32 . LAKES OF NORTH AMERICA. 



Basins of small size, due to chemical precipitation, occur in connection 

 with springs that deposit calcareous tufa or siliceous cinter. Many ex- 

 amples of pools formed in this way occur in the Yellowstone National 

 Park and in other hot spring regions of the Cordilleras. Near the west 

 shore of Mono lake, California, there is a castle-like bowl of calcareous 

 tufa, fully 50 feet high and from 150 to 200 feet in diameter, with several 

 long aqueduct-like branches, which was formed from the water of a spring 

 that has now ceased to flow. Far out on the desert valleys of Utah and 

 Nevada one sometimes finds circular basins with rims of tufa from a few 

 inches to three or four feet high, and holding beautifully clear water with 

 a temperature approaching the boiling point. In other instances, these 

 deposits rise several feet above the adjacent surface and resemble volcanic 

 craters. In their summits there are frequently steaming caldrons. 



In regions underlain by gypsum, rock salt, and other easily soluble 

 substances, depressions are formed on account of the removal in solution 

 of the rocks beneath and the subsidence of the surface. 



Gypsum is thought by some geologists to owe its formation to the 

 alteration of limestone by the passage through it of sulphurous gases or 

 of sulphurous waters. When this occurs, the volume of the deposit is 

 increased and the ground above may be elevated into mounds, and thus 

 obstruct the drainage. 



CONCLUSION. 



In this chapter an attempt has been made to describe briefly the 

 principal types of lake basins occurring in North America, to indicate the 

 processes by which they have been formed, and to show to some extent, 

 where they severally belong in the history of topographical development. 



Many basins have resulted from the action of more than one agency, 

 and in not a few instances several agencies have cooperated in their 

 production. Basins of a composite character have thus originated, but 

 the principal cause leading to their existence is usually so pronounced 

 that when carefully studied, they may without great violence be referred 

 to some one of the types here described. 



The study of lakes has shown that they frequently have a long and 

 varied history, which is no less interesting and instructive than the story 

 of the origin and decadence of the hills that are reflected in their glassy 

 depths. Some of the phases of their not uneventful lives are described 

 in the succeeding chapters. 



