THE WORK OF THE OCEAN. 391 



If the lake be bordered by a low marsh, the ice and frozen earth of 

 the latter are really continuous with the ice of the lake, and the push 

 of the latter sometimes arches up the former into distinct anticlines, 

 the frozen part only being involved in the deformation. A succession 

 of colder and less cold periods may give rise to a succession of such anti- 

 clines.^ If the shore be steep and of non-resistant material, the crowd- 

 ing of the ice produces different but not less striking results. Where 

 the thrust of the ice is against a low chff of yielding material, such 

 as clay, it disturbs all above the shore-line. Where the cHff is suf- 

 ficiently resistant, it withstands the push of the ice, and the ice itself 

 is warped and broken. 



Saline lakes. — A few lakes, especially in arid or semi-arid regions, 

 are salt, and others are '^ bitter.'^ Beside sodium chloride, salt lakes 

 usually contain magnesium chloride, and magnesium and calcium sul- 

 phates. ^'Bitter" lakes usually contain much sodium carbonate, as 

 well as some sodium chloride and sulphate, and sometimes borax. 

 The degrees of saltness and bitterness vary from freshness on the one 

 hand to saturation on the other. The water of the Caspian Sea (lake) 

 contains, on the 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 (eastern Turkestan), the densest body of water known, 

 about 33%. See accompanying table. 



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

 scended from fresh-water ancestors, while others, Uke the Caspian and Aral 

 Seas, 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 to be salt when the aridity 

 is such that evaporation from the lake exceeds its inflow. In this case 

 the inflowing waters bring in small amounts of saline and alkahne mat- 

 ter, which is concentrated as evaporation takes place. The concen- 

 tration may go on until the point of saturation is reached, or until 

 chemical reactions cause precipitation. In general the least soluble 

 minerals are precipitated first. Thus gypsum begins to be deposited 

 from sea-water when 37% of it has been evaporated; but the satu- 

 ration-point for salt is not reached until 93% of the water has been 

 evaporated (see p. 375). The relations in lakes are similar,and gypsum 



» Buckley. Wis. Acad, of Sci., Vol. XIII, Pt. I, 1900. A study of ice ramparts 

 formed about the shores of Lake Mendota, Wis., in 1898-99. 



