MATERIALS OF THE EARTH 81 



from chrysolite or olivine, but also from other magnesian minerals; some- 

 times constitutes the bulk of rock masses. 



Siderite. Iron carbonate; rhombohedral; H. 3.5-4.5; luster vitreous, 

 more or less pearly, ash-gray, yellowish or greenish, also brownish; occurs 

 as extensive iron deposits and in crystalline rocks. 



Steatite. Soapstone, a variety of talc (q. v.) ; hydrous magnesium silicate. 



Sulphur. A well-known element occurring native in volcanic regions; 

 also formed by the decomposition of sulphides, particularly pyrites. 



Talc. A hydrous magnesium silicate; usually in folise; granular or fi- 

 brous forms; also compact; easy cleavage into thin flexible lamina?, but not 

 elastic; luster pearly on cleavage surface; apple-green to silvery white; H. 

 1-2; a secondary product from the alteration of magnesian minerals; dis- 

 tinguished by its soft, soapy feel, soapstone being one variety ; whitish form 

 is known as French chalk. 



Topaz. An aluminum silicate, with part of the oxygen replaced by 

 fluorine; orthorhombic ; H. 8; luster vitreous; colorless, straw-yellow 

 verging to various pale shades, grayish, greenish, bluish, and reddish; dis- 

 tinguished by its hardness and infusibility ; occurs in crystalline rock. 



Tremolite. A calcium-magnesium amphibole; a common constituent 

 of certain crystalline rocks. 



Zeolites. A group of minerals derived from the alteration of various 

 aluminous silicates. 



REFERENCE LIST OF THE COMMONER ROCKS * 



Adobe. A fine silty or clayey deposit formed by gentle wash from slopes 

 and subsequent lodgment on flats; especially applied to clayey accumu- 

 lations in the basins and on the plains of the western dry region. 



Agglomerate. An aggregate of irregular, angular or subangular blocks 

 of varying sizes, usually of volcanic origin; distinguished from conglomerate, 

 in which the constituents are rounded. 

 i Alluvium. Sediment deposited by streams. 



Amygdaloid. A vesicular igneous rock whose cavities have been filled 

 with minerals deposited from aqueous solutions; the fillings are called 

 amygdules, because sometimes almond-like in form. 



Andesite. An igneous rock consisting essentially of the plagioclase feld- 

 spar andesine (sometimes oligoclase) and pyroxene (or some related ferro- 

 magnesian mineral); sometimes cellular, porphyritic, or even glassy; usually 

 rich in feldspar micro lites. 



Anorthosite. A rock consisting mainly of the feldspar labradorite. 



1 The following definitions are given, as nearly as practicable, in accord- 

 ance with present common usage, which is, however, more or less varying 

 and inconsistent. 



