326 
the power in heat, of promoting the solution ; yet he remarks, that 
experience has shown, that an actual solution of silex in simple 
water takes place, not only in springs, the natural temperature of 
which is much inferior to the ebullient heat of those in Iceland, but 
also in several other mineral waters : and even that some common 
sweet-water springs contain dissolved silicious earth. 
In corroboration of the opinion, that the induration of various 
substances may have been derived from the deposition of silicious 
matter, from its solution in water, I must call your attention to the 
silicious tufa which is deposited by the Icelandic hot-springs : in 
the same manner as the calcareous tufa is deposited by mineral 
springs impregnated with lime, as in those of Matlock. A bason 
of 60 feet in diameter, the outer border of which is 9 feet above 
the opening of the pipe, as well as the pipe itself, which is 19 feet 
in diameter, and which, from its depth being unknown, is consi- 
dered by the Icelanders as the gate of hell, are entirely formed by 
deposition from the boiling water of the Geyser, which is sometimes 
thrown out, from this enormous opening, to above the height of 100 
feet. This tufa having been analysed by Mr. Klaproth, was found 
to consist of 
Gr. 
Silex 98 
Alumine 1.50 
Oxide of iron 0.50 
100 * 
M. Gensanne relates, that in the mines of Cramaillot, in Tranche 
Compte, stalactitic concretions of silex form on the vaults of the 
works, from the transuding water; and that they are sometimes 
found on the timbers of the minef. 
* Analytical Essays, p. 408. 
f Histoire de Languedoc, tom. ii. p. 28. 
