These shells belong mostly to marine testacea, but in some places exclusively to
forms characteristic of lakes and rivers. Hence it is concluded that some
ancient strata were deposited at the bottom of the sea, and others in lakes and
estuaries.

We have now pointed out one great class of rocks, which, however they may vary
in mineral composition, colour, grain, or other characters, external and
internal, may nevertheless be grouped together as having a common origin. They
have all been formed under water, in the same manner as modern accumulations of
sand, mud, shingle, banks of shells, reefs of coral, and the like, and are all
characterised by stratification or fossils, or by both.

VOLCANIC ROCKS.

The division of rocks which we may next consider are the volcanic, or those
which have been produced at or near the surface whether in ancient or modern
times, not by water, but by the action of fire or subterranean heat. These rocks
are for the most part unstratified, and are devoid of fossils. They are more
partially distributed than aqueous formations, at least in respect to horizontal
extension. Among those parts of Europe where they exhibit characters not to be
mistaken, I may mention not only Sicily and the country round Naples, but
Auvergne, Velay, and Vivarais, now the departments of Puy de Dome, Haute Loire,
and Ardeche, towards the centre and south of France, in which are several
hundred conical hills having the forms of modern volcanoes, with craters more or
less perfect on many of their summits. These cones are composed moreover of
lava, sand, and ashes, similar to those of active volcanoes. Streams of lava may
sometimes be traced from the cones into the adjoining valleys, where they have
choked up the ancient channels of rivers with solid rock, in the same manner as
some modern flows of lava in Iceland have been known to do, the rivers either
flowing beneath or cutting out a narrow passage on one side of the lava.