AGE OF GYPSUM OF CENTRAL IOWA 743 



It is quite conceivable that the opening could be so restricted 

 that the outflow would be greatly diminished and the density of 

 1.05 to I.I 3 which is necessary for the deposition of limestone 

 be reached. If this were maintained for a long time and the 

 outflow were enough to prevent further concentration, a thick 

 bed of limestone without gypsum and salt would be formed. 

 If the opening were still further restricted, gypsum would be 

 precipitated and at length salt. In this case, however, the cal- 

 cium carbonate in the inflowing sea water would be precipitated 

 with the gypsum, unless converted into gypsum or a more soluble 

 salt by reaction with other salts, or isolated during deposition, as 

 is the case today in Great Salt Lake. The amount of the calcium 

 carbonate (one-tenth as much as the gypsum), if present, would 

 be easily recognized. If, instead of a small opening, the inland 

 sea were shut off from the ocean by a low barrier, over which the 

 sea water passed only in time of great storms, the deposits might 

 be more varied. The water would be diluted at times so that 

 precipitation of the more soluble salts would cease, and after a 

 period of evaporation, if the amount of calcium carbonate in the 

 newly added water were considerable, there would be a deposit 

 of limestone succeeded by gypsum. A series of limestone and 

 gypsum beds occurs in the northern peninsula of Michigan near 

 St. Ignace. 



In applying "inclosed sea" conditions like those now prevail- 

 ing about Great Salt Lake to the Iowa gypsum, two questions 

 arise. Was there a supply of gypsum in the rocks of the region 

 subject to the solvent action of stream water sufficient to yield 

 the existing deposit? If this question may be answered in the 

 affirmative, do the deposits formed in inclosed seas structurally 

 and chemically resemble those of Webster county? The Coal- 

 Measure shales and sandstones, with here and there a limited 

 area of St. Louis limestone, formed the land surface when the 

 gypsum was deposited. There is a considerable amount of gyp- 

 sum in all of these strata which appears frequently in large 

 selenite crystals. Rivers flowing over this surface would carry a 

 large percentage of gypsum in solution, provided the gypsum 

 now contained in these strata was present at that time. It is 



