744 FRANK A. WILDER 



hardly probable that the gypsum of the Coal-Measure shales was 

 formed at the time of their deposition, for the presence of great 

 numbers of ferns indicate fresh water. A more probable origin 

 lies in the action of water on pyrites, giving rise to ferrous sul- 

 phate, which in turn changed part of the lime carbonate of the 

 shales into gypsum. This may have taken place before the great 

 gypsum deposit was made, and, if so, the gypsum dissolved out 

 of the Coal Measure shales may have been sufficient to form it. 

 The same waters which carried the gypsum would, however, carry 

 much lime carbonate and mud, and it is difificult to conceive of 

 fifteen feet of pure gypsum forming in an inclosed basin fed by 

 streams. It is true that at the top in one or two localities thin 

 layers of limestone, sandstone, or shale occur with thin layers 

 of gypsum, but the presence of fifteen feet of gypsum containing 

 only I per cent, of sand and clay practically precludes the pos- 

 sibility of its origin in an inland basin fed by land streams. 



Turning to the "Mediterranean hypothesis," there are two 

 apparent difificulties. In the series of deposits due to deposition 

 on account of evaporation in such a basin, limestone would be 

 the lowest member. If the amount of calcium carbonate in the 

 waters tributary to the basin was small, limestone might not 

 appear beneath the gypsum as a distinct formation, but, mixed 

 with the finer impurities, would still be present as a notable cal- 

 careous element in a clay or shale. The Iowa gypsum overlies 

 a fire clay, the analysis of which shows but a very limited amount 

 of lime. Moreover, the lime carbonate in the inflowing water, 

 after the density necessary for the deposition of gypsum had 

 been reached in the basin, would, it would seem, be deposited 

 with the gypsum. The phenomena observed about Great Salt 

 Lake perhaps relieve us of these difificulties. As already stated, 

 the water of the lake is almost free from calcium carbonate, 

 while deposits of calcareous tufa and oolite have been and still 

 are forming along the shores where water action is violent. 

 This localizing of the calcium carbonate, if it were complete, 

 would render possible deposits of pure gypsum like that of Iowa, 

 in which no calcium carbonate appears. Unfortunately, calcium 

 carbonate due to precipitation from solution appears widely dis- 



