336 GEOLOGY 



average, less salt than that of the sea; that of Great Salt Lake 

 contains about 18%; that of the Dead Sea, about 24%; and that 

 of Lake Van, in eastern Turkestan, the densest body of water 

 known, about 33%. 



Many salt lakes, such as the Dead Sea and Great Salt Lake, 

 are descended from lakes which were fresh, while others, like the 

 Caspian Sea, are probably isolated portions of the ocean. Lakes 

 of the former class have usually become salt through a decrease in 

 the humidity of the region where they occur. The water begins 



Fig, 271. Tufa domes, Pyramid Lake, Nevada. (Fairbanks.) 



to be salty when the aridity is such that evaporation from the lake 

 exceeds its inflow. The inflowing waters bring in small amounts 

 of saline and alkaline matter, which is concentrated as evaporation 

 takes place. The concentration may go on until saturation is 

 reached, or until chemical reactions cause precipitation. 



Deposits of salt and other mineral matters once in solution are 

 making in some salt lakes at the present time, and considerable 

 formations of the same sort have been so made in the past. Buried 

 beneath sediments of other sorts, beds of common salt or of other 

 precipitates are preserved for ages. Lime carbonate has been 

 precipitated in quantity from some extinct lakes (Fig. 271). 



Lakes which originate by the isolation of portions of the sea 

 are salt at the outset. If inflow exceeds evaporation, they become 



