154 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

inference from this hypothesis would be that successive rings would
increase in density from without toward the centre, and that as each
ring became broken there would be a rearrangement of the parts
according to the density of the materials out of which it was formed.
This view is to a large extent substantiated by the fact that the planets
and their individual satellites for the most part conform to this law.
The materials of the earth thus become gradually more dense as its
centre is approached.

If we accept the nebular hypothesis and consider that condensation
and cooling have taken place, then as our globe slowly changed from
a state of igneous fusion the first rocks must have been formed by
solidification at the surface of the molten mass, while as yet the oceans
and many of the more volatile substances existed in the dense cloudy
atmosphere. Whether or not any portion of this first cooling crust
now remains where it is accessible to man is a matter of doubt. It is
probable, however, that ages must have elapsed before the crust had
so far cooled as to allow the concentration of the oceans upon it; and
ages more must have passed before this hot and chemically surcharged
ocean had so far cooled and purified itself as to allow the develop-
ment of life within it. We get a still further conception of the vast
lapses of time which these early rocks imply, when we discover that,
even after the waters had become suited for living beings, a great part
of the development and differentiation of organic life went on in be-
ings which have left no trace of their existence. Hardly a more
remarkable fact confronts us in geology than the variety and com-
plexity of types in the earliest rocks which contain any trace of life
at all. This fact, which is all the more remarkable for being attested
by the best of evidence from all parts of the earth's surface, compels us
to assign to the history of life before its first permanent record was
deposited a longer period perhaps than all that has since elapsed.
These earliest forms were either unsuited for preservation or else they
have been obliterated in the subsequent alteration of the rocks con-
taining them.

All of the oldest rocks which are to-day entirely without, or with
only slight traces of former life, are referred to the first great division