tory is not yet unravelled.

Before proceeding farther, I will enumerate the geological epochs in
their succession, confining myself, however, to such as are perfectly
well established, without alluding to those of which the limits are
less definitely determined, and which are still subject to doubts and
discussions among geologists. As I do not propose to make here any
treatise of Geology, but simply to place before my readers some pictures
of the old world, with the animals and plants that 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.

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 county 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
clue 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 c