NORTH AMERICA IN THE ICE PERIOD. 9 



precipitation on the land. Hence we seem driven to the acceptance of 

 the other alternative — diminished evaporation — for the filling of the 

 reservoirs of the Great Basin. And when we search for a cause of 

 diminished evaporation only one presents itself, but that offers an easy 

 and natural solution of the problem. A depression of temperature 

 would certainly reduce evaporation (since the power of air to absorb 

 moisture varies directly with the temperature), and at the same time 

 form lakes in the valleys, and glaciers on the mountains. To prove 

 this, we have only to cite the phenomena presented by summer and 

 winter in the Western Territories. In winter the snow-fall on the 

 highlands is heavy, and the accumulation of moisture in this form is 

 large ; the skies are cloudy, and the evaporation is small. In summer 

 the sky is cloudless, the heat intense, evaporation and desiccation rapid. 

 In the spring the snows melt, flood the valleys and form temporary 

 lakes, which in midsummer dry up to playas. A climatic change 

 which would perpetuate the conditions of winter and spring would in- 

 evitably produce glaciers and lakes, and these would be in the main 

 synchronous ; and thus all we find recorded in the past history of this 

 region would be repeated. But to intensify and prolong the summer 

 w r ould not produce either lakes or glaciers. 



From the facts which have been enumerated above, it will be seen 

 that from all sides we get evidence confirmatory of the theory that 

 a certain period in the history of this continent was marked by the 

 spread of ice and snow over a very much lai-ger portion of the surface 

 than they now occupy ; and that we are fully justified in designating 

 this time as an ice or glacial period ; also that this was a period during 

 which, from some extraneous cause, the climate was made colder, and 

 the conditions which now prevail on Alpine summits perpetually, and in 

 winter elsewhere temporarily, were more wide-spread and continuous. 



That the Ice period was cold and not warm is also proved by the 

 presence of the remains of an arctic flora and fauna in all regions near 

 the old glaciers ; the arctic shells of the Champlain, the arctic plants in 

 the Quaternary clays, the reindeer, the musk-ox, the woolly elephant, 

 and woolly rhinoceros, all tell the same story. 



On the preceding pages the Ice period is spoken of as a single geo- 

 logical epoch of the Quaternary age : and so it must be reckoned in 

 any general division of geological time. But the evidence is conclu- 

 sive that the Ice period was double ; that is, there were two maxima of 

 cold separated by a long interval in which the climate was ameliorated, 

 and over large areas which had been for ages occupied by glaciers and 

 snow-fields, the ice and snow were withdrawn, and the surface was 

 covered with vegetation, again to be partially taken possession of 

 by glaciers. 



Just how far north the glaciers retreated during the interglacial 

 warm period we do not yet know, but probably not far beyond the 

 Great Lakes ; since the vegetation which covered Southern Ohio, dur- 



