64 LITHOLOGICAL GEOLOGY. 



break in another, and are inelastic. Also fibrous, with satin lustre. 

 Also earthy, massive, of dull gray and other colors. Very soft, 

 so as easily to be cut with a knife. 



The white, compact kind is ala- Fig ' U ' Fig " 55 * 



baster. When heated, burns white 

 and crumbles, losing its water, and 

 becoming a powder, which is com- 

 mon "plaster of Paris." Composition: Sulphuric acid, 46.51, lime, 

 32.56, water, 20.93. 



(27.) Anhydrite. — The same in composition as gypsum, except that it con- 

 tains no water. Crystals rectangular, with three distinct and nearly equal 

 cleavages, affording cubical and rectangular blocks, and thus easily distin- 

 guished from those of gypsum. Color, white, grayish, bluish, and other pale 

 shades. Hardness near that of calcite. G. '== 2.8-3.01. Before the blowpipe, 

 does not exfoliate like gypsum. Composition : Sulphuric acid, 58.8, lime, 41.2. 



6. Additional mineral species. 



71. (28.) Graphite, called also Plumbago or Black Lead. The lustre, soft- 

 ness, and the tracing it leaves are well seen in the common "lead-pencil." The 

 structure is either foliated or granular ; and sometimes it gives a slaty structure 

 to a rock. It is essentially pure carbon. 



(29.) Pyrites (Iron Pyrites). — In cubes, the adjacent faces Fig- 56. 



of which are often striated at right angles with one another, as 

 in the annexed figure (56) ; also in other forms; also massive. 

 Color, pale brass-yellow. Hard enough to strike fire with 

 steel. G. = 4.8-5.1. Composition : Sulphur, 53.3, iron, 

 46.7. 



(30.) Chalcopyrite (or Copper Pyrites). — Resembles iron 

 pyrites, but is of a deeper yellow color, much softer, being scratched with a 

 knife, and giving a dark-greenish powder, and when wet with nitric acid a 

 knife-blade put in the acid becomes coated with copper. Contains sulphur, 34.9, 

 copper, 34.6, iron, 30.5. There is also a gray sulphuret of copper, called copper- 

 glance, which has a steel or iron lustre, and consists of sulphur, 20.2, copper, 79.8. 

 There is still another, which tarnishes readily, and is sometimes called horse- 

 flesh ore, from the color of the tarnish, which consists of sulphur, 23.7, copper, 

 62.5, iron, 13.8. 



(31.) Blende. — In crystals; also massive, cleav able, with a brilliant lustre; 

 the lustre resinous ; the yellow and brown varieties look much like a resin, 

 and the black variety approaches metallic in lustre ; a touch of the point of a 

 knife affords a white or whitish scratch or powder. Blende is a sulphuret of 

 zinc, containing sulphur 33, zinc 67. 



(32.) Galena. — In cubic and other related crystalline forms, and breaking 

 readily by cleavage into small cubes ; also massive and granular. Color, lead- 

 gray. Readily cut by a knife, although brittle. G. = 7.2-7.7. Galena is 

 the common lead-ore, a sulphuret of lead, consisting of sulphur, 13.4, lead, 86.6. 



