How do we know when the various classes of animals and plants were
established on the earth? How do we know the order of their appearance
and the succession of their advances? The answer is: by reading the Rock
Record. In the course of time the crust of the earth has been elevated
into continents and depressed into ocean-troughs, and the surface of the
land has been buckled up into mountain ranges and folded in gentler
hills and valleys. The high places of the land have been weathered by
air and water in many forms, and the results of the weathering have been
borne away by rivers and seas, to be laid down again elsewhere as
deposits which eventually formed sandstones, mudstones, and similar
sedimentary rocks. Much of the material of the original crust has thus
been broken down and worked up again many times over, and if the total
thickness of the sedimentary rocks is added up it amounts, according to
some geologists, to a total of 67 miles. In most cases, however, only a
small part of this thickness is to be seen in one place, for the
deposits were usually formed in limited areas at any one time.