CHEMICAL DEPOSITS IN LAKES. 81 



excess. In these regions alone, therefore, can salt lakes exist. Such 

 regions occur in the interior of Asia, on the plateau of Mexico, in the 

 basin of Utah, and in several other places. 



Even in case a salt lake is originally formed by the isolation of a 

 portion of sea- water, whether it remains a salt lake or gradually becomes 

 fresh will depend upon the conditions we have already mentioned. 

 For example : if the Mediterranean should be separated from the At- 

 lantic at the straits of Gibraltar, it would not only remain a salt lake, 

 but would diminish in area, and finally deposit salt. This we conclude, 

 because the water of the Mediterranean seems to be a little more salt 

 than that of the Atlantic. If, on the contrary, the Black Sea were sepa- 

 rated from the Mediterranean, or the Baltic from the Atlantic, or the 

 bay of San Francisco from the Pacific, the supply by rivers, in the case 

 of these inland seas being greater than their loss by evaporation, they 

 would rise until they found an outlet, and then would be gradually 

 rinsed out, and become fresh. Lake Champlain was, in very recent 

 geological time, an arm of the sea. When first isolated it was salt. It 

 has become fresh by this process. 



Deposits ill Salt Lakes. — The nature of the chemical deposits in salt 

 lakes will depend upon the manner in which these lakes have been 

 formed. We will take the simplest case, viz., that of a lake formed by 

 the isolation of sea- water, and its concentration by evaporation. In 

 this case the substance first deposited would be gypsum ; for this sub- 

 stance is insoluble in a saturated brine, and therefore always deposits 

 first in the artificial evaporation of sea-water in salt-making. Upon 

 the gypsum would be deposited salt. Meanwhile, however, the rivers 

 during their flood-season would bring down sediments. During the 

 flood-season, the supply of water being greater than loss by evaporation, 

 the deposit of salt or gypsum would cease ; while during the dry season 

 the deposit of sediment would cease, and the evaporation being now in 

 excess, the deposit of salt would recommence. Thus the deposits in the 

 bottom of salt lakes probably consist of alternations of salt and sedi- 

 ment, the whole underlaid by layers of gypsum. These views have been 

 confirmed by observation. During the dry season Lake Elton deposits 

 annrally a considerable layer of salt. Wells dug near the margin of 

 this lake revealed a hundred alternations of salt and mud, the salt-beds 

 being many of them eight or nine inches thick.* Most of the salt has 

 already deposited ; for the water of this lake is an almost pure bittern. 

 The great predominance of chloride of magnesium in Dead Sea water 

 shows that it is a mother-liquor, from which immense quantities of 

 common salt have already been deposited. Similar alternations, there- 

 fore, no doubt exist in the bottom of this sea.f The Great Salt Lake, 



* Bischof, Chemical and Physical Geology, vol. i, p. 405. f Ibid., p. 400. 



