358 PRINCIPLES OF STRATIGRAPHY 



97.82 per cent., and below this lies a coarsely crystalline salt 1.5 m. 

 thick and of perfect transparency containing 98.7 per cent. NaCl. 

 The upper layers consist of small crystals which are dissolved dur- 

 ing the spring rains. The larger crystals of the deeper layers re- 

 main undissolved and further increase in size during the summer, 

 and thus the deeper layers are constantly becoming poorer in mag- 

 nesium chloride and as the salt becomes purer it also increases in 

 density. 



This region is characterized by more than a hundred lakes of 

 varying salinity, many of which deposit very pure rock salt. Many 

 of these are covered by drifted sands which have dried up the lakes 

 and drawn away the bitterns, so that the salt under the sand is per- 

 fectly dry and very pure. Some of these lakes of the Astrakhan 

 district deposit magnesium chloride, others magnesium or sodium 

 sulphate, and others still, gypsum. 



South of the Caspian, in Persia, is an extensive desert nearly 800 

 miles in length and in places over 200 miles in width, covered with 

 Salinas and dry salt, and dune sand areas. Of the salinas, that of 

 Darya in Namak may be noted. For 2 km. from the margin 

 of the salina the ground is muddy and contains the skeletons of 

 animals which probably died of thirst ; beyond this a zone of earthy 

 salt follows, with the appearance of a frozen swamp during a thaw. 

 Six to eight kilometers from the border begins the pure salt, in the 

 form of irregular polygonal masses 20 to 90 cm. in length. The 

 crust of salt beyond this extends for 40 km. and is from 2 to 3 

 meters thick, and very hard. 



The Great Salt Plain of Lop, in Eastern Turkestan, is described 

 by Huntington (28) as resembling "the choppiest sort of sea with 

 whitecaps a foot or two high ; and frozen solid." He says : "When 

 we camped in what we hoped was a soft spot and tried to drive in 

 the iron tent-pegs, most of them bent double. We had to use an 

 axe to hew down hummocks of rock salt a foot high before we 

 could get places smooth enough for sleeping" (p. 251). The rough- 

 ness of this salt plain is explained as follows : "During the long- 

 continued process of drying up the ancient lake of Lop deposited 

 an unknown thickness of almost pure rock salt. When the salt 

 finally became dry it split into pentagons from five to twelve feet 

 in diameter, the process being similar to that which gives rise to 

 mud-cracks. The wind, or some other agency, apparently de- 

 posited dust in the cracks ; when rain or snow fell, the moisture 

 brought up new salt from below ; and thus the cracks were solidly 

 filled. When next the plains became dry the pentagons appeared 

 again. This time the amount of material was larger and the penta- 



