quantity of silica in solution, it seems necessary that it should
be raised to a high temperature.[321] The hot springs of the Valle das
Fernas, in the island of St. Michael, rising through volcanic rocks,
precipitate vast quantities of siliceous sinter. Around the circular
basin of the largest spring, which is between twenty and thirty feet in
diameter, alternate layers are seen of a coarser variety of sinter mixed
with clay, including grass, ferns, and reeds, in different states of
petrifaction. In some instances, alumina, which is likewise deposited
from the hot waters, is the mineralizing material. Branches of the same
ferns which now flourish in the island are found completely petrified,
preserving the same appearance as when vegetating, except that they
acquire an ash-gray color. Fragments of wood, and one entire bed from
three to five feet in depth, composed of reeds now common in the island,
have become completely mineralized.

The most abundant variety of siliceous sinter occurs in layers, from a
quarter to half an inch in thickness, accumulated on each other often to
the height of a foot and upwards, and constituting parallel, and for the
most part horizontal, strata many yards in extent. This sinter has often
a beautiful semi-opalescent lustre. A recent breccia is also in the act
of forming, composed of obsidian, pumice, and scoriae, cemented by
siliceous sinter.[322]

_Geysers of Iceland._--But the hot springs in various parts of Iceland,
particularly the celebrated geysers, afford the most remarkable example
of the deposition of silex.[323] The circular reservoirs into which the
geysers fall, are lined in the interior with a variety of opal, and
round the edges with sinter. The plants incrusted with the latter
substance have much the same appearance as those incrusted with
calcareous tufa in our own country. They consist of various grasses, the
horse-tail (_Equisetum_), and leaves of the birch-tree, which are the
most common of all, though no trees of this species now exist in the
surrounding country. The petrified stems also of the birch occur in a
state much resembling agatized wood.[324]

By analysis of the water, Mr. Faraday has ascertained that the solution
of the silex is promoted by the presence of the alkali, soda. He
suggests that the deposition of silica in an insoluble state takes place
partly because the water when cooled by exposure to the air is unable
to retain as much silica as when it iss