CHAPTEK XIX. 



THE CAUSE OF THE GLACIAL PERIOD, 



For the past few years speculation concerning the cause 

 of the Glacial period has been largely dominated by an 

 astronomical theory. But geologists have in general felt the 

 impropriety, in such an important matter, of abandoning 

 their own field to accept the glittering results of celestial 

 mathematics. At any rate, it would be improper for them 

 to let the astronomical solution go unchallenged. If the 

 geologist suffers himself to be lifted into the air, like the 

 fabled Antaeus, he labors at a disadvantage, and can be easily 

 overcome. For this reason the glacialists of America have, 

 of late, limited their labors chiefly to the collection of ter- 

 restrial facts, and when asked, as they often are, " TThat was 

 the cause of the Glacial period ? " the first answer has fre- 

 quently been, " That is none of our business." Still, it is by 

 the interpretation of facts that causes are discovered, and the 

 collection of facts concerning the glaciation of North America 

 has advanced so rapidly during the past few years, that it is 

 now high time to consider more fully their meaning and 

 discuss the subject anew, if for no other reason than for the 

 sake of finding out how little is known about it. 



It is easily seen that a glacier is the combined product of 

 cold and moisture. A simple lowering of the temperature 

 will not produce an ice age. Before an area can maintain a 

 glacier, it must first get the clouds to drop down a sufficient 

 amount of snow upon it. A climate which is cold and dry 

 mav not be so favorable to the production of glaciers as one 

 which is temperate, but whose climatic conditions are such 



