('()M)jTi()i\s ()i<^ DErobiTioN; 235 



it should 1)0 horuv in mind that tlie tliick deposits arc only a few square 

 miles in extent in any basin, and that the waters would necessarily remain 

 deep enough to keep all sodiuju chloride and more soluble salts of the 

 original ocean water in solution. 



With a number ol' deeper pools m which the thick gy[)siim deposits 

 are made it seems highly improbable that none of them would evaporate 

 lo the stage where sodium chloride and the more soluble salts would be 

 deposited. Perfect Ijalance between inflow and evaporation would be 

 ha 1(1 to maintain in a considerable number of unconnected pools, and 

 l)robably some of them would have no inflowing streams and would dry 

 u|) very r-apidly. But the ^Vyoming deposits seem never to be associated 

 with salt beds. 



A 40-foot bed of gypsum, resulting from the evaporation of 57,000 

 feet of normal sea-water, should have nearly •'! feet of limestone below it 

 if the evaporation all took jdace in a I'estricted basin; but if the waters 

 were wide-spread in the beginning, al)Out half of the limestone might be 

 deposited over the widei' area, as more than half of the CaCO,, precipi- 

 tates when the Noluine of sea-water is reduced about 50 per cent, and the 

 limestone below the gypsmn might be less than 2 feet in thickness. The 

 writer has not seen limestone immediately below the gypsum at any place 

 in the Eed Beds. 



Grabau discusses^ the abundance of animals that are brought into in- 

 closed basins that ha\o partial connection with bodies of Avater that supply 

 as fast as eva]ioratioii depletes their waters, and concludes that gypsum 

 and salt deposits, formeil where normal sea-waters supplv, across a bar. 

 the watei's taken liy e\aporation, should be highly fossiliferous. 'I'hiek 

 gypsum and salt de|)osits are usually non-l'ossiliferous. 



drahau" emphasizes the agency (d' streams and winds in cari'ving 

 enioi'esccnf gypsum lo inlei'ior basins, hut it is e\'ident that such deposits, 

 ir they are e.xtensixc. can not be pure, as the sti'eams and winds would 

 cari'y silt, sand, and otluu' minerals with the gy])sum. The gypsum of 

 ihe Wyoming i?ed Beds is remarkably free from sediments and other 

 minerals, and I hough the entire r(\a'ion surrounding must have been of 

 led sands and cdays. the gypsum beds are white and pui'o. 



Modi m i:n \\\u Ih I'OTFiKsrs 



.\ modified bar theory seems to explain the phenomena of thick gypsum 

 and salt deposits, and the niodifleation consists of supplying the receiving 



^ I'rinclples of stnitiprii-ih.v. p. :^66. 

 » I'liiKiplcs of Klriillgraidi.v. p. :!f!7. 



