Piliary,

JAN, 811900

WEST JOL

Branch,

VOL. XLIX.

No. 1.

Aside from the origin of the earth there is perhaps no question relating to our planet of more general interest than its age. At the outset let it be understood that we can not hope to make anything like an exact statement of this, and that we must content ourselves with a rough approximation.

To determine this estimate we have at present two methods, one of these may be styled the Geological, since it is based on data pertaining to that science; and the other the Physical, because it uses the principles of Physics. How these estimates are actually made will be stated later in this paper. Let us first examine the estimates themselves, as made by the followers of these two sciences.

THE GEOLOGISTS' ESTIMATES.

James Croll thought 72,000,000 years have elapsed since the deposition of the first sedimentary

rocks (sandstones, limestones, shales, etc.); while Sir Alfred R. Wallace thought 28,000,000 years sufficient time to allow. Some years ago Professor Alexander Winchell announced 3,000,000 years as the time required, this being the lowest geological estimate of which the writer has knowledge. In 1893 Mr. C. D. Walcott, the director of the United States geological survey, announced 45,000,000 as the time necessary for the formation of the sedimentary rocks; and in the same year Mr. W. J. McGee stated that in his opinion, 6,000,000,000 years are necessary. These figures show two things: (1) That geologists themselves vary greatly in their estimates of the age of the earth; and (2) that our planet is very old, for if the smallest of the above

figures should be adopted even then our planet would have an antiquity greater than would have

been thought possible a few generations ago.

Pro

THE PHYSICISTS' ESTIMATES. Lord Kelvin, who has taken the lead in this mode of computations, announced in 1862 that the crust of the earth could not have been formed less than 20,000,000 years ago nor more than 400,000,000 years ago. At the same time he stated that 100,000,000 would be sufficient to include the total geological history of our planet. But later, after reviewing the evidence, he announced that only about 20,000,000 years could be granted. Professor Tait still further restricts geological time, and can not allow more than 10,000,000 years. fessor Simon Newcomb, the eminent American astronomer, states that owing to the great heat of the early crust water in the liquid form has not existed on our planet more than 10,000,000 years, and he estimates the age of the solar system at about 20,000,000. Professor George Darwin has computed that the moon separated from the earth about 57,000,000 years ago, and since this happened while the planet was still in a liquid condition it was long before the beginning of geological time, which, we may say, dates from the formation of a solid crust.

HOW THE PHYSICIST HAS CHECKED

THE GEOLOGIST.

If the estimates of the geologists and physicists just named be

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examined, it will be found that those of the former are generally much larger than those of the latter. In fact until comparatively recent years geologists were customed to deal with time as of unlimited extent. If 100,000,COO years sufficed for the production of a certain phenomenon, very good; but if it did not, then this period or trebled. might be doubled Thus it happened that the geologist became accustomed to dealing with vast periods, and time became to him an important item of his stock in trade.

But in the sixties the physicist began to call a halt to these heavy demands on the "bank of time.' Instead of geological time being of unlimited extent it was declared to be just the reverse; and while it is doubtless very long when judged by human standards, it is nevertheless short when compared to the periods demanded by the geologists. Not content, however, with this general restriction, the physicist has made reduction after reduction in his estimates, until the time allowed for the evolution of our planet appears altogether inadequate to the geologist.

While these restrictions can hardly be accepted by the geologist, it must be admitted that they have performed a useful service in restraining him from the use of vast ages for which there is neither authority nor necessity.