Page:Popular Science Monthly Volume 16.djvu/857

 Besides, such a collocation affords a sufficient period for the development of the rich and characteristic fauna and flora of the forest-bed—a widespread fossiliferous and carbonaceous deposit that must represent a lapse of time of geological importance. The belief that any glacier would necessarily remove all the débris formed by an antecedent one—a belief which has done much to prevent the acceptance of theories of successive glaciation—is best shown to be fallacious by the unequivocal evidence that the ice of the last glacial epoch did pass over older deposits of unconsolidated materials without removing them. Some such evidence is cited by Dr. Croll in the last-named chapters.

Among the interesting cognate subjects taken up in the remaining eleven chapters of the work are—methods of measuring the rate of subaërial denudation, and of determining the mean thickness of the rocks of the globe; the age and origin of the sun (which is an able effort to reconcile the existing disagreement between the geologist and the astronomer and physicist as to the age of the earth); the physical cause of continental submergence and emergence during the glacial period; the influence of the obliquity of the ecliptic on terrestrial climate; some glacial phenomena of Scotland and England; and the physical cause of glacier-motion. Appendices and an index are added.

It has been the aim in the foregoing pages to convey a general idea of the nature and scope of the work under review, and at the same time to indicate those points which do not seem to be sustained; and, as is natural in view of this double object, justice has not been done to the work as a whole. "Climate and Time" represents years of study and an almost incredible amount of conscientious labor by perhaps the most competent living man to deal with this obscure subject, which occupies a position intermediate between geology, physics, and astronomy, and requires a thorough knowledge of all of these branches of science for its adequate comprehension. As a geologist, Dr. Croll occupies an important and responsible position; and, as an astronomer and physicist, his reputation in scientific circles is even more enviable. Owing to the confusion in which he found the subject, to the absence of reliable data, and perhaps to a rather radical disposition, he seems to have fallen into a few errors; but, with some reservations, his ingenious theory has been received with much favor, and has been pretty widely adopted, especially on the other side of the Atlantic. Here, it is comparatively unknown, and, in too many cases, lack of acquaintance with the principles on which it is based has led to its being unfavorably regarded; but even those who reject the theory would do well to familiarize themselves with its details before they undertake to investigate the subject anew.