The long history of the earth and its inhabitants is conveniently
divided into eras. Thus, just as we speak of the ancient, mediæval, and
modern history of mankind, so we may speak of Palæozoic, Mesozoic and
Cenozoic eras in the history of the earth as a whole.
Geologists cannot tell us except in an approximate way how long the
process of evolution has taken. One of the methods is to estimate how
long has been required for the accumulation of the salts of the sea,
for all these have been dissolved out of the rocks since rain began to
fall on the earth. Dividing the total amount of saline matter by what is
contributed every year in modern times, we get about a hundred million
years as the age of the sea. But as the present rate of
salt-accumulation is probably much greater than it was during many of
the geological periods, the prodigious age just mentioned is in all
likelihood far below the mark. Another method is to calculate how long
it would take to form the sedimentary rocks, like sandstones and
mudstones, which have a _total_ thickness of over fifty miles, though
the _local_ thickness is rarely over a mile. As most of the materials
have come from the weathering of the earth's crust, and as the annual
amount of weathering now going on can be estimated, the time required
for the formation of the sedimentary rocks of the world can be
approximately calculated. There are some other ways of trying to tell
the earth's age and the length of the successive periods, but no
certainty has been reached.
The eras marked on the table (page 92) as _before the Cambrian_
correspond to about thirty-two miles of thickness of strata; and all the
subsequent eras with fossil-bearing rocks to a thickness of about
twenty-one miles--in itself an astounding fact. Perhaps thirty million
years must be allotted to the Pre-Cambrian eras, eighteen to the
Palæozoic, nine to the Mesozoic, three to the Cenozoic, making a grand
total of sixty millions.