236 E. B. BKAKSON THICK GYPSUM AND SALT DEPOSITS 



basins with liighly concentrated waters instead of normal sea-water. In 

 the drying up of a large interior sea the waters might come to lie in 

 separate basins if the bottom were uneven. Evaporation over the full 

 expanse of the interior sea might be rapid enough to decrease the depth 

 and area in spite of the inflow of some stream, but when considerable 

 areas of bottom had become exposed the total evaporatiou would have 

 become less and the inflow nearer to the amount of evaporation. As- 

 suming that isolated basins would be formed, separated by low barriers, 

 and that the main streams would empty into the marginal basins, the 

 inflow might be sufiicient to cause these basins to overflow and supply 

 the inner basins, that had no direct stream connections, with highly 

 charged Avaters as fast as their own waters evaporated. As licds of gyp- 

 sum 10 feet in thickness are wide-spread, a depth of water great enough 

 In contain the salt of sea-water evaporated to deposit them must be as- 

 sumed, and the evaporation must not be carried beyond nine-tenths of 

 the original amount if the salt is to remain in solution. The depth of a 

 basin for 10 feet of gypsum would have to be at least 1,400 feet and 

 ])0ssibly 1,500. 



The concentration might have reached one-fifth the original mjIuuk' 

 of sea-water when the isolated seas were formed, and the waters contain- 

 ing .3 per cent of calcium sulphate would bring 1 foot of gypsum for 

 every 333 feet of water. If the excess evaporation from the inner pools 

 were 5 feet per year, a foot of gypsum would be brought in every 67 

 years. With the overflow of the outer basins the salinity would decrease 

 and the amount of gypsum brought in would become progressively 

 smaller; but, correcting for this, 5 feet of calcium sulphate Avould be 

 brought in in less than 400 years. The waters of the basin 1,500 feet 

 (h^ep wduhl contain al)0ut 5 feet of gypsum before the other waters came 

 ill, and 10 feet in thickness is thus accounted for. With this raj^id 

 deposition and freedom from stream inflow the deposits would l)e less 

 likely to contain impurities of a sedimentary nature than witli slower 

 deposition and inflow from streams. 



In the waters of the Caspian Sea calcium sulphate is as about 1 to 8 

 compared to salt, while in ocean water it is about as 1 to 1 7. The 

 evaporation of water of this salinity would give gypsum deposits twice 

 as thick as postulated. 



The writer has not been able to find data on the rate and completeness 

 of mixing of inflowing fresh waters with the saline waters already in the 

 basins. The surface waters of the Caspian are relatively fresh near the 

 mouths of the large inflowing rivers, but the salinity seems to be almost 

 imiform from top to bottom at no great distance from the rivers. The 



