AGE OF GYPSUM OF CENTRAL IOWA 737 



in a cubic foot of sea water today is three-fiftieths of one pound. 

 The amount of water necessary to yield a cubic foot of gypsum, 

 then, is 2,333.3 cubic feet. If the sides of the containing basin 

 were vertical, the depth of the water necessary to produce fifteen 

 feet of gypsum must have been 35,000 feet. If the average 

 thickness of the Iowa gypsum be taken as fifteen feet and the 

 gypsum area seventy square miles in extent, the amount of sea 

 water necessary to deposit it, assuming that its content of gypsum 

 was the same as in sea water today, was sixty-eight trillions of 

 cubic feet. If a basin twenty miles wide be assumed, with two 

 shores sloping to a center at an angle of 10 degrees, the length 

 of the basin which would contain this amount of water must have 

 been twenty-six miles and the depth at the center more than 

 9,000 feet, diminishing uniformly in depth from the center. 

 Such a trough manifestly did not exist at the locality in question 

 at the time of the deposition, and the hypothesis that the gypsum 

 was deposited in a detached arm of the ocean, unaided by con- 

 siderable supplies of salt from rivers or from the main body of 

 salt water, is untenable. 



There remain to be considered: (i) arms of the sea which 

 were at least part of the time connected with the ocean, and 

 which received more or less water from land; (2) inclosed seas 

 fed wholly by rivers and without outlet except by evaporation. 



Taking up the second case first, it will be instructive to review 

 the conditions actually existing in inclosed salt seas in Asia and 

 America. The nature of salt deposits jnade in a lake ?iot co?mected 

 with the ocean a?id without outlet, where evaporation is as great as, 

 or greater than, inflow, may vary as widely as do the relative 

 proportions of salts in the inflowing streams. The variation in 

 the nature and amount of salts carried in solution by different 

 streams is a natural consequence of differences in the mineral 

 constitution of their drainage areas. In the Elbe and Thames 

 chlorides predominate^ (in the latter with gypsum), and the 

 evaporation of these waters would give rise to lakes containing 

 a large percentage of common salt. In the Seine, sulphate of 

 lime (gypsum) predominates, while the waters of the Rhine, 



'T. Sterry Hunt, Chemical and Geological Essays. 



