[Illustration: FOSSIL SCORPION.--SILURIAN PERIOD.]
First, we have the Azoic period, _devoid of life_, as its name
signifies,--namely, the earliest stratified deposits upon the heated
film forming the first solid surface of the earth, in which no trace
of living thing has ever been found. Next comes the Silurian period,
when the crust of the earth had thickened and cooled sufficiently to
render the existence of animals and plants upon it possible, and when
the atmospheric conditions necessary to their maintenance were already
established. Many of the names given to these periods are by no means
significant of their character, but are merely the result of accident:
as, for instance, that of Silurian, given by Sir Roderick Murchison to
this set of beds, because he first studied them in that part of Wales
occupied by the ancient tribe of the Silures. The next period, the
Devonian, was for a similar reason named after the country of
Devonshire in England, where it was first investigated. Upon this
follows the Carboniferous period, with the immense deposits of coal
from which it derives its name.
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