Art. XLIX.—Traces of Volcanic Dust-showers at Napier, Petane, and generally throughout the East Coast Districts, North of Cape Kidnappers.
[Read before the Hawke's Bay Philosophical Institute, 12th July, 1886.]
The results of some recent experiments made by me upon the dark soils covering the Napier and Petane Hills, as also upon a dark sand deposit near Mr. Villers' hotel at Petane, may not prove uninteresting to members of this Society. For some time past I have been collecting data as to the extent and character of the pumice deposits of the East Coast District between Poverty Bay and the Manawatu Gorge, in the Seventymile Bush. Within this wide area there is ample evidence of comparatively recent volcanic products; but until examining specimens of the ejectamenta of the recent eruptions at Tarawera, in the Lake District, the thought had not occurred to me that possibly there might be evidence in our Napier hills of dust showers similar to those which have been experienced in the district extending from Tologa Bay to Tauranga, including the whole of the Bay of Plenty.
The Napier hills, or, at least, those portions of them that have not suffered from extreme denudation, are covered with a remarkable cap of what at first inspection seems to be a dark vegetable soil. When first broken up this soil is very productive; but this quality quickly disappears unless manures are plentifully used, there being little or no “body” in the soil. I had often wondered how such a cap of black soil came to be formed upon the hills, for only a small percentage is of vegetable origin; but on seeing several specimens of the volcanic dust and sands which fell upon the deck of the “Southern Cross,” on her way down from Auckland at the time of the Tarawera eruption, the thought at once occurred to me that possibly the black soil of Napier and surrounding district might be the result of similar showers of volcanic dust, at a time when the volcanic cones of Ruapehu, Ngauruhoe, Tongariro, Tauhara, and others were in a condition of activity. The results of my tests confirm this opinion; for I find that among the many tests I have made of the soils in and around Napier, a very large percentage, in fact the greater portion, are of volcanic origin.
My experiments were carried out in the following manner: I obtained from the edge of the Napier Bluff, and immediately underlying the turf, a small parcel of black soil, containing altogether about 20 ounces. This I moistened with water, and made up into a kind of paste. I then arranged five different receivers, one inside the other, so that the overflow of water
from the smallest vessel might pass into the one next in size, and so on. Water was then allowed to run slowly upon the pasty mixture, which was stirred continuously, so as to drive off the vegetable matter and lighter products. The process was continued until nothing remained of the original soil except the heavier and insoluble sands and grits. After allowing the mud and lighter sands which had overflowed into the different vessels to settle, the water was drawn off, and the sediment or deposit contained in each was carefully collected and placed in an oven to dry. The same plan was followed with a number of other specimens from the hills where the lands had not been broken up, as also from other places outside Napier, and in each instance the results were very similar. The products, as far as I have been able to make out with any degree of certainty, are: pumice sands, magnetic iron sands (magnetite), lava, ashes, felspar, nepheline, leucite, and olivine. Under the microscope beautiful specimens of minute glass-like rod crystals of leucite were common, having a faint black or dotted line running through them similar to those described by Rutley.
It is a curious fact that the whole of the East Coast between Poverty Bay and Cape Kidnappers has a black soil similar to that covering the Napier hills, the only difference being in the thickness of the deposit, which varies from 4 inches to about 16 inches.
Since writing the foregoing, I have found at Petane a peculiar black sand or soil deposit, about 8 inches in thickness, interbedded with fine sands like those which form the highest beds at Battery Point, Napier. This black sand has a close resemblance to the black soil covering the surrounding hills, and but for the somewhat greater compactness of the former, due, no doubt, to pressure, it would be difficult to distinguish it from the present surface soils. I have washed specimens of this black sand, and I find that it also is of volcanic origin. Scoria, lava, obsidian, olivine, perlite, felspar, mica, and a trace of magnetite are distinguishable; but some of the sands I am still unable to identify. After washing, the sand is not unlike emery powder in appearance.
From the results of my experiments I feel convinced that the East Coast District of this island has been subject, at a not very remote date, to dust showers of volcanic ejectamenta. Had the wind been blowing from the north-west at the time of the recent eruptions, it is a matter of certainty that the dust showers which fell in the district extending in a north-easterly direction for about 120 miles from the seat of the volcanic outburst, would have fallen throughout the East Coast District as far as Napier and the Hawke's Bay river system. Within 75 miles of Napier there are many volcanic cones, including the semi-dormant Tongariro and the not– altogether–extinct cone of
Ruapehu—the highest point of elevation in the North Island; and although this district is separated by the Ruahine chain of mountains, and other minor ranges, from what may be termed the zone of active volcanic phenomena, as represented by hot springs, solfataras, geysers, and burning mountains, it is certainly not outside the zone of volcanic influences, the effects of which may be seen at any time along the East Coast. A recurrence of activity in and about the district of which Lake Taupo is the natural centre, would undoubtedly bring showers of volcanic dust and débris as far as Napier, should the wind be blowing in this direction at the time; but I cannot agree with those who say that such showers would be detrimental to vegetation. They may cause temporary inconvenience, but of their beneficial effects in the production and formation of soils I think there can be no question for a doubt. To me, volcanic dust showers are blessings in disguise. They may cause loss and inconvenience at the time of their deposition; but they contain within their particles the elements of fertility, and only need, like wine, age to make them valuable adjuncts in the formation of rich soils.