A CENTURY OF GEOLOGY.
By Prof. JOSEPH LE CONTE.
[Concluded.]
THE AGE OF THE EARTH.
Until almost the beginning of the present century the general belief in all Christian countries was that not only the earth and man, but the whole cosmos, began to exist about six thousand to seven thousand years ago; furthermore, that all was made at once without natural process, and have remained substantially unchanged ever since. This is the old doctrine of the supernatural origin and substantial permanency of the earth and its features. Among intelligent and especially scientific men this doctrine, even in the eighteenth century, began to be questioned, although not publicly; for in 1751 Buffon was compelled by the Sorbonne to retract certain views concerning the age of the earth, published in his Natural History in 1749.[1] Remnants of the old belief lingered even into the early part of the present century, and may even yet be found hiding away in some of the remote corners of civilized countries. But with the birth of geology, and especially through the work of Hutton in Scotland, Cuvier in France, and William Smith in England, the much greater—the inconceivably great—antiquity of the earth and the origin of its present forms, by gradual changes which are still going on, was generally acknowledged. Indeed, as already said, this is the fundamental idea of geology, without which it could not exist as a science.
Geology has its own measures of time—in eras, periods, epochs, ages, etc.—but it is natural and right that we should desire more accurate estimates by familiar standards. How old, then, is the earth, especially the inhabited earth, in years? Geologists have attempted to answer this question by estimates based on the rates of sedimentation and erosion, or else on the rate of changes of organic forms by struggle for life and survival of the fittest. Physicists have attempted to answer the same question by calculations based on known laws of dissipation of energy in a cooling body, such as the sun or the earth. The results of the two methods differ widely. The estimates of the geologists are enormous, and growing ever greater as the conditions of the problem are better understood. Nothing less than several hundred million years will serve his purpose. The estimates of the physicists are much more moderate, and apparently growing less with each revision. The latest results of King and Kelvin give only twenty to thirty millions.[2] This the geologist declares is absurdly inadequate. He can not work freely in so narrow a space—he has not elbow room.
The subject is still discussed very earnestly, but with little hope of definite conclusion. One thing, however, must be remarked. Both parties assume—the geologist tacitly, the physicist avowedly—the nebular hypothesis of the origin of the solar system, and therefore the early incandescent fluid condition of the earth as the basis of all his reasonings. Now, while this is probably the most reasonable view, it is not so certain that it can be made the basis of complex mathematical calculation. There is a possible alternative theory—viz., the meteoric theory—which is coming more and more into favor. According to this view, the planets may have been formed by aggregation of meteoric swarms, and the heat of the earth produced by the collision of the meteors in the act of aggregation. According to the one view (the nebular), the heat is all primal, and the earth has been only losing heat all the time. According to the other, the aggregation and the heating are both gradual, and may have continued even since the earth was inhabited. According to the one, the spendthrift earth wasted nearly all its energy before it became habitable or even a crust was formed, and therefore the habitable period must be comparatively short. According to the other, the cooling and the heating, the expenditure and the income, were going on at the same time, and therefore the process may have lasted much longer.
The subject is much too complex to be discussed here. Suffice it to say that on this latter view not only the age of the earth, but many other fundamental problems of dynamical geology, would have to be recalculated. The solution of these great questions must also be left to the next century. In the meantime we simply draw attention to two very recent papers on the subject—viz., that of Lord Kelvin,[3] and criticism of the same by Chamberlin.[4]