Art. XXXIX.—On a Deposit of Diatomaceous Earth at Pakaraka, Bay of Islands, Auckland.
[Read before the Wellington Philosophical Society, 13th February, 1891.]
Last March I examined the geology of the district surrounding Pakaraka, Bay of Islands, Auckland, and in the course of this work had an opportunity of examining a deposit of diatomaceous earth about half a mile to the east of the residence of the Hon. Henry Williams.
Of this I brought samples from the upper surface and from about 1ft. below the surface of the deposit, which were submitted to Mr. Maskell, who found only recent species in the samples from the upper part, and fossil forms only in the samples taken at about 1ft. from the surface of the deposit. Such being the result of the examinations made by him, on my describing the conditions under which the deposit had accumulated, by way of explanation of the facts Mr. Maskell
suggested that probably an older diatomaceous deposit had been denuded for supply of the lower part of that under description, and in which only fossil forms are found, while the higher and last formed were manifestly due to diatoms (of recent forms only) which had lived and died within the area wherein their remains had accumulated. But this is not the only explanation that may be advanced, and, this not disposing of all the objections to it, before advancing any of my own I deem it necessary to describe more closely the position of the deposit and the conditions under which it has accumulated.
Between the valley of the Kawakawa River (which from Pakaraka lies to the east and south) and the fall westward into the Hokianga River the drainage is carried by the Waitangi River into the Bay of Islands. Between Waimate and Ohaeawai on the west, to the hills south-east and east of Pakaraka, and thence east and north to Black Bridge, the upper Waitangi basin approaches to a circular form, and, with the exception of the volcanic cones within it, may be described as a depression surrounded by hills on all sides. Before the outbreak of the late Tertiary volcanoes, Cretaceotertiary beds covered most of the area from Turntable Hill to Waimate and from Black Bridge to the southern watershed. Probably the earliest eruptions began in late Pliocene times; but, although at different places the volcanic forces must have been active during the Recent period, there are not now (except in the neighbourhood of Ohaeawai, where yet there are thermal springs) any signs of activity. From Waimate a number of crater-cones encircle the western and southern limits of the Waitangi watershed. The most beautiful and perfect of these is Paeroa, situate—or rather, built up—on the southern side of the Pakaraka Plain.
Prior to the first eruptions of Paeroa, the site of the future mountain was a nearly level plain extending from Ohaeawai east through Pakaraka to the north-west of Turntable Hill and the range of hills of which it forms a part. The cone itself, and the lesser scoria hills that lie at its base to the north-east and east, are chiefly, if not wholly, scoria, which appears as a coarse breccia or scoria-ash of finer grain. The solid lava-streams from this crater lie on the south and south-west sides of the mountain. To the east, for the distance of a mile from its base, the scoria-beds form numerous small hills, and together form a ridge of higher ground, the eastern end of which abuts against a higher ridge of Cretaceo-tertiary rocks and a second volcanic cone which has arisen in that direction. This barred the flow of water which originally had its course to the north between the site of Paeroa and the Cretaceous hills to the eastward. Barred thus, the waters from the hills
to the south-east passed the southern base of Paeroa and joined the eastern branch of the Waitangi west of the crater.
But the lava-flows of later date choked this outlet also, and in this manner formed the lake to the south-east of Paeroa. This has no visible outlet, and without question its surplus waters for the most part escape through the barrier of loose scoria-blocks and scoria-ash that lies to the north of the lake and east and north-east of Paeroa itself.
Several heavy springs, evidently drainage from the lake, appear at low levels among the scoria hills described. Of these, that which most concerns us at the present time appears from a brecciated rocky face half a mile east of Pakaraka House. It opens into a small nearly circular depression less than an acre in extent. On the north side of this the waters escape by a narrow passage, which has either been cut or usurped by the stream in its course north to join the larger creek, which in that direction flows north-west along that boundary of the Pakaraka Flat. In summer and during dry weather the amount of water issuing from the underground channel is very limited; but after a continuance of wet weather it is considerable, and not merely fills the narrow channel cut through the little patch of level ground below the outlet, but floods the whole basin to a depth of 3ft. or 4ft. When dry weather again sets in the basin is drained, and then there is left on the grass, ferns, and stones which form the bottom and sides of the basin, a white or greenish deposit of what on examination proves to be mainly diatoms. What of this falls on the bottom and level parts of the little grass-covered basin is soon washed through the grass and indistinguishably added to a thick deposit of the same material which has in this manner been accumulating since first the underground waters filled the basin.
