d marl, which contain large beds of rock salt. The
origin of the brine, therefore, may be derived in this and many other
instances from beds of fossil salt; but as muriate of soda is one of the
products of volcanic emanations and of springs in volcanic regions, the
original source of salt may be as deep seated as that of lava.

Many springs in Sicily contain muriate of soda, and the "fiume salso,"
in particular, is impregnated with so large a quantity, that cattle
refuse to drink of it. A hot spring, rising through granite, at Saint
Nectaire, in Auvergne, may be mentioned as one of many, containing a
large proportion of muriate of soda, together with magnesia and other
ingredients.[328]

_Carbonated springs.--Auvergne._--Carbonic acid gas is very plentifully
disengaged from springs in almost all countries, but particularly near
active or extinct volcanoes. This elastic fluid has the property of
decomposing many of the hardest rocks with which it comes in contact,
particularly that numerous class in whose composition felspar is an
ingredient. It renders the oxide of iron soluble in water, and
contributes, as was before stated, to the solution of calcareous matter.
In volcanic districts these gaseous emanations are not confined to
springs, but rise up in the state of pure gas from the soil in various
places. The Grotto del Cane, near Naples, affords an example, and
prodigious quantities are now annually disengaged from every part of the
Limagne d'Auvergne, where it appears to have been developed in equal
quantity from time immemorial. As the acid is invisible, it is not
observed, except an excavation be made, wherein it immediately
accumulates, so that it will extinguish a candle. There are some springs
in this district, where the water is seen bubbling and boiling up with
much noise, in consequence of the abundant disengagement of this gas. In
the environs of Pont-Gibaud, not far from Clermont, a rock belonging to
the gneiss formation, in which lead-mines are worked, has been found to
be quite saturated with carbonic acid gas, which is constantly
disengaged. The carbonates of iron, lime, and manganese are so
dissolved, that the rock is rendered soft, and the quartz alone remains
unattacked.[329] Not far off is the small volcanic cone of Chaluzet,
which once broke up through the gneiss, and sent forth a lava stream.

_Supposed atmosphere of carbonic acid._--Prof. Bischoff in his history
of volcanoes,[330] has shown what