§48 THE GEYSERS OF ICELAND. 97 



basin of the " great Geyser" is an irregular oval, about 

 fifty-six feet by forty-six, formed of a mound of siliceous 

 deposits about seven feet high ; the channel through which 

 the water is ejected being sixteen feet in diameter at the 

 opening, but contracting to ten feet lower down ; its per- 

 pendicular depth is estimated at sixty feet. 



Sir G. S. Mackenzie has proposed an ingenious expla- 

 nation 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 

 (Lig?i. 13, a,) into a cavity in the rock (b), and heated 

 steam, produced by volcanic agency, rises through fissures 

 in the lava beneath (c, c). The steam becomes in part 

 condensed, and the water filling the lower part of the 

 cavity (d) is raised to a boiling temperature, while vapour 

 under high pressure occupies the upper part of the chasm. 

 The expansive force of the steam becomes gradually aug- 

 mented, and at length drives the water up the fissure or 

 pipe (ej, and a boiling fountain with jets of vapour is pro- 

 duced; this continues playing till all the water in the 

 reservoir is expended, when the steam itself escapes with 

 great violence, till the supply is exhausted.* 



The siliceous concretions formed by these springs cover 

 an extent of four leagues. M. Eugene Eobert 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 bircli-tree, and por- 

 tions of the stems, are distinctly perceptible, presenting the 

 appearance of the agatized woods of the AVest Indies. 

 Rushes, and various kinds of mosses, converted into a white 

 siliceous rock, in which the minutest fibres are preserved, 

 also occur ; but on the margin of the Geysers, from the 



* Travels in Iceland, p. 229. 

 H 



