In the first
division are the _Igneous Rocks_--such as the lavas and ashes of
volcanoes--which are formed within the body of the earth itself,
and which owe their structure and origin to the action of heat.
The Igneous Rocks are formed primarily below the surface of the
earth, which they only reach as the result of volcanic action;
they are generally destitute of distinct "stratification," or
arrangement in successive layers; and they do not contain fossils,
except in the comparatively rare instances where volcanic ashes
have enveloped animals or plants which were living in the sea
or on the land in the immediate vicinity of the volcanic focus.
The second great division of rocks is that of the _Fossiliferous,
Aqueous_, or _Sedimentary_ Rocks. These are formed at the surface
of the earth, and, as implied by one of their names, are invariably
deposited in water. They are produced by vital or chemical action,
or are formed from the "sediment" produced by the disintegration
and reconstruction of previously existing rocks, without previous
solution; they mostly contain fossils; and they are arranged
in distinct layers or "strata." The so-called "aerial" rocks
which, like beds of blown sand, have been formed by the action
of the atmosphere, may also contain fossils; but they are not
of such importance as to require special notice here.
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