136 PHYSICAL GEOLOGY 



" bittern," contains chiefly Epsom and Glauber salts. The Dead 

 Sea is such a lake. 



(3) The salt of some salt lakes has been attributed to an accumu- 

 lation of wind-blown salt. Perhaps the best example of a salt lake 

 in which this origin is evident is furnished by a lake in northern 

 India (Sambhar Lake). This lake is situated in an inclosed basin 

 more than 400 miles inland and appears to receive the greater part, if 

 not all, of its salt from dust-laden winds which sweep over the plains 

 between it and an arm of the sea during the dry months. Analysis 

 of the air during the dry season shows that at least 3000 metric tons 

 of salt are carried over the lake annually, an amount sufficient to 

 account for the accumulations of salt in the lake. 



Alkaline Lakes. — Alkaline and borax lakes differ from salt lakes 

 in that they contain a predominance of sodium carbonate or borax. 

 The source of this carbonate and borax, as in the case of common 

 salt, is the rocks over which the streams which feed such lakes flow. 



Origin of Rock Salt. — Deposits of salt underlie many hundreds 

 of square miles of sedimentary rocks in New York and other states. 

 The thickness of the salt beds varies greatly, the thickest reported 

 in New York consisting of 325 feet of solid salt. The greatest salt 

 deposit known is that at Stassfurt, Germany, which is 4794 feet deep. 

 Since salt and gypsum occur together, it is believed that such deposits 

 have been formed as a result of the evaporation of salt lakes. One 

 objection to this theory is the great thickness of some beds and their 

 purity. In the case of such deposits it is believed that an estuary or 

 lagoon was separated from the sea by a bar over which water was 

 carried during storms or perhaps at high tide. If the region in which 

 this occurred was hot and arid, it is conceivable that salt might be 

 deposited to the depth of the lagoon or estuary. If such a basin 

 should slowly subside, a bed of salt of great thickness could result. 

 Such remarkably thick deposits as those in Louisiana, where the 

 bottom has not been reached at a depth of 2000 feet, requires a still 

 further modification of the theory. 



Extinct Lakes. — Upon their disappearance lakes leave behind 

 them proofs of their former existence. If they were of comparatively 

 short duration, as would be the case if they had been formed by ice 

 jams (p. 186), their former presence might be attested by (1) the 

 deltas deposited by the streams which flowed into them, as well as 

 by (2) the stratified sand and clay which were spread over their beds. 

 When their life was long, (3) wave-cut terraces, (4) sand bars and 



