THE ORIGIN AND DESCENT OF ROCKS. 463 



of these are present in abundance, and the varieties are characterized by the 

 leading constituent; isometric, usually in dodecahedrons or trapezohedrons ; 

 H. 6.5-7.5; luster vitreous to resinous; commonly red or brown, sometimes 

 yellow, white to blue, green or black ; common in mica schist, gneiss, hornblende 

 schist; also in granite, syenite, and metamorphosed limestone. 



Geyserite — a concretionary deposit of silica in the opal condition; formed 

 about geysers; white or grayish. 



Glauconite — green-sand, a hydrous potassium-iron silicate usually impure, 

 amorphous, or earthy; dull olive-green or blackish, yellowish, or grayish green; 

 opaque, commonly occurs as grains or small aggregations. 



Graphite — plumbago, black lead; a form of carbon, usually impure; rhom- 

 bohedral, but rarely appearing as a crystal; more often as thin laminse of greasy 

 feel; yields a black adhesive powder; hence its common use for lead pencils; 

 occurs in granite, gneiss, mica schist, crystalline limestone; sometimes results 

 from alteration of coal by heat ; occasionally occurs in basaltic rocks and meteor- 

 ites. 



Gypsum — a hydrous calcium sulphate; monoclinic; perfect cleavage into 

 smooth polished plates; occurs in a variety of forms, including fibrous and 

 granular; H. 1.5-2; luster pearly and shiny; white, sometimes gray, flesh- 

 red, yellowish, and blue; impure varieties dark; crystallized varieties include 

 selenite, satinspar, alabaster, etc.; easily recognized by its softness and want 

 of effervescence with acids; occurs in beds; calcined and ground constitutes 

 plaster of Paris. 



Haiiynite — a complex sodium-aluminum silicate and calcium sulphate; crystals 

 dodecahedrons; luster vitreous or somewhat greasy; bright blue, sky-blue, or 

 greenish blue, or green; occurs in certain igneous rocks, commonly associated 

 with nephelite and leucite. 



Hematite — ferric oxide, Fe203, iron-sesquioxide ; rhombohedral, more com- 

 monly columnar, granular, botryoidal, or stalactitic; luster metallic, some- 

 times earthy; iron-black, dark steel-gray, red when earthy; gives red streak 

 or powder; a leading iron ore, 70 percent, metallic iron when pure; the chief 

 source of the red color of soils and rocks generally. 



Hornblende — an amphibole; name sometimes used as a synonym for amphi- 

 bole; sometimes to designate a variety under amphibole (q.v.). 



Hyalite — a variety of silica in the opal condition; clear and colorless like 

 glass, consisting of globular concretions or crusts. 



Hypersthene — one of the pyroxenes ; a ferromagnesian silicate; orthorhombic ; 

 H. 5-6; luster somewhat pearly on cleavage; surface often iridescent; dark 

 bro^vnish green, grayish, or greenish black and brown; a frequent constituent 

 of crystalline rocks. 



Iceland spar — a form of transparent calcite (q.v.). 



Ilmenite — menaccanite; a titanium iron oxide; rhombohedral; resembles 

 hematite; luster submetallic; iron-black; powder black or brownish red; occurs 

 frequently in crystalline rocks associated with magnetite. 



Iron pyrites — pyrite (q.v.). 



Kaolin — kaolinite; essentially a hydrous aluminum silicate; usually in clay- 

 like or earthy form; white or grayish white; often tinged with impurities; com' 



