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Art. LII.—On a Diatom Deposit near Pakaraka, Bay of Islands, Auckland.
[Read before the Wellington Philosophical Society, 3rd August, 1892.]
Two years ago I collected samples of a diatomaceous deposit from the bed of a small lake near Pakaraka, Bay of Islands. On my return to Wellington Mr. Maskell kindly undertook the examination of the samples, which were from the surface, and a foot below the surface, of the deposit.
Mr. Maskell found that in the sample taken a foot below the surface there were forms that did not appear in the surface-deposit, some of which he considered as belonging to extinct forms only found in a fossil state.
Subsequently I read a short paper on the subject, and in that I expressed the opinion that the deposit was not, as suggested by Mr. Maskell, in part the result of the breaking-up of pre-existing beds of diatoms, from which were derived the extinct forms found in the samples examined.
I, on the contrary, favoured the supposition that from the top to the bottom of the deposit the different species of diatoms had lived and died where their remains are now found; and I suggested that were samples obtained from the lower or deeper parts of the deposit other genera might be shown to appear in and be confined to the lower part of the deposit.
Through the kindness of the Hon. Mr. Williams, of Pakaraka, I was again enabled to submit to Mr. Maskell samples taken at intervals of a foot from the surface to the bottom of the deposit, which proved to be 9ft. in thickness.
Mr. Maskell examined five specimen-slides from each of the ten samples taken at a foot apart, and he has favoured me with the following summary of the results obtained:—
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Diatomaceous Earth.—Lake Intemita, Pakaraka, Auckland,
Summary of Observations on Ten Samples at each Foot in Depth. (Five Specimens examined of each Sample.)
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
| — | Achnanthes. | Surirella. | Pinnularia. | Epithemia. | Navicula. | Synedra. | Melosira. | Colour. |
|---|---|---|---|---|---|---|---|---|
| Nine feet | Very rare | Several | Several | Several | Plentiful | Several | Very plentiful | White: rather dirty. |
| Eight feet | Very rare | Plentiful | Plentiful | Plentiful | Plentiful | Several | Very plentiful | White: rather dirty. |
| Seven feet | Plentiful | None | Very rare | Rare | Plentiful | None | Very plentiful | White: clean. |
| Six feet | Very plentiful | None | None | Rare | Plentiful | None | Very plentiful | White: very clean. |
| Five feet | Very plentiful | None | None | Rare | Plentiful | None | Very plentiful | White. |
| Four feet | Plentiful | None | None | Some | Plentiful | None | Very plentiful | White: very clean. |
| Three feet | Fairly common | None | None | Some | Plentiful | None | Very plentiful | White. |
| Two feet | Rare | None | None | Rare | Plentiful | None | Very plentiful | White: clean. |
| One foot | Plentiful | None | None | Rare | Plentiful | None | Very plentiful | White: very clean. |
| Surface | Very rare–two only seen | None | None | None | Plentiful | None | Very plentiful, and in filaments | Green: consistence fluffy. |
Melosira forms the principal part of the deposit from all depths.
The disappearance of Surirella above eight feet, at which spot it is found in considerable numbers, is peculiar; also, the same may be said of Pinnularia and Synedra.
Epithemia is practically present in all except the surface deposit.
Achnanthes reaches its greatest frequency at five to six feet, I am not sure whether the two specimens seen in the surface deposit really belong to it.
Navicula (very small) is present all through.
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Owing to the absence of intermediate forms connecting the wide distinctions of the different genera, and other reasons, it is evident that no evolutionary process has taken place. The forms present are all well known to students of this branch of natural science. But, while this is so, it is also true that the theory that much of the deposit is derived from an older preexisting deposit of diatoms is not borne out by Mr. Maskell's last examination. None the less, the results obtained are exceedingly interesting.
I may here point out that the conditions under which, from first to last, the deposit accumulated were unvaryingly the same, there being special provision in the regularity of the intermittence of the supply-waters to this end. Yet, while two genera, Navicula and Melosira, flourished in predominant numbers throughout the whole period of the deposit, one, Synedra, is found only in the first-formed foot of the deposit, but in no great abundance, while three other genera show several specimens at 9ft., are plentiful at 8ft., but in the middle and higher part are absent or rare.
The cause or causes of the disappearance of these forms is a matter respecting which I do not feel warranted in expressing a decided opinion. Nor was this paper written with such a purpose. As stated in the first paper on the subject, further facts were needed in order that the subject might be fairly discussed. It seems to me probable that variations of temperature, not seasonal, but climatic, may account for the gradual or comparatively sudden appearance and disappearance of the four genera not found in all parts of the deposit. Any other theory, it would seem, is either insufficient to account for the facts, or demands too much by way of organic modification or physical change.