l springs, volcanic craters, and
other sources, until the component elements of any given number of
coal-seams had been evolved from below, without any variation taking
place in the constitution of the atmosphere. It has been too common, in
reasoning on this question, to compute the loss of carbon by the volume
of coal stored up in the ancient strata, and to take no account of the
annual gain, by the restoration of carbonic acid to the atmosphere,
through the machinery above alluded to.[331]

_Disintegrating effects of carbonic acid._--The disintegration of
granite is a striking feature of large districts in Auvergne, especially
in the neighborhood of Clermont. This decay was called by Dolomieu, "la
maladie du granite;" and the rock may with propriety be said to have
_the rot_, for it crumbles to pieces in the hand. The phenomenon may,
without doubt, be ascribed to the continual disengagement of carbonic
acid gas from numerous fissures.

In the plains of the Po, between Verona and Parma, especially at Villa
Franca, south of Mantua, I observed great beds of alluvium, consisting
chiefly of primary pebbles, percolated by spring-water, charged with
carbonate of lime and carbonic acid in great abundance. They are for the
most part incrusted with calc-sinter; and the rounded blocks of gneiss,
which have all the outward appearance of solidity, have been so
disintegrated by the carbonic acid as readily to fall to pieces.

The subtraction of many of the elements of rocks by the solvent power of
carbonic acid, ascending both in a gaseous state and mixed with
spring-water in the crevices of rocks, must be one of the most powerful
sources of those internal changes and rearrangements of particles so
often observed in strata of every age. The calcareous matter, for
example, of shells, is often entirely removed and replaced by carbonate
of iron, pyrites, silex, or some other ingredient, such as mineral
waters usually contain in solution. It rarely happens, except in
limestone rocks, that the carbonic acid can dissolve all the constituent
parts of the mass; and for this reason, probably, calcareous rocks are
almost the only ones in which great caverns and long winding passages
are found.

_Petroleum springs._--Springs of which the waters contain a mixture of
petroleum and the various minerals allied to it, as bitumen, naphtha,
asphaltum, and pitch, are very numerous, and are, in many cases,
undoubtedly connected with subterranean f