LITMIR.BIZ

Скачать книгу

dated its beginning with the advent of

       man, swept utterly away this beautiful imagining. We can, indeed,

       find no beginning of the world. We trace back events and come to

       barriers which close our vista—barriers which, for all we know,

       may for ever close it. They stand like the gates of ivory and of

       horn; portals from which only dreams proceed; and Science cannot

       as yet say of 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

      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.

      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).

      4

      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. We go all

       over the explored world, recognising the successive deposits by

       their fossils and by their stratigraphical relations, measuring

       their thickness and selecting as part of the data required those

       beds which we believe to most completely represent each

       formation. The total of these measurements would tell us the age

       of the Earth if their tale was indeed complete, and if we knew

       the average rate at which they have been deposited. We soon,

       however, find difficulties in arriving at the quantities we

       require. Thus it is not easy to measure the real thickness of a

       deposit. It may be folded back upon itself, and

Скачать книгу