ition of animals and plants.

As these springs derive their chief importance to the geologist from
the quantity and quality of the earthy materials which, like volcanoes,
they convey from below upwards, they may properly be considered in
reference to the ingredients which they hold in solution. These consist
of a great variety of substances; but chiefly salts with bases of lime,
magnesia, alumine, and iron, combined with carbonic, sulphuric, and
muriatic acids. Muriate of soda, silica, and free carbonic acid are
frequently present; also springs of petroleum, or liquid bitumen, and of
naphtha.

_Calcareous springs._--Our first attention is naturally directed to
springs which are highly charged with calcareous matter, for these
produce a variety of phenomena of much interest in geology. It is known
that rain-water collecting carbonic acid from the atmosphere has the
property of dissolving the calcareous rocks over which it flows, and
thus, in the smallest ponds and rivulets, matter is often supplied for
the earthy secretions of testacea, and for the growth of certain plants
on which they feed. But many springs hold so much carbonic acid in
solution, that they are enabled to dissolve a much larger quantity of
calcareous matter than rain-water; and when the acid is dissipated in
the atmosphere, the mineral ingredients are thrown down, in the form of
porous tufa or of more compact travertin.[314]

_Auvergne._--Calcareous springs, although most abundant in limestone
districts, are by no means confined to them, but flow out
indiscriminately from all rock formations. In central France, a district
where the primary rocks are unusually destitute of limestone, springs
copiously charged with carbonate of lime rise up through the granite and
gneiss. Some of these are thermal, and probably derive their origin from
the deep source of volcanic heat, once so active in that region. One of
these springs, at the northern base of the hill upon which Claremont is
built, issues from volcanic peperino, which rests on granite. It has
formed, by its incrustations, an elevated mound of travertin, or white
concretionary limestone, 240 feet in length, and, at its termination,
sixteen feet high and twelve wide. Another encrusting spring in the same
department, situated at Chaluzet, near Pont Gibaud, rises in a gneiss
country, at the foot of a regular volcanic cone, at least twenty miles
from any calcareous rock. Some masses of tufaceous deposit, p