ily 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.

For all practical purposes, we may consider that the Aqueous
Rocks are the natural cemetery of the animals and plants of bygone
ages; and it is therefore essential that the palaeontological
student should be acquainted with some of the principal facts as
to their physical characters, their minute structure and mode of
origin, their chief varieties, and their historical succession.

The Sedimentary or Fossiliferous Rocks form the greater portion of
that part of the earth's crust which is open to our examination, and
are distinguished by the fact that they are regularly "stratified" or
arranged in distinct and definite layers or "strata." These layers
may consist of a single material, as in a block of sandstone, or
they may consist of different materials. When examined on a large
scale, they are always found to consist of alternations of layers
of different mineral composition. We may examine any given area,
and find in it nothing but one kind of rock--sandstone, perhaps,
or limestone. In all cases, however, if we extend our examination
sufficiently far, we shall ultimately come upon different rocks;
and, as a general rule, the thickness of any particular set of
beds is comparatively small, so that different kinds of rock
alternate with one another in comparatively small spaces.

[Illustration: Fig. 4.--Sketch of Carboniferous s