MINERALOGY. 441 



Tufa, formed by deposition from waters resembles Rock- 

 milk, but is more cellular or porous and not so soft ; 

 occasionally used for light walls, making mortars for 

 druggists, and, lastly, as medicine. Dolomite, the white 

 variety, used as marble, is the Magnesian Carbonate of 

 Lime. 



Gypsum Plaster of Paris Sulphate of Lime; 

 mostly of compact texture, seldom crystallized, but when 

 it is, and pure, it is as pellucid as glass, and has a pearly 

 luster. Occurs likewise in laminated masses of large size, 

 with a satin luster ; in radiating or stellated forms ; also 

 affording foliated, flexible laminse ; color gray, reddish, 

 brownish, and the compact variety opaque. Found in 

 great beds among the limestone, marl, clay, and sand- 

 stone formations ; very often associated with rock-salt. 

 The pure and fine-grained snowy gypsum, also called 

 Alabaster, and used for statuettes and small sculpture, 

 comes from Florence. Gypsum when burnt and ground 

 forms a white powder, which, being mixed with water, 

 on drying becomes hard and compact. Known as Plas- 

 ter of Paris, and is used for making models and stucco 

 ornaments, taking casts, and giving a finish to walls. 

 Gypsum is also ground and, employed as compost, is 

 considered an important agent in improving clover fields. 

 Anhydrite is a relative species, compact, hard, and also 

 a sulphate of lime, like gypsum, but differs in contain- 

 ing no water. 



II. Fluor spar Flusspath Fluoride of Cal- 

 cium. 



Fluor spar or Fluate of Lime. Structure some- 

 times, although rarely, in crystal form or columnar ; 

 mostly massive, granular, coarse or fine. Luster vitre- 

 ous, transparent or translucent ; colorless, but also fre- 

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