268 PHYSICAL GEOLOGY 



NONFRAGMENTAL DEPOSITS IN LAKES 



The more important points concerning the distribution 

 and structure of lake-laid beds of gravel, sand, mud, etc., 

 are indicated above. The leading chemical and organic de- 

 posits of lakes may be described further. 



Salt lakes. Many lakes in arid regions lose as much or 

 more water by evaporation into the dry air than they receive 

 as rain on their surfaces and as run-off from the tributary 

 slopes. Such a lake cannot maintain an outlet. Streams 

 bring minerals into the lake, often calcium carbonate, 

 gypsum, and common salt, which have been dissolved during 

 the passage of the water through or over the rocks. As 

 water is evaporated from the surface of the lake, these things 

 are left behind, and as the process continues, the waters of 

 the lake become more and more saline. When they become 

 over-saturated, the minerals begin to be precipitated from 

 solution. The deposition of calcium carbonate often pre- 

 cedes that of gypsum, which is followed in turn by common 

 salt. Certain salt lakes and seas, such as the Caspian and 

 Aral seas, represent portions of former arms of the ocean, 

 now isolated by diastrophism. Such lakes begin their 

 careers with the saltness of the sea. Most salt lakes, how- 

 ever, owe their salinity to the gradual concentration of salt 

 leached by ground waters from the rocks and brought in by 

 streams. 



One hundred pounds of average sea water contain nearly 

 3| pounds of mineral matter in solution, of which more 

 than three fourths is common salt. The waters of many 

 lakes are much salter than this. Those of Great Salt Lake, 

 for example, contain about 18 per cent by weight of dis- 

 solved salts. This dissolved material is chiefly common salt, 

 of which the lake is estimated to contain some 400,000,000 

 tons. Lake Van, in eastern Asiatic Turkey, contains 33 per 

 cent of salt, and is the densest lake known. 



Extensive salt beds which were deposited in ancient lakes 



