AGE OF GYPSUM OF CENTRAL IOWA 747 



140 grams of NaCl per liter the solubility of gypsum is affected 

 but slightly by changes of temperature. In a sodium-chloride 

 solution the presence of calcium carbonate up to 8-10 per cent, 

 is scarcely a factor in determining the amount of gypsum that 

 will be dissolved.^ 



At low temperature in Salt Lake a double decomposition takes 

 place between magnesium sulphate and sodium chloride, result- 

 ing in the formation of sodium sulphate which is deposited and 

 magnesium chloride which remains in solution.^ 



All of the facts cited in regard to the solubility of gypsum 

 and calcium carbonate in mixed solutions and double decom- 

 positions probably have little bearing on chemical deposits from 

 sea water, though they might be determining factors in the order 

 of deposition in inland lakes. Assuming that the composition of 

 sea water in the past did not differ greatly from that of the 

 present, in the course of its evaporation there is little reason for 

 supposing a departure from the normal order of precipitation 

 found in all salt pans where sea water is used. If a marked 

 difference in composition existed, however, it might become a 

 prime factor in the problem. Given an excess of sulphates (such 

 as NagSO^); or an excess of calcium ions (as regards calcium 

 carbonate) from calcium sulphate, and it 'tnay be found that the 

 curves plotted to illustrate the solubility of calcium carbonate 

 and calcium sulphate in mixed solutions, will cross each other in 

 such a way as to allow the calcium sulphate (gypsum) to come 

 down first. 



A change in the amount of atmospheric carbon dioxide, or a 

 considerable change in temperature, might seriously change the 

 order of deposition from that which now occurs when sea water 

 is evaporated under ordinary conditions. A further study of 

 these conditions during periods of great gypsum deposition may 

 make clear the reason for vast deposits of gypsum due to evapo- 

 ration of sea water which contain at the most only traces of 

 calcium carbonate. 



The same line of reasoning which is used to explain great 



'Cameron and Seidell, in Journal of Physical Chemistry, Vol. V, No. 9, p. 653. 

 ^Russell, Lakes of North Atnerica, p. 75. 



