AGE OF THE EARTH. The evidence from geology all points toward an age for the earth to be reckoned in millions of years. One line of evi dence upon which this conclusion is based may be illustrated as follows: there are, in some places, great accumulations of rock layers which were deposited in the ocean. These layers are known to reach a depth of many thousands of feet, in some instances over 40,000 feet. A study of these beds indicates that they were accumulated slowly, as similar beds of limestone, clay, sand, and gravel are now being accumulated in the sea. If anything like the present rate prevailed, the time required for their formation is very great, probably not less than 100,000,000 years. This estimate is, of course, open to doubt because of the question whether the past and the present have been so closely alike; but even if this doubt is warranted, the deduction must still be made that the age of the earth is very great. From a study of the wearing away the land, and the planing down of mountains, a similar conclusion may be reached. A second class of evidence pointing to a great age for the earth is supplied from a study of the fossils pre served in the rocks. The evolution of plant and animal life seems, in general, to have been gradu al, as it is in the present time; and this conclu sion harmonizes with the evidence from the rocks themselves.
Physicists have also estimated the age of the earth in several ways. One of these estimates is
based on the rate of cooling of the heated interior of the earth. Another estimate is based on the effect of the tides in retarding the rotation of the earth by the friction of the tide wave. Still a third line of argument is based upon the rate of cooling of the sun, whose light, according to Lord Kelvin, will not last more than five or six million years longer. The facts concerning the earth's heat, the sun's heat, and the earth form, together with the rate of cooling of the sun and the earth and the effect of tidal friction, have led Lord Kelvin and other physicists to the con clusion that the age of the earth is not greater than 20,000,000 years. Great though this esti mate of time is, it is not great enough to satisfy geologists; for the evidence from geology seems to point to a far longer history for the earth.
From what has been said, it is evident that we are not in a position to state even approximately the age of the earth in years; but all lines of evidence agree in pointing to the conclusion that geological time is to be reckoned in millions of years; and geologists are practically unanimous in the belief that the time since the oldest stratified rocks were deposited cannot be much less than 100,000,000 years.