On the stones and fern-fronds which are under water when the basin is full, the green living diatoms are deposited, forming a coating from ⅛in. to ½in. thick, according to circumstances. This deposit round the margin of the basin soon bleaches white on the surface, but is found to be green immediately below the surface. According to Mr. Maskell it is almost wholly composed of living forms of Diatomaceæ. Very probably the same species would be found among the grass-roots, and for the first few inches into the deposit filling the basin itself. Unfortunately, I did not bring samples to prove that such is the case; but it is so self-evident that this must be so that no doubts need be ventured on the subject. The deposit in the middle of the basin is 6ft. to 8ft. thick, and was exposed by the cutting-down of a cattle-track crossing the creek at this place. I took a sample from about 1ft.
below the surface. Some of this also was examined by Mr. Maskell, whose decision as to the fossil nature of the species forming this part of the deposit has already been stated. Subsequently samples were forwarded to England, and examined by one of the chief authorities on diatoms, whose decision was in accordance with the conclusion Mr. Maskell had already arrived at.
Such are the facts of the case, and such the conclusions arrived at by competent authorities. And yet I am not satisfied that the true explanation has been hit upon; and here I venture a theory of explanation to which objections may possibly be raised as grave as those which lie against Mr. Maskell's theory, yet they are in a different category; and I have written this paper so that the Society may have an opportunity of debating the probabilities of each.
Considering the conditions under which the diatomaceous deposit has accumulated, it is reasonable to expect that recent forms of diatoms would be found in the lowest, as well as the highest, beds of the deposit; and it is certainly surprising that the upper beds, or latest part of the deposit, should be wholly composed of recent species which are absent from the middle and lower parts. It is quite a possibility that the fossil species forming the bulk of the deposit have been derived from an older deposit, either forming the bed of the lake or now buried beneath the scoria hills to the east of Paeroa. But it seems to me that, in order to account for the facts of the case, it must be supposed that at first only fossil species carried along the underground channel were deposited in the little basin whence the specimens were obtained. And, as the deposit is entirely composed of fossil species to within 1ft. of the present surface, the introduction or appearance of living forms is of very recent date.
As, however, the whole deposit is manifestly of quite recent date, and as at first the conditions were as fit for the existence of recent forms of diatoms as they now are, it seems extraordinary that throughout the deposit there is not a mixture of fossil and living species.
Taking these facts into account, I would prefer to account for the difference in the species found in the top and bottom beds of the deposit by supposing that the species first living in the pond gave place to other forms, either modified descendants of the original species or species introduced from a different stock, and in this way would avoid the necessity of hypothecating an older deposit, the existence of which has not been proved, and, at the same time, accounting for the separateness of the living and extinct forms as they are found in the higher and lower parts of the deposit.
P.S.—I would here add that, as the surface-layers are formed wholly of living forms, and all are extinct at about 1ft. from the surface, it seems reasonable to suppose that at, say, 6ft. from the surface other and quite distinct species may be found. And, as Mr. Williams informed me he dug into the deposit to a yet greater depth without passing through it, other and quite distinct species, it is probable, will be found in the first-formed and lower part of the deposit.
If samples were taken not more than 6in. apart in the section of the deepest part of the deposit, an examination of these would be likely to set at rest any doubts as to the true origin and mode of accumulation of the deposit, since, if it is mainly a derived and secondary deposit, then from about 1ft. from the surface to the greatest depth there should be little variation of the specific forms, while, on the other hand, if the species changed more than once, that would go far to prove the correctness of my theory on the subject.