96 ' NEW YORK STATE MUSEUM 



tite are found intimately associated with one another. Whether 

 these minerals are formed directly by volcanic action or are 

 secondary minerals, can not in all cases be defimtely stated. 

 It is however observed in many specimens that the aragonite 

 and sulfur crystals are inclosed in clear plates of selenite. This 

 fact goes to show that gypsum was formed later than the other 

 minerals, and its formation was possibly due to the action of 

 sulfuric acid, either as a volcanic gas or in solution, as is the 

 case in thermal springs, on crystals of calcite or beds of lime- 

 stone, though the probability is that the selenite was deposited 

 from water containing gypsum in solution. 



(4) Formation through the action of pi/rite on limestone. The 

 sulfid of iron in the form of pyrite and marcasite is found 

 in many clays, and, when oxidized, forms iron sulfate, which, 

 when brought in contact with the carbonate of lime, brings 

 about a double decomposition, changing the sulfate of iron into 

 siderite (or iron protocarbonate) and the carbonate of lime into 

 sulfate of lime (or gypsum). 



(5) Solution and deposition 'by spring loaters. In regions 

 where beds of rock gypsum occur within range of ground water 

 circulation, the solvent action of the latter may lead to the 

 formation of secondary deposits at the surface. This theory of 

 origin has been advanced particularly by G.P.Grimsley to account 

 for the earthy gypsum or gypsite that occurs in central Kansas. 

 The secondary gypsum is a soft, incoherent, granular mass, found 

 in low, swampy ground in the vicinity of bedded deposits. On 

 microscopic examination the grains are found to consist of angu- 

 lar crystals that have apparently been formed in place. The 

 secondary deposits of Kansas have much economic importance. 

 The seams or veins which traverse beds of gypsum are also usually 

 of secondary origin. 



In the author's opinion, the theory of deposition from sea water 

 is the only tenable explanation of the formation of large, inter- 

 bedded deposits of gypsum. The reasons for this supposition are 

 as follows : (1) Most of the great deposits are found interstrati- 



