Page:Popular Science Monthly Volume 49.djvu/376

358 ice ages, therefore, would be marked by a length of some two hundred and fifty to five hundred feet for each on the assumed scale, and they would be separated by an interval approximately two hundred or four hundred times as long, according to the range in the estimates of the length of the intervening Mesozoic and Tertiary eras.

The chief astronomic theories of the causes of glaciation, proposed by Dr. James Croll and General A. W. Drayson, would require the frequent recurrence of glacial epochs during all the vast interval dividing the two times of actual widely extended glaciation of which geology bears record. It seems quite certain, therefore, that we must look rather to unusual conditions of the earth itself than to its astronomic relations as the causes of the Ice age.

Another theory, which supposes changes in the earth's attitude toward the sun, is the suggestion, first made in 1866 by Sir John Evans, that, while the earth's axis probably remained unchanged in its direction, a comparatively thin crust of the earth may have gradually slipped as a whole upon the much larger nucleal mass, so that the locations of the poles upon the crust have been changed, and that the Glacial period may have been due to such a slipping or transfer by which the regions that became icecovered were brought very near to the poles. The same or a very similar view has been recently advocated by Dr. Fridtjof Nansen, who writes:

The easiest method of explaining a Glacial epoch, as well as the occurrence of warmer climates in one latitude or another, is to imagine a slight change in the geographical position of the earth's axis. If, for instance, we could move the north pole down to some point near the west coast of Greenland, between 60° and 65° north latitude, we could, no doubt, produce a Glacial period both in Europe and America.

Very small changes of latitude which had been detected at astronomical observatories in England, Germany, Russia, and the United States, seemed to give some foundation for this theory, which in 1891 was regarded by a few American glacialists as worthy of attention and of special investigation by astronomers, with temporary establishment of new observatories for this purpose on a longitude about 180° from Greenwich or from Washington. During the year 1892, however, the brilliant discoveries by Dr. S. C. Chandler of the periods and amounts of the observed variations of latitude, showing them to be in two cycles respectively of twelve and fourteen months, with no appreciable secular change, forbade reliance on this condition as a cause or even as an element among the causes of the Ice age. This theory is now entirely out of the field. Sir Robert S. Ball, after reviewing Dr.