PHYSIOGRAPHY. 243 



Gypsum. 



3. Axis of revolution perpendicular ; face of composi- 

 tion parallel to T. According to this law are formed the 

 arrow-shaped twins, consisting of lenticular crystals. Glob- 

 ular masses, generally formed of discernible individuals. 

 Dentiform. Massive : composition granular, passing into 

 impalpable, sometimes scaly ; also columnar, often as thin 

 as hair, long, generally straight and parallel. Sometimes 

 without cohesion of the single particles, in the state of pow- 

 der. 



1. Before the blow-pipe, it exfoliates and melts, though with difficul- 

 ty, into a white enamel, which after a short time falls into powder. In 

 a lower degree of heat, it loses its water and becomes friable, so as to be 

 easily reduced to an impalpable powder. If mixed with water, this pow- 

 der becomes warm, and soon hardens into a solid mass. 



2. Analysis. 

 By BUCHOLZ. 



Lime .... 33-0 



Sulphuric acid ... - 44-8 

 Water 21-0 



3. Compound varieties of Gypsum form beds in secondary mountains, 

 more sparingly in the older classes of rocks ; and they are generally pos- 

 sessed of a considerable thickness. Its principal repositories are sand- 

 stones and clay, in which it is associated with limestone and Common 

 Salt. Brine springs very often issue from the rocks in its vicinity. Sim- 

 ple varieties are chiefly found in clay, in salt works ; also in abandoned 

 mines and old heaps, where they are often products of recent origin. 



4. Gypsum is found in almost all countries, both crystallized and mas- 

 sive ; for example, in Mansfeld, Thuringia, Bavaria, Franconia, Swa- 

 bia, Switzerland, in the Tyrol, Stiria and Austria ; also Poland, Hungary 

 and Transylvania ; in England, France, Spain. Beautiful crystals are 

 found near Oxford, at Bex in Switzerland, Hall in the Tyrol, and at 

 Ischel ; large lenticular crystals, generally twins, occur at Montmartre, 

 near Paris. Gypsum occurs in great quantities in Nova Scotia, both 

 the earthy varieties and the scaly. In the United States, this species is 

 abundant in Arkansaw, Illinois, Tennessee, Virginia, Ohio and N.York. 

 At Poland, in Trumbull co. (Ohio,) exceedingly perfect and transparent 



