232 E. P>. BRANSON THICK GYPSUM AND SALT DEPOSITS 



salts in the deposits; 3. In accounting for the absence of salt deposits 

 above the gypsum; 4. In explaining the absence of sedimentary impuri- 

 ties; and 5. In accounting for the absence of fossils. The diflBculties in 

 explaining thick deposits of almost pure salt are : 1. Prohibitive depth of 

 water required to contain the salt in solution; 2. Lack of alternation of 

 salt and gypsum in many thick deposits ; 3. Thick salt deposits without 

 gypsum below ; 4. Absence of fossils in the deposits. 



Some Conditions for Salt and Gypsum Deposition 



Sea-water contains 1.7488 parts per thousand calcium sulphate in 

 solution, but as salt begins to precipitate rapidly soon after 1.4 parts of 

 gypsum have been deposited, only about 1.4 parts per thousand is avail- 

 able for deposition in pure gypsum beds. Usiglios' experiments^ show 

 gypsum beginning to deposit from sea-water after .81 of the volume has 

 been removed by evaporation and sodium chloride beginning to precipitate 

 after .905 has been removed in that way. The total amount of gypsum 

 precipitated in the evaporation from .19 original volume to .095 original 

 volume is 1.466 parts per thousand. As gypsum contains 18 parts water 

 for every 127 parts calcium sulphate and its specific gravity is slightly 

 above 2.3, the volume of gypsum pi'ecipitated would be about as 1 to 2 

 compared to the weight of calcium sulphate in solution. From 1,000 

 feet of normal sea-Avater about .7 feet of gypsum would be precipitated 

 before the point of saturation for sodium chloride would be reached, and 

 it would require a depth of 57,000 feet of water for 40 feet of gypsum. 

 Thick deposits would probably result from the drying up of extensive 

 interior seas, and M'ith the drying up the waters would occupy smaller 

 and smaller areas and become more and more concentrated. Gypsum 

 would not begin to precipitate until four-fifths of the water had been 

 evaporated, and the depth of -water to contain 40 feet would need to be 

 only about 11,500 feet; but this is still much greater than the depth of 

 any continental depressions. 



The bar theory of Ochsenius helps this out to some extent, for if a 

 new supply of sea-water were brought in over a bar as fast as evaporation 

 took place, not all of the water would need to be in the basin at the same 

 time, and the minimum depth of the basin would be regulated finally 

 by the water required to keep the sodium chloride in solution. As sodium 

 chloride begins to deposit when sea-water has been reduced by evapora- 

 tion to .095 its original volume, the evaporation of the 57,000 feet must 



» stated In several recent English works. See U. S. Geol. Surv. Bull. 491. Clarke : 

 The data of geochemistry, p. 208, and Grabau : Principles of stratigraphy, p. 349. 



