Page:Popular Science Monthly Volume 16.djvu/854

820 any were seriously affected by the cause specified. It must be admitted, however, that the physical agencies which have already been described seem competent to inaugurate a glacial epoch during any period of high eccentricity of the terrestrial orbit, without any assistance from the meteorological influences. These periods, recurring at long and irregular intervals, must have alternately refrigerated and revivified the circumpolar regions again and again during the immeasurable ages whose lapse is so dimly recorded in the rocky strata. The theory is, therefore, in the highest accord with the modern uniformitarian doctrine which rejects all hypothetical explanations of phenomena which are not in harmony with the present course of nature.

The present low temperature of the southern hemisphere as compared with the northern is explained by the assertion that the warm waters of the southern hemisphere are borne into the northern, while the cold waters of the northern hemisphere are conveyed into the southern. The argument supporting this conclusion is that of a radical advocate of the wind theory, and is as strongly opposed to the first principles of the physical theory of climate as the most conservative critic could wish. Space will not permit the discussion of this chapter.

Chapter XVIII. contains a résumé of the evidence of former glacial eras which had been collected up to the time of the preparation of the volume. There had already been placed on record more or less decisive evidence of former glaciers, not only in the Quaternary but in the Miocene, the Eocene, the Cretaceous, the Oölitic, the Permian, the Carboniferous, the Old Red Sandstone, the Silurian, and even the Cambrian. In some of these cases, notably in the formations of the Permian age, the evidence is so voluminous, so distinct, and from such widely separated localities, that it seems impossible not to conclude that our Pleistocene ice age was but the homologue of long antecedent secular winters. The reasons for the paucity of evidence regarding these early glacial eras are summarized in the preceding chapter.

A statement of the method of computing the eccentricity of the terrestrial orbit, elaborate tables (laboriously computed by the author, with the exception of about a dozen periods) showing the eccentricity for 3,000,000 years in the past and 1,000,000 years in the future, and conclusions as to the probable date of the glacial epoch, constitute Chapter XIX. It has since been pointed out, by an undoubted authority in such matters (Professor Newcomb), that Leverrier's formulæ, which were employed in making the computations embraced in the tables, are defective, and hence that the figures given are not rigidly correct; but for the present these minor inaccuracies may be disregarded. The dates given may, therefore, be assumed to be correct, though they are undoubtedly only approximations.

It has already been stated that periods of high eccentricity occurred 210,000 and 850,000 years ago, respectively. Sir Charles Lyell, the founder of the uniformitarian school of geology, was inclined to