CAUSE OF THE ICE AGE. 517 



SO ably inaintaiuod by Croll and (leikie; and some otliei- adequate cause 

 or causes nuxst be souglit for the glaeiation of these great continental areas 

 and other districts of smaller extent, both in the northern and southern 

 hemispheres, during- the Pleistocene period. 



The principal cause of the Ice age seems to tlie wi-iter to be pr(>l)ably 

 fouiid by the clew supplied in the relations already stated of the earth's 

 crust and interior whereby they become somewhat distorted from the 

 spheroidal form while tlu^ process of contraction goes forward, the lateral 

 pressure bearing down some portions of the earth's surface and uplifting 

 other extensive areas. The protuberant plateaus, swept over by moisture- 

 laden winds, would be the gathering grounds of vast ice-sheets. A similar 

 explanation of the Glacial period was long ago proposed by Lyell and 

 Dana, but without referring the elevatory movements to the earth's defor- 

 mation In- conti-action and accunudating lateral pressure while approaching 

 an epoch of mountain-building, which fundamental principle was first suo-- 

 gested to me in an article from the ])en of Prof W. O. Crosby on the origin 

 and relations of continents and ocean basins.' 



During the periods immediately preceding great plications and short- 

 ening of segments of the earth's crust involved in the formation of lofty 

 mountain ranges, the broad crustal movements causing glaciation would be 

 most widespread and attain their maximum vertical extent. The accumu- 

 lation of ice-sheets may have brought about the depression of their areas, 

 with corresponding elevation of other plateaus, which in turn would become 

 ice-covered, so that the epochs of glaciation of the Northern and Southern 

 hemispheres, or of North America and Europe, may have alternated with 

 each other.- More probably, however, as shown by the observations and 

 studies of Salisbury, Geikie, Chandjerlin, the present writer, and others, 

 the glaciations of North America and Europe were approximately synchro- 

 nous; and even the successive stages of the Ice age on these two continents 



'Proc, Boston Soc. Nat. Hist., 1883, Vol. XXII, pp. 45.5-460. 



-Compare tlio opiuious of Capt. F. W. Hiittoii, cited in A. Geikie's Text-book of Geology, 2d ed., 

 p. 912, that the former gre.ater extension of glaciers iu New Zealand was caused by au increase in the 

 elevation of the land, and that it belonged to a much earlier time than the Ice age in the northern 

 hemisphere, probably to the Pliocene period. 



