16 CEMENTS, LIMES, AND PLASTERS. 



thicker beds of red shales. Such gypsum beds may vary greatly in 

 extent as well as in thickness. Beds now worked in different American 

 localities, for example, vary from six to sixty feet in thickness. The 

 gypsum occurring in the beds frequently contains a considerable percent- 

 age of impurities, as is shown by the analyses given in Table 8, page 53. 



Deposits of rock gypsum have been formed by the gradual evapora- 

 tion, in lake basiris or shallow arms of the sea, of waters carrying lime 

 sulphate in solution. If any natural water be evaporated to a suf- 

 ficient extent, it will deposit the salts which it contains, the order in 

 which the various salts are deposited depending principally upon their 

 relative proportions in the water and their solubility. A normal water, 

 whether from stream, lake, or ocean, will carry as its three commonest 

 constituents lime carbonate, lime sulphate, and sodium chloride. If 

 such a water be evaporated, therefore, deposits of limestone, gypsum, 

 and common salt would result : and, as above noted, these three minerals 

 are very common associates in gypsum deposits. 



Gypsum-earth deposits consist of masses of small crystals or grains 

 of gypsum, intermingled usually with much clayey matter, sand, etc. 

 Such deposits occur in depressions, and are supposed to be formed by 

 the evaporation of spring-waters which have taken up lime sulphate 

 in solution from underlying beds of rock gypsum, only to deposit it 

 again on reaching the surface and being subjected to evaporation. 



In certain areas in the West, notably in Arizona and New Mexico, 

 deposits of gypsum sand occur. These deposits are made up of fine 

 grains of gypsum, worn off from outcrops of rock gypsum and carried 

 by the wind to the place of deposition. 



Geologic distribution of gypsum deposits. Gypsum has a very wide 

 geological range, but the workable gypsum deposits of the United States 

 occur at only a few geological horizons. The Salina group of the Silurian 

 carries large gypsum deposits which are worked in New York, Ontario, 

 Ohio, and Michigan. The Lower Carboniferous carries workable gypsum 

 deposits in Virginia, Michigan, and Montana. Most of the deposits 

 west of the Mississippi occur in rocks of Permian or somewhat later age. 

 Three geological series, therefore, carry almost all of the workable 

 gypsum of the United States. 



Distribution of gypsum in the United States. The gypsum-producing 

 localities of the United States are indicated on the accompanying map. 

 This map is taken from the publication cited below,* to which the reader 



* "Gypsum Deposits of the United States," by George I. Adams and others. 

 Bulletin No. 223, U. S. Geological Survey. Washington, D. C. 



