748 FRANK A. WILDER 



deposits of gypsum may be applied to many limestones. Cal- 

 cium carbonate in sea water is one-tenth as abundant as cal- 

 cium sulphate, and for every twenty feet of gypsum two feet 

 of limestone must be precipitated, unless the calcium carbonate 

 is converted into some other substance. Since the density 

 required to precipitate limestone is far below that required for 

 deposition of gypsum, it is highly probable that in many shallow 

 seas but slightly connected with the abyssmal ocean limestone 

 was continuously and abundantly deposited. Such deposits must 

 be more wide-spread than gypsum, for the same reasons that gyp- 

 sum deposits must be more abundant than salt. While laying 

 stress on this point, the fact probably remains that most of the 

 limestone of the earth is of organic origin. 



While conditions like those now existing in the Mediterranean 

 Sea, when somewhat intensified, may in the main be regarded as 

 giving rise to gypsum deposits, this sea presents one peculiarity 

 which could not have characterized many of the regions where 

 gypsum occurs. Structural conditions indicate that most of the 

 gypsum deposits were formed in arms of a shallow epi-conti- 

 nental sea. The Mediterranean Sea with its average depth of 

 1,000 fathoms is truly abyssmal. 



Although there may be some doubt as to the exact manner in 

 which the calcium carbonate is removed from the brine during 

 concentration, the fact that it is removed in some one or more of 

 the ways suggested, or by some process not yet brought to light, 

 may be assumed. This removes the only serious difficulty in 

 conceiving of extensive and very pure deposits of gypsum form- 

 ing in basins only slightly, yet continuously through long periods, 

 connected with the ocean. The Mediterranean hypothesis, with 

 the modifications pointed out, may be accepted as accounting for 

 the Iowa gypsum, as well as similar deposits in various periods 

 of geological history. It must be admitted, however, that further 

 chemical investigations in regard to the reactions between salts 

 in solution during the process of brine concentration must be 

 undertaken before the problem can be regarded as fully solved. 



State University of Iowa, Frank A. WiLDER. 



Iowa City, 



