78 GEOLOGY 



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 meteorites. 



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, satin-spar, alabaster, etc. ; easily recognized by its softness 

 and want of effervescence with acids; occurs in beds; calcined and ground, 

 constitutes plaster of Paris. 



Hematite. Ferric oxide, Fe 2 O 3 ; iron sesquioxide; rhombohedral, more 

 commonly columnar, granular, botryoidal, or stalactitic; luster metallic, 

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

 streak or powder; a leading iron ore, 70 per cent metallic iron when pure; 

 the chief source of the red color of soils and rocks generally. 



Hornblende. The most important amphibole (q. v.); name sometimes 

 used as a synonym for amphibole. 



Hypersthene. One of the pyroxenes (q. v.); a ferromagnesian silicate; 

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

 iridescent; dark brownish green, grayish, or greenish black and brown; a 

 frequent constituent of crystalline rocks. 



Iceland spar. A form of transparent calcite (q. v.) 



Ilemnite. A titanium iron-oxide; rhombohedral; resembles hematite; 

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

 quently 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 im- 

 purities; commonly arises from decomposition of aluminous silicates, espe- 

 cially the feldspars; basis of pottery and china. 



Labradorite. A plagioclase feldspar; essentially an aluminum-calcium- 

 sodium silicate; composition intermediate between that of albite and ;in- 

 orthite; triclinic; H. 6; luster pearly or vitreous, gray, brown, or greenish; 

 sometimes colorless or white; frequently shows play of colors; important 

 constituent of various crystalline rocks, especially of the basic class; usually 

 associated with a pyroxene or amphibole. 



Leucite. Essentially an aluminum-potassium silicate, allied to the feld- 

 spars; H. 5-6; luster vitreous, white, ash-gray, or smoke-gray; occurs in 

 certain volcanic rocks, particularly lavas of Vesuvius. 



Limonite. Brown hematite, ocher; a hydrous iron oxide; commonly 

 earthy; also concretionary, stalactitic, botryoidal, and mamillary, with 

 fibrous structure; H. 5-5.5; luster silky, sometimes submetallic, but com- 

 monly dull and earthy ; brown, ocherous yellow; streak and powder yellowish 



