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Pumice Deposits.
I have frequently referred to the superficial pumice deposit in the district. It is found most persistently in all the valleys, terraces, and flats within a radius of 40 miles round, from the south-west to the north-east, from the centre of Taupo Lake, sometimes covering the surface with a deep deposit of large and small particles, and rendering land which otherwise would be fertile quite arid and useless. It is particularly detrimental in the valleys of the Tuhua District and Upper Whanganui, where large areas of level land along the river sides and on the terraces are rendered unfit for cultivation. This pumice deposit appears everywhere more or less within the limits mentioned above; but on the mountain tops and down their slopes it is seen for the most part merely as a thin sprinkling, and accumulates more in the little valleys and flats on the hillsides, frequently to a depth of 25 to 50 feet, carried there, no doubt, by the surface waters and the wind. In the Tuhua District, a few miles to the south-east of Taumarunui, a fine tract of about 10,000 acres of good soil, derived from the blue papa rock, is partly spoiled by the pumice filling the valleys and covering the rolling country on the slopes of the hills. I have carefully studied this pumice deposit with a view to ascertaining the mode of distribution. Between Taupo Lake and the valley of the Ongarue, a distance of
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25 miles, the pumice is most persistent, and does not appear to alter much in the form of distribution or in the size of the particles. In the valley of the Pungapunga, 15 miles west of Taupo Lake, blocks of pumice measuring over 2 feet in diameter are found, and all the valleys and flats are covered deeply with the deposit. As we recede from Taupo the pumice deposit thins out gradually, and the particles decrease in size, until at a distance of 50 miles from the centre of the lake very little is seen, and that in very small particles. I have frequently seen pumice mixed with the earth brought up by the roots of trees which were blown over by the wind. I have also found it, at a depth of 3 feet or more, when constructing trigonometrical stations on the mountain tops, but only in such manner as I think may be accounted for by its becoming mixed up with the earth in the ordinary course of surface-soil formation, its movements by the roots of trees and other vegetation and by the action of earth-worms. The pumice must, I think, be regarded as a superficial deposit, and probably the product of some of the latest volcanic efforts. I saw no trace of the remains of a crater within the district in which it is found although having, in the course of my duty, visited nearly all the higher mountains. Round Tongariro and Ruapehu very little pumice is found in comparison with other localities; whilst at the Waimarino Plains and westward of Ruapehu scarcely any is seen. The Assistant Surveyor-General, in his “Geology of the Northern Portion of Hawke's Bay,” read before the Auckland Institute, 27th November, 1876,* mentions the pumice as occurring in that district under somewhat similar conditions. He says: “Towards the east the deposit gradually thins out, until, approaching the vicinity of Poverty Bay, very little is seen.” The portion of Poverty Bay referred to by Mr. Percy Smith would probably be about 50 miles from Taupo Lake; thus the limit of the pumice deposit in that direction corresponds with that on the west of the lake.
Mr. Percy Smith further states: “The general opinion appears to be that this pumice was ejected from Tongariro and adjacent volcanoes, and was spread over the surface of the country by the wind. There are certain considerations which favour this view, such for instance as finding the greatest thickness of the sand on the lee sides of high ranges, where it would naturally accumulate, and also from the fact that the size of the particles appears to diminish as we recede from the supposed centre of distribution. The only other hypothesis which would account for the presence of pumice over such an extent of country is, that it has been carried into its present position by water.” This hypothesis seems to me quite out of
[Footnote] * “Trans. N.Z. Inst.,” vol. ix., p. 565.
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the question as applied to the King Country. The deep beds of pumice in the valleys of the Whanganui, Pungapunga, and elsewhere, are unquestionably alluvial deposits, the blocks of pumice and loose sand being washed off the hill-sides, and deposited partly, it may be, in the still waters of long narrow lakes, which might be formed by the river beds being temporarily choked up by trunks of trees and detritus carried down in heavy floods.
The large area of country, and great height at which pumice is found, precludes the idea that it is due to the action of water, and all the evidence so far goes to show that it was spread in the air, and, I think points to the region of Lake Taupo as the centre of distribution. It is true that many of the particles, even amongst those found on the mountain tops, have an abraded appearance; but would not this be the natural result of their mode of ejection from the crater, being carried by the violent tornado of escaping gases high up into the air, and for miles in any direction that the prevailing wind may take them, their attrition in the air rounding off their angles?