THE CAUSE OF THE GLACIAL PERIOD. 509 



ferring the glacial climate to conditions brought about by a 

 period of increased eccentricity of the earth's orbit from two 

 hundred and forty thousand to eighty thousand years ago, 

 which has been so ably maintained by Croll and Geikie ; and 

 some other adequate cause or causes must be sought for the 

 successive Quaternary glacial epochs of these great continental 

 areas and other districts of smaller extent both in the northern 

 and southern hemispheres ; also for the rare occurrence of gla- 

 cial conditions in various portions of the earth during past 

 geologic ages, especially in the Carboniferous and Permian 

 periods. The principal cause of all these epochs of glaciation 

 seems to the writer to be probably found 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 the process of contraction goes forward, the lateral 

 pressure bearing down some portions of the earth's surface, 

 and uplifting other extensive areas. Protuberant plateaus, 

 swept over by moisture-laden winds, would be the gathering- 

 grounds of vast ice-sheets, which would probably wax and 

 wane with the changes of the earth's attitude toward the sun, 

 by which the earth's place in any season, as summer, alternates 

 from aphelion to perihelion, and back to aphelion in cycles of 

 twenty-one thousand years. A similar explanation of the Gla- 

 cial period was long ago proposed by Lyell and Dana, but 

 without referring the elevatory movements to the earth's de- 

 formation by contraction and accumulating lateral pressure 

 while approaching an epoch of mountain -building, which 

 fundamental principle was first suggested to me in an article 

 from the pen of Professor W. 0. Crosby, on the origin and 

 relations of continents and ocean basins.* 



During the periods immediately preceding great plications 

 and shortening of segments of the earth's crust involved in the 

 formation of lofty mountain -ranges, the broad crustal move- 

 ments causing glaciation would be most wide-spread and attain 

 their maximum vertical extent. The accumulation of ice- 

 sheets may have brought about the depression of their areas. 



* ''Proceedings of the Boston Society of Natural History," 1883, vol. xxii, 

 pp. 455-460. 



