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Gypsum deposits are classified according to origin, mode of occur- 

 rence, relation to the earth's surface, etc. 



According to mode of occurrence there are the following classes: 

 (i) Interbedded deposits alternating with shales, limestones, and sand- 

 stones, laid down in seas or lakes. (2) Efflorescent deposits due to 

 evaporation of water which has come to the earth's surface through 

 gypsum deposits. The result is gypsite, a finely crystalline form of 

 gypsum. (3) Periodic lake deposits due to deposition of gypsum from 

 the waters of intermittent lakes. These are granular and crystalline, 

 and vary much in size. (4) Gypsum veins due to re-working of deposits 

 of the bedded type by ground water, and redeposition in veins. The 

 deposits are crystalline, occurring as satinspar or selenite. (5) Gypsum 

 dunes due to disintegration of massive gypsum or effiorescent deposits, 

 and their transport by the wind. (6) Isoltate crystals and flakes due to 

 formation of sulphuric acid from pyrite and the reaction of the latter 

 on limestone. 



According to origin the deposits are classified as: (i) surface-water 

 deposits; (2) ground-water deposits. 



In age gypsum deposits vary from Silurian to Quaternary. In 19 18 

 gypsum was produced in eighteen states and in Alaska. New York 

 was the largest producer, Iowa second, and Michigan third. 



In New York gypsum has been mined more than one hundred years, 

 during which time it is estimated that about ten million tons have been 

 produced. The deposits are in the Salina (Silurian) formation. The 

 gypsum is in a series of lenses and was deposited by evaporation from an 

 inland sea. The future of the industry in this region is difficult to pre- 

 dict, since the gypsum beds run under younger rocks to the south and the 

 distance they can be profitably mined is uncertain. 



In Iowa there are two areas of gypsum, the area around Fort Dodge 

 being the more important. Here the gypsum occurs in bedded deposits 

 which overlie the Mississippian and Pennsylvanian unconformably. 

 They are thought possibly to be Permian. These beds have been 

 worked since 1872. At Centerville gypsum was discovered a few years 

 ago at a depth of 500 feet, but is not yet developed. At this place the 

 gypsum is in limestone of Mississippian age. 



In Michigan the deposits are in the Michigan formation of the 

 Mississippian system, and in the Bass Islands formation, and Salina 

 formation of the Silurian. The gypsum is of the massive rock variety, 

 and constitutes lenses in shale and limestone. The deposits are almost 

 inexhaustible. The chief development is in the central part of the 

 southern peninsula at Grand Rapids, and north of Saginaw. 



