process by which any large proportion of flint can be held in solution by water, yet we have unquestionable proofs, that the solution of silicious earth has been effected by natural processes, on a very extensive scale. At the present moment, Nature, in her secret laboratories, is still carrying on a modification of the same process; and of this fact we have a remarkable instance in the Geysers of Iceland, and in the springs of Carlsbad, in Bohemia. Professor Silliman remarks, that "the sulphuret of silicon, which is the base of silex, is very soluble, and that silicious earth itself is taken up by fixed alkalies, and by fluoric acid; and that these agencies, like most of those which are chemical, are rendered more active by heat." A high temperature therefore appears necessary to enable water to dissolve a large proportion of silex, &c.; hence, we find that the thermal springs of volcanic regions are the principal agents by which silicious depositions and incrustations, are at present produced.

58. THE GEYSERS.-The Geysers, or boiling fountains, of Iceland, have long been celebrated for possessing this property in an extraordinary degree; holding a large proportion of silex in solution, and depositing it, when cooling, on vegetables and other substances, in a manner similar to that in which carbonate of lime is precipitated by the incrusting springs of which we have already spoken. Iceland may be considered as a mass of volcanic matter; the only substances not of volcanic origin in the

whole island, being beds of surturbrand, or bituminous wood, in which occur leaves, trunks, and branches of trees, with clay and ferruginous earth. These strata support alternating beds of basalt, tufa, and lava, which form the summit of the hill in which the vegetable remains occur. The Geysers, of which there are a considerable number, are springs, or rather intermittent fountains of hot water, which issue from crevices in the lava. A fountain of boiling water, accompanied with a great evolution of vapour, first appears, and is ejected to a considerable height; a volume of steam succeeds, and is thrown up with great force, and a terrific noise like that produced by the escape of steam from the boiler of an engine. This operation continues sometimes for more than an hour; an interval of repose of uncertain duration succeeds, after which the same phenomena are repeated. If stones are thrown into the mouth of the cavity, from which the fountain has issued, they are ejected with violence, after a short interval, and again jets of boiling water, vapour, and steam appear in succession. The eruptions of the "great Geyser," witnessed by Sir G. S. Mackenzie,* were preceded by a sound like the distant discharge of heavy ordnance, and the ground shook sensibly; the sound was rapidly repeated, when the water in the basin, after heaving several times, suddenly rose in a large column,

* Travels in Iceland, in the summer of the year 1810, by Sir George Steuart Mackenzie, Bart.

accompanied by clouds of steam, to the height of ten or twelve feet. The column seemed to burst, and sinking down, produced a wave, which caused the water to overflow the basin. A succession of

eighteen or twenty jets now took place, some of which rose to a height of fifty or sixty feet. After the last eruption, which was the most violent, the

water suddenly left the basin, and sunk into the pipe in the centre to a depth of ten feet. After a few hours the eruption was repeated; the jets sometimes attaining ninety feet in altitude. The basin of the "great Geyser" is an irregular oval, about fifty-six feet by forty-six, formed of a mound of silicious depositions about seven feet high; the pipe through which the water is ejected being sixteen feet in diameter at the opening, but contracting to ten feet lower down; its perpendicular depth is estimated at sixty feet. Sir G. S. Mackenzie has proposed an ingenious explanation of these phenomena, which the diagram in the preceding page will serve to illustrate. It is supposed that the water from the surface percolates through crevices (Tab. 13, a) into a cavity in the rock (b), and heated steam, produced by volcanic agency, rises through fissures in the lava (cc). The steam becomes in part condensed, and the water filling the lower part of the cavity (d) is raised to a boiling temperature, while steam under high pressure occupies the upper part of the chasm. The expansive force of the steam becomes gradually augmented, till at length the water is driven up the fissure or pipe (e), and a boiling fountain with an escape of vapour is produced, which continues playing till all the water in the reservoir is expended, and the steam itself escapes with great violence till the supply is exhausted.*

* Travels in Iceland, p. 229.

The silicious concretions formed by these springs cover an extent of four leagues; these are specimens of the more friable varieties, presented me by Professor Babbage: M. Eugene Robert, who has recently visited Iceland, states that this curious formation may be seen, passing by insensible gradations, from a loose friable state, the result of a rapid deposition, to the most compact and transparent masses, in which impressions of the leaves of the birch-tree, and portions of the stems, are distinctly perceptible, presenting the appearance of the agatized woods of the West Indies. Rushes, and various kinds of mosses converted into a white silicious rock, in which the minutest fibres are preserved, also occur; but on the margin of the Geysers, from the splashing of the water, the depositions resemble large cauliflowers; and on breaking these masses, vegetable impressions are often discovered. Numerous thermal springs occupy the valley in the interior of the island, in the midst of which the Geysers are situated. It is evident that these waters arise from deep crevices, in which they have been heated by volcanic fires. The rivers proceeding from the springs often resemble milk in appearance, owing to the argillaceous bole which they take up in their passage among the silicious concretions: such are the white rivers of Olassai. Mount Hecla, like all the mountains of Iceland, is entirely covered with snow, and no smoke appears on its summit. Accumulations of rolled masses of obsidian and pumice