The Outline of Science, Volume 1

The long history of the earth and its inhabitants is conveniently divided into eras. Thus, just as we speak of the ancient, medi val, and modern history of mankind, so we may speak of Pal ozoic, Mesozoic and Cenozoic eras in the history of the earth as a whole. Geologists cannot tell us except in an approximate way how long the process of evolution has taken. One of the methods is to estimate how long has been required for the accumulation of[Pg 90] the salts of the sea, for all these have been dissolved out of the rocks since rain began to fall on the earth. Dividing the total amount of saline matter by what is contributed every year in modern times, we get about a hundred million years as the age of the sea. But as the present rate of salt-accumulation is probably much greater than it was during many of the geological periods, the prodigious age just mentioned is in all likelihood far below the mark. Another method is to calculate how long it would take to form the sedimentary rocks, like sandstones and mudstones, which have a total thickness of over fifty miles, though the local thickness is rarely over a mile. As most of the materials have come from the weathering of the earth's crust, and as the annual amount of weathering now going on can be estimated, the time required for the formation of the sedimentary rocks of the world can be approximately calculated. There are some other ways of trying to tell the earth's age and the length of the successive periods, but no certainty has been reached. The eras marked on the table (page 92) as before the Cambrian correspond to about thirty-two miles of thickness of strata; and all the subsequent eras with fossil-bearing rocks to a thickness of about twenty-one miles—in itself an astounding fact. Perhaps thirty million years must be allotted to the Pre-Cambrian eras, eighteen to the Pal ozoic, nine to the Mesozoic, three to the Cenozoic, making a grand total of sixty millions.