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 strata at Kinghorn,
in Fife, showing stratified beds (limestone and shales) surmounted
by an unstratified mass of trap.
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