But all of the foregoing is fundamentally part of
this department of knowledge and it is absolutely essential for any one
who desires to understand what the fossils themselves demonstrate.
The oldest sedimentary rocks are devoid of fossil remains and so they are
called the Azoic or Archaean. They comprise about 30,000 feet of strata
which seem to have required at least 20,000,000 years for their formation.
This period is roughly two-fifths of the whole time necessary for the
formation of _all_ the sedimentary rocks, and this proportion holds true
even if the entire period of years should be taken as 100,000,000 instead
of 50,000,000 or less. The earth during this early age was slowly
organizing in chemical and physical respects so that living matter could
be and indeed was formed out of antecedent substances--but this process
does not concern us here. The important fact is that the second major
period, called the Palaeozoic, or "age of ancient animals," saw the
evolution of the lowest members of the series,--the invertebrates,--and
the most primitive of the backboned animals, like fishes and amphibia. The
rocks of this long age include about 106,000 feet of strata, demanding
some 21,000,000 or 22,000,000 years for their deposition.
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