell introduces to the Whanganui district a submarine volcano, and it may be at once perceived that I cannot leave this novel and unexpected visitor from the lower regions unchallenged.
I have further reasons for taking this step, as I understood that Mr. Duigan has also in a paper, read this year, introduced volcanoes into the Whanganui district. As I did not hear this paper read, and do not know its argument, I will confine my remarks to Mr. Purnell's paper, which is now before me, as a discussion of it will fully settle the question of Whanganui volcanos.
If any one is responsible for a correct statement of the igneous rocks of this Province, I am that person, and it would certainly be a curious fact if I had overlooked such a prominent matter as volcanoes at Whanganui, a district which I have traversed in all directions.
The necessity which Mr. Purnell seems to feel for establishing a submarine volcano is really quite unnecessary. The whole process of conveying volcanic material is going on every day and all day long before the eyes of any one who chooses to look.
Pumice floats down the Whanganui River in such quantities that it would be no difficult matter for a ship anchored in the river to intercept it by putting out nets, and so load the ship, and make the pumice an article of commerce. Volcanic ashes are no doubt also washed down, and the harder volcanic rocks are rolled down as boulders. When the Whanganui River stood at a higher level—that is to say, before it cut itself so deep a channel—its waters would spread more to right and left over the surrounding country, and would there leave deposits of pumice, of volcanic ashes, or even of hard igneous boulders.
Around the volcanic groups of Ruapehu and Tongariro there are immense areas covered with pumice. From this, pumice has been washed down the Whanganui River, and there, in many parts, forms thick deposits in the narrow valley. The river cuts through these deposits, and wearing away the banks, constantly floats immense quantities down stream, which are either deposited further down or carried out to sea.
When Ruapehu and Tongariro were in full blast, we may suppose the quantities of pumice and volcanic ashes brought down the river to have been enormous.
There are plenty of volcanic products at Whanganui; but none in original situ. From Taranaki volcanic boulders are rolled along the beach. From Tongariro similar boulders are brought down the river, and at last carried out to sea, while in addition, as above stated, pumice and possibly volcanic ashes are always travelling down the river.
It would require very strong argument to establish a volcano in the absence of any igneous rocks in situ. Now, in the Whanganui district, the whole country is composed of marine tertiaries up to the base of Ruapehu. There are no volcanic cones; there are no igneous dykes of any kind; there is not a particle of evidence that a volcano ever existed in the district.
With the exception of the volcanic group of Ruapehu and Tongariro, I think I may safely say that there is no evidence of any volcanic cone in the Province of Wellington, and even of igneous dykes few have been found. The same remarks will apply to the Province of Hawkes Bay without exception. Taranaki has its own volcano, but it is a magnificent one; while Auckland is thickly studded with volcanic cones of all sizes. Although Wellington has only the central group to boast of, yet this group, comprising Ruapehu, Tongariro proper, Ngauruhoe, and Puke Onake is superior to all the rest, even to Mount Egmont. There seems to be a strong tendency in the human mind to place volcanoes in the Whanganui district. Even Hochstetter has been persuaded by somebody to put one at Taupiri, where no volcano exists. Taupiri is composed of the usual marine tertiaries.
I hope Mr. Purnell will excuse my criticism. If he can find any volcanic or igneous rocks in situ, he may have a case for argument. I assert that there are none to be found thus in the Whanganui district, neither in the shape of volcanic cones, nor of dykes of dolerite, trachyte, basalt, greenstone, or any other igneous rock whatsoever.
One might as well place a volcano in London as at Whanganui. I will undertake to find plenty of igneous rock in London, brought as ballast. I have seen chalk flints in the road metal of the streets of Sydney. I did not, therefore, suppose that we must find a cretaceous formation there, because I saw that the flints had come from London as ballast.
If we find volcanic products, such as pumice or tufa in any district, and are unable to find a volcano in situ, from which these may have come, then it may be a fair argument to suppose that, at some time or another, a volcano must have existed, although all direct traces of it had disappeared. But when we find the clearest cause and effect before our eyes, there is not the slightest necessity for adopting hypotheses.
I am also extremly doubtful whether it would be competent for a submarine, volcano to eject pumice. I never heard of such a thing. Pumice is simply fluid glass (obsidian) expanded by rapid passage into the atmosphere. An eruption under water, and therefore under great pressure, is not likely to permit the formation of this product.
I should be happy to oblige Mr. Purnell in any reasonable way; but a volcano at Whanganui is too much to ask for. It would be a great and unnecessary expenditure of hypothetical power.
Apart from the question of the volcano, I have read Mr. Purnell's paper with much interest, and I hope he will continue his researches, and give us further accounts of them.