322 MAN AND THE GLACIAL PERIOD. 



Ocean to the Canadian highlands is solved by the lava- 

 flows west of the Rocky Mountains. This immense exu- 

 dation of molten matter was accompanied by an enormous 

 liberation of heat, which must have produced significant 

 changes in the meteorological conditions. 



The moisture of the atmosphere is precipitated by 

 means of the condensation connected with a lowering of 

 its temperature. Ordinarily, therefore, when moist winds 

 from an oceanic area pass directly over a lofty mountain- 

 chain, the precipitation takes place immediately, and the 

 water finds its way back by a short course to the sea. 

 This is what now actually occurs on the Pacific coast. 

 The Sierra Nevada condense nearly all the moisture ; so 

 that very little falls on the vast area extending from their 

 summits eastward to the Rocky Mountains. All that re- 

 gion is now practically a desert land, where the evapora- 

 tion exceeds the precipitation. In Professor Carpenter's 

 view the heat radiated from the freshly exuded lava is 

 supposed to have prevented the precipitation near the 

 coast-line, and to have helped the winds in carrying it 

 farther onward to the northeast, where it would be con- 

 densed upon the elevated highlands, upon which the 

 snows of the great Laurentide Glacier were collected. 



It is not necessary for us to attempt to measure the 

 amount of truth in this subsidiary hypothesis of Professor 

 Carpenter, but it illustrates how complicated are the con- 

 ditions which have to be considered before we rest securely 

 upon any particular hypothesis. The unknown elements 

 of the problem are so numerous, and so far-reaching in 

 their possible scope, that a cautious attitude of agnosti- 

 cism, with respect to the cause of the Glacial period, is 

 most scientific and becoming. Still, we are ready to go so 

 far as to say that Mr. Upham's theory comes nearest to 

 giving a satisfactory account of all the phenomena, and it 

 is to this that Professor Joseph Le Conte gives his cau- 

 tious approval. 



