358 POPULAR SCIENCE MONTHLY. 



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, pro- 

 posed by Dr. James Croll and General A. W. Drayson, would re- 

 quire the frequent recurrence of glacial epochs during all the vast 

 interval dividing the two times of actual widely extended glacia- 

 tion of which geology bears record. It seems quite certain, there- 

 fore, 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 ih the earth's atti- 

 tude toward the sun, is the suggestion, first made in 1866 by Sir 

 John Evans, that, while the earth's axis probably remained un- 

 changed in its direction, a comparatively thin ^crust of the earth 

 may have gradually slipped as a whole upon the much larger nu- 

 cleal 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 ice- 

 covered 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 occur- 

 rence 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 pur- 

 pose on a longitude about 180° from Greenwich or from Wash- 

 ington. 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 respect- 

 ively 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. 



* The First Crossing of Greenland (1890), vol. ii, p. 454. 



