THE ORIGIN AND DESCENT OF ROCKS. 461 



Augite — one of the pyroxenes (q.v.) ; an aluminum-calcium-magnesium-iron 

 silicate; H. 5-6; monoclinic, crystals usually thick and stout; sometimes lamel- 

 lar; also granular; black, greenish black, deep green; an important rock-forming 

 mineral. 



Beauxite — essentially hydrated alumina; occurs in concretionary grains of 

 clay-like form, whitish to bro^\•n; valuable as a source of aluminum. 



Beryl — a beryllium-aluminum silicate; hexagonal; prismatic; H. 8; luster 

 vitreous or resinous; marl -green, pale passing into whitish; closely resembles 

 apatite, but distinguished by superior hardness and in composition. 



Barite — barites, heavy-spar, barium sulphate; orthorhombic, H. 5-3.5; 

 luster vitreous to resinous, sometimes pearly, white, inclining to yellow, gray, 

 blue, red, or broAATi; very heavy, sp. gr. 4.3-4.7. 



Biotite — black mica, a potash-aluminum-magnesium-iron silicate; monoclinic; 

 easy basal cleavage into thin laminae ; sometimes occurs as a massive aggregation 

 of cleavable scales; H. 2.5-3; luster splendent on cleavage surface; black to 

 dark green; cleavage surfaces smooth and shining; a very common constituent 

 of crystalline rocks. 



Bitumen — the same as asphaltum (q.v.). 



Bismuth — a metal of whitish color and rather brittle nature; occurring oc- 

 casionally native, usually as an ore. 



Bronzite — a variety of enstatite (q.v.) ; grayish green to olive-green and 

 brown with luster on cleavage surface often adamantine, pearly or bronze-like 

 and submetallic. 



Calcite — calcspar; calcium carbonate; rhombohedral, perfect rhombohedral 

 cleavage; often taking the forms kno^Mi as dogtooth spar, nail-head spar; 

 frequently stalactitic and stalagmitic; H. 2.5-3.5; luster vitreous; white, 

 occasionally pale shades of gray, red, green, blue, violet, yellow, brown; strong 

 double refraction; embraces variety called Iceland spar; a very common min- 

 eral; the essential basis of limestone. 



Cassiterite — tinstone; an oxide of tin; tetragonal; luster adamantine, usually 

 splendent; brown or black, sometimes red, gray, white, or yellow; an important 

 source of tin. 



Catlinite — essentially a hardened red clay, rather a rock than a mineral; much 

 prized by Indians for pipes. 



Chalcedony — a crypto-crystalline variety of quartz having a wax-like luster, 

 either transparent or translucent; white, grayish, pale bro^NTi to dark brown, black, 

 sometimes delicate- blue, occasionally other shades; frequently occurs as the 

 lining or filling of cavities, taking on a botryoidal or mamillary form. 



Chiastolite — andalusite (q.v.) . 



Chlorite — the type of an important group of secondary minerals usually 

 charact2rized by a green color, softness and smoothness or unctuousness of feeling; 

 they are usually aluminum-magnesium-iron silicates, with chemically combined 

 water; derived from several other species, as pyroxene, amphibole, biotite, 

 garnet, etc. ; embraces a number of species, among which are clinochlore, pen- 

 ninite, prochlorite, and delessite. 



Clwomite — chromic iron; essentially an iron chromate; isometric; luster 



