THE NONMETALLIC MINERALS. 407 



The following is an analysis of a commercial gypsum from Ottawa 

 County, Ohio, as given by Professor Orton: 1 



Lime 32. 52 



Sulphuric acid 45. 56 



Water 20. 14 



Magnesia 0. 56 



Alumina 0. 16 



Insoluble residue. . . . 0. 68 



99.62 



Origin. Gypsum in considerable quantities occurs associated only 

 with stratified rocks and is regarded mainly as a chemical deposit 

 resulting from the evaporation of waters of inland seas and lakes; it 

 may also originate through the decomposition of sulphides and the 

 action of the resultant sulphuric acid upon limestone; through the 

 mutual decomposition of the carbonate of lime (limestone) and the 

 sulphates of iron, copper, and other metals; through the hydration of 

 anhydrite and through the action of sulphurous vapors and solutions 

 from volcanoes upon the rocks with which they come in contact. 

 According to Dana, 2 the gypsum deposits in western New York do not 

 form continuous layers in the strata, but lie in imbedded, sometimes 

 nodular masses. In all such cases, this authority says, the gypsum was 

 formed after the beds were deposited, and in this particular instance 

 are the product of the action of sulphuric acid from springs upon the 

 limestone. "This sulphuric acid, acting on limestone (carbonate of 

 lime), drives off its carbonic acid and makes sulphate of lime, or 

 gypsum; and this is the true theory of its formation in New York." 

 Dr. F. J. H. Merrill, however, regards a portion at least of the New 

 York beds as a product of direct chemical precipitation from sea water. 3 



The gypsum deposits of northern Ohio are regarded by Professors 

 Newberry and Orton as deposits from the evaporation of landlocked 

 seas, as was also the rock salt which overlies it. By this same process 

 must have originated a large share of the more recent gypsum deposits 

 of the Western States. 



Geological age and mode of occurrence. As may be readily inferred 

 from what has gone before, beds of gypsum have formed at many 

 periods of the earth's history and are 3till forming wherever proper 

 conditions exist. The deposits of New York State occur in a belt ex- 

 tending eastward from Cayuga Lake and in beds belonging to the 

 Salina period of the Upper Silurian age. The rock is often earthy 

 and impure, and is used nearly altogether for land plaster. It is asso- 

 ciated with dark, nearly black, limestones and shales and beds of rock 

 salt. In southwest Virginia, along the Holston River, are also beds 



1 Geology of Ohio, VI, 1888, p. 700. 



2 Manual of Geology, p. 234. 



3 Bulletin No. 11, of the New York State Museum, April, 1893. 



