16 GLACIAL AND SUEFACE GEOLOGY OF THE PACIFIG COAST. 



area along the base of the Azoic nucleus of Northeastern America, an area 

 which has not partaken of the uplift hy which the Appalachian chain has 

 been foi'nied, or of that much more recent one which has given rise to the 

 system of the Cordilleras. These lakes are all either entirely included 

 within the Palaeozoic, or else partly in that formation and partly in the 

 Azoic, and in no case do they extend into the more recent groups of strata 

 which are found, on going south or west from the older rocks, to overlap 

 these latter formations. The orographic character of these large bodies of 

 water is therefore the most marked feature of their occurrence, and there 

 is nothing about them which renders it necessary, or even possible, to refer 

 their origin to glacial erosion ; there is nothing in their position or the di- 

 rection of their axes which in any way lends countenance to the idea of 

 their basins having been scooped out by the action of ice. 



Climatological causes act together with oi'ographic ones in the formation 

 and maintenance of the lakes in question. The depressions of the surface 

 are kept filled by the annual jirecipitation, which more than overcomes the 

 evaporation ; for the region is one of considerable rainfall, and of pretty 

 high northern latitude. The surplus water is chiefly carried off by two 

 mighty rivers, the St. Lawi^ence and the Mackenzie, of which rivers all the 

 Great Lakes — with the exception of Winnipeg and WinneiDCgosis — are 

 expansions. 



Very different from this is the condition of things in the so-called '' Great 

 Basin." Here was once a region of large lakes, many in number, and some 

 of them little, if at all, inferior in size to the Great Lakes themselves. But 

 the climate has changed ; the lakes have shrunk up and nearly or quite dis- 

 appeared, and in most cases there is nothing left but old lake bottoms, cov- 

 ered with alkaline or saline deposits in the dry season, and with mud or 

 shallow water in the wet. No one could by any possibility assign any other 

 than an orographic origin to these lake-basins ; that they are not now filled 

 with water is due to climatological causes, to the nature of which reference 

 will be made further on. 



That lakes of large area are essentially nothing more than portions of the 

 earth's surface depressed by crust movements below the genei-al level of 

 drainage, and kept full and running over by the excess of precipitation over 

 evaporation, seems to be perfectly clear ; to admit that they are the result 

 of erosive action would require us also to believe that large, nearly closed 

 seas, like Hudson's Bay and the Mediterranean, are areas of erosion ; and it 



