110 AQUEOUS ROCKS 



resultant sulphuric acid upon limestone; through the mutual 

 decomposition of the carbonate of lime (limestone) and the sul- 

 phates 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. 



The gypsum deposits of northern Ohio form apparently con- 

 tinuous beds over thousands of square miles, and are regarded 

 by Professors Newberry and Orton as deposited by the evapo- 

 ration of landlocked seas at the same time as was the rock-salt 

 which overlies them. 



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 still 

 forming wherever proper conditions exist. 



In New York there are extensive deposits belonging to the 

 Salina period of the Upper Silurian. In Ohio, gypsum asso- 

 ciated with limestones and shales of Lower Helderberg age occur 

 over areas comprising thousands of square miles. The follow- 

 ing section of beds in Ottawa County, this state, will serve to 

 show the conditions under which the rock may occur : 



Drift clays 12 to 14 feet 



Gray roek carrying impure gypsum 5 to 14 feet 



Blue shale J to 14 feet 



Boulder bed carrying gypsum embedded in shaly limestone . 5 to 14 feet 



Blue limestone 1 to 14 feet 



Main gypsum bed 7 to 14 feet 



Gray limestone 1 to 14 feet 



Gypsum 3 to 5 feet 



Anhydrite is an anhydrous variety of calcium sulphate some- 

 what less common than gypsum. Barite, or heavy spar, the 

 sulphate of barium, also occurs in nature, but less abundantly 

 than the calcium sulphates. It is found commonly in con- 

 nection with metallic ores (silver, lead, and zinc), or as a 

 secondary mineral associated with limestone, sometimes in 

 distinct veins, or, as in southwest Virginia, filling irregular 

 fractures in certain beds of the Cambrian limestones, or in 

 part replacing the limestone itself. It is easily distinguished 

 from coarsely crystalline calcite, for which it might possibly 

 be mistaken, by its weight, the specific gravity being about 

 4.5 as against 2.7 for the latter. 



