of our solar system, and of the heavens, becomes comparable with
that of the dynasties of ancient nations. If by millions of
years, the sun and stars are proportionately venerable. If by
hundreds or thousands of millions of

2

years the human mind must consent to correspondingly vast epochs
for the duration of material changes. The geological age plays
the same part in our views of the duration of the universe as the
Earth's orbital radius does in our views of the immensity of
space. Lucretius knew nothing of our time-unit: his unit was the
life of a man. So also he knew nothing of our space-unit, and he
marvels that so small a body as the sun can shed so much, heat
and light upon the Earth.

A study of the rocks shows us that the world was not always what
it now is and long has been. We live in an epoch of denudation.
The rains and frosts disintegrate the hills; and the rivers roll
to the sea the finely divided particles into which they have been
resolved; as well as the salts which have been leached from them.
The sediments collect near the coasts of the continents; the
dissolved matter mingles with the general ocean. The geologist
has measured and mapped these deposits and traced them back into
the past, layer by layer. He finds them ever the same;
sandstones, slates, limestones, etc. But one thing is not the
same. _Life_ grows ever less diversified in character as the
sediments are traced downwards. Mammals and birds, reptiles,
amphibians, fishes, die out successively in the past; and barren
sediments ultimately succeed, leaving the first beginnings of
life undecipherable. Beneath these barren sediments lie rocks
collectively differing in character from those above: mainly
volcanic or poured out from fissures in

3

the early crust of the Earth. Sediments are scarce among these
materials.[1]

There can be little doubt that in this underlying floor of
igneous and metamorphic rocks we have reached those surface
materials of the earth which existed before the long epoch of
sedimentation began, and before the seas came into being. They
formed the floor of a vaporised ocean upon which the waters
condensed here and there from the hot and heavy atmosphere. Such
were the probable conditions which preceded the birth-time of the
ocean and of our era of life and its evolution.

It is from this epoch we date our geological age. Our next
purpose is to consider how long ago, measured in years, that
birth-time was.