The Birth-Time of the World | Page 4

this or
that dream if it proceeds from the gate of horn or from that of ivory.
In short, of the Earth's origin we have no certain knowledge; nor can
we assign any date to it. Possibly its formation was an event so gradual
that the beginning was spread over immense periods. We can only trace
the history back to certain events which may with considerable
certainty be regarded as ushering in our geological era.
Notwithstanding our limitations, the date of the birth-time of our
geological era is the most important date in Science. For in taking into
our minds the spacious history of the universe, the world's age must
play the part of time-unit upon which all our conceptions depend. If we
date the geological history of the Earth by thousands of years, as did
our forerunners, we must shape our ideas of planetary time accordingly;
and the duration 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
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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
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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.

That the geological age of the Earth is very great appears from what we
have already reviewed. The sediments of the past are many miles in
collective thickness: yet the feeble silt of the rivers built them all from
base to summit. They have been uplifted from the seas and piled into
mountains by movements so slow that during all the time man has been
upon the Earth but little change would have been visible. The
mountains have again been worn down into the ocean by denudation
and again younger mountains built out of their redeposited materials.
The contemplation of such vast events
[1] For a description of these early rocks, see especially the monograph
of Van Hise and Leith on the pre-Cambrian Geology of North America
(Bulletin 360, U.S. Geol. Survey).
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prepares our minds to accept many scores of millions of years or
hundreds of millions of years, if such be yielded by our calculations.
THE AGE AS INFERRED FROM THE THICKNESS OF THE
SEDIMENTS
The earliest recognised method of arriving at an estimate of the Earth's
geological age is based upon the measurement of the collective
sediments of geological periods. The method has undergone much
revision from time to time. Let us briefly review it on the latest data.
The method consists in measuring the depths of all the successive
sedimentary deposits where these are best developed.