SULPIIATKS. CIHIOMATI.S, ETC. 



537 



tin- arrowhead twins, is common at Montmartre IK :ir Paris. 

 Simple crystals are more often found in days, as at Poland, < >hio. 

 Ail crystalline gypsum which shows the perfect cleavage is known 

 a- selrnite. The fibrous variety with a satiny or pearly luster and 

 a fibrous fracture is satin- spar, while the granular massive variety 

 is alabaster. Rock gypsum is an impure granular form, oi'im 

 cart hy. Gypsum is deposited from solution and is associated wit h 

 sedimentary rocks, limestones, and clays, from which the soluble 

 calcium sulphate has been leached out. It is also associated with 

 salt deposits, being deposited from the concent rated brines before 



FIG. 521. Gypsum. Poland, Ohio. 



the more soluble sodium or magnesium sulphates and chlorides, 

 and therefore in the usual position underlying the salt, or is near it 

 in position ; at times, when there have been several distinct periods 

 of concentration, it may be interbedded with salt and shales. 

 Large beds of rock gypsum are found in the Salina formations of 

 New York, but here the gypsum beds are above the salt and are 

 probably independent of it, the concentration of the solution hav- 

 ing been interrupted before the salt was deposited. 



Large beds of gypsum are found in Nova Scotia, Newfoundland, 

 Michigan, and in the borax lakes regions of California and Nevada. 

 Gypsum is also formed near volcanoes and fumaroles ; small crys- 

 tals of gypsum cover the walls of the lava caves of Kilauea. 



Commercially rock gypsum is ground and used as a fertilizer. 

 The purer varieties, when heated at a temperature below 130 C., 



