treatise of Geology, but simply to place before my readers some
pictures of the old world, with the animals and plants that have
inhabited it at various times, I shall avoid, as far as possible, all
debatable ground, and confine myself to those parts of my subject
which are best known, and can therefore be more clearly presented.

[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. Then comes the Permian period, named,
again, from local circumstances, the first investigation of its
deposits having taken place in the province of Permia in Russia. Next
in succession we have the Triassic period, so called from the trio of
rocks, the red sandstone, Muschel Kalk (shell-limestone), and Keuper
(clay), most frequently combined in its formations; the Jurassic, so
amply illustrated in the chain of the Jura, where geologists first
found the clew to its history; and the Cretaceous period, to which the
chalk cliffs of England and all the extensive chalk deposits belong.
Upon these follow the so-called Tertiary formations, divided into
three periods, all of which have received most characteristic names in
this epoch of the world's history we see the first approach to a
condition of things resembling that now prevailing, and Sir Charles
Lyell has most fitly named its three divisions, the Eocene, Miocene,
and Pliocene. The termination of the three words is made from the
Greek word _Kainos_, recent;