PHILOSOPHICAL SOCIETY OP WASHINGTON. 45 



89th Meeting. May 22, 18T5. 



Vice-President Hilgard in tbe Chair. 



Thirty-eight members and visitors present. . 

 The discussion of the 



CAUSES OF THE GLACIAL PERIOD 



was continued from the last meeting. 



Mr. J. W. Powell gave a description of the various gravels 

 found in the geological formations of our northwestern valleys. 

 These consisted of shore gravels on sea beds aud on lake shores ; 

 ice gravels, deposited by floating ice and by morains; sub-aerial 

 gravels formed by living streams ; desert gravels and residuary 

 gravels; pre-quarternary gravels, consisting of shore gravels, 

 floating ice gravels, sub-aerial gravels modified, and, perhaps, 

 morainal gravels modified. Without committing himself to 

 either the astronomical or geographical theory concerning the 

 origin of these, he argued that when we separate that which is 

 due to glaciation from that which is due to other causes, the 

 problem becomes greatly simplified, and there was not required 

 that great change in the meteorological condition of the earth 

 which some have supposed. 



Mr. DuTTON replied to certain points made by Mr. Taylor 

 at the previous meeting in favor of Mr. Croll's theory, and 

 argued its rejection on the ground of insufficiency. The changes 

 in amount and distribution of solar radiation dependent upon the 

 eccentricity of the earth's orbit seemed altogether too small for the 

 very large conclusions that had been drawn from them. He had 

 always supposed that the thermal equator, instead of being shifted 

 by the eccentricity of the earth's orbit, was dependent npon the re- 

 lative distribution of land and water upon the earth's surface. He 

 passed in review several points in the astronomical theory, and 

 concluded with saying that the attempt to fix definite limits of 

 time within which glacial action has taken place must always be 

 futile, as the idea of indefinite duration of time for the accom- 

 plishment of these changes was thrust upon the geologist with a 

 force he could not resist. 



