ues from the earth at a
temperature of 180 degrees or 190 degrees Fahr.; and partly because the
evaporation of the water decomposes the compound of silica and soda
which previously existed. This last change is probably hastened by the
carbonic acid of the atmosphere uniting with the soda. The alkali, when
disunited from the silica, would readily be dissolved in and removed by
running water.[325]

Mineral waters, even when charged with a small proportion of silica, as
those of Ischia, may supply certain species of corals, sponges, and
infusoria, with matter for their siliceous secretions; but there is
little doubt that rivers obtain silex in solution from another and far
more general source, namely, the decomposition of felspar. When this
mineral, which is so abundant an ingredient in the hypogene and trappean
rocks, has disintegrated, it is found that the residue, called porcelain
clay, contains a small proportion only of the silica which existed in
the original felspar, the other part having been dissolved and removed
by water.[326]

_Ferruginous springs._--The waters of almost all springs contain some
iron in solution; and it is a fact familiar to all, that many of them
are so copiously impregnated with this metal, as to stain the rocks or
herbage through which they pass, and to bind together sand and gravel
into solid masses. We may naturally, then, conclude that this iron,
which is constantly conveyed from the interior of the earth into lakes
and seas, and which does not escape again from them into the atmosphere
by evaporation, must act as a coloring and cementing principle in the
subaqueous deposits now in progress. Geologists are aware that many
ancient sandstones and conglomerates are bound together or colored by
iron.

_Brine springs._--So great is the quantity of muriate of soda in some
springs, that they yield one-fourth of their weight in salt. They are
rarely, however, so saturated, and generally contain, intermixed with
salt, carbonate and sulphate of lime, magnesia, and other mineral
ingredients. The brine springs of Cheshire are the richest in our
country; those of Northwich being almost saturated. Those of Barton
also, in Lancashire, and Droitwich in Worcestershire, are extremely
rich.[327] They are known to have flowed for more than 1000 years, and
the quantity of salt which they have carried into the Severn and Mersey
must be enormous. These brine springs rise up through strata of
sandstone and re