THE ORIGIN AND DESCENT OF ROCKS. 467 



feel greasy; 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. 



Titanite — calcium -titano-silicate ; monoclinic; luster adamantine to resinous; 

 brown, gray, yellow, green, and black; H. 5-5.5; occurs in various crystalline 

 rocks. 



Topaz — an aluminum silicate, with part of the oxygen replaced by fluorine; 

 orthorhombic ; H. 8; luster vitreous; colorless, straw-ydlow verging to various 

 pale shades, grayish, greenish, bluish, and reddish; distinguished by its hardness 

 and infusibility; occurs in crystalline rock. 



Tremolite — a calcium-magnesium amphibole; a common constituent of 

 certain crystalline rocks. 



Viridite — a general term used for green products of rock alteration, usually 

 hydrous silicates of iron and magnesia-, mainly chlorite. 



"Wad — bog manganese; a variety of psilomelane (q.v.). 



Zeolite — a group of minerals derived from the alteration of various aluminous 

 silicates. 



Zircon — zirconium silicate; H. 7.5; luster adamantine; pale yellowish, 

 grayish, yellowish green, brownish yellow, and reddish brown ; infusible ; occurs 

 characteristically in square prismatic forms; found in crystalline rocks and 

 granular limestone. 



REFERENCE LIST OF THE MORE COMMON ROCKS. ^ 



Adobe — a fine silty or loamy deposit formed by gentle wash from slopes 

 and subsequent lodgment on flats; especially applied to silty accumulations 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. 



Alluvium — sediment deposited by streams. 



Amygdaloid — a vesicular igneous rock whose cavities have become filled 

 with minerals; the fillings are called amygdules, because sometimes almond -like 

 in form. 



Andesite — an aphanitic igneous rock consisting essentially of the plagioclase 

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

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

 in feldspar microlites. 



Anorthosite — a rock consisting mainly of the feldspar labradorite. 



Aphanite — a rock whose constituents are so minute as to be indistinguishable 

 to the naked eye; rather a condition of various rocks than of any specific 

 rock. 



Aqueous rocks — a general term applied to rocks deposited through the agency 

 of water. 



^ The following definitions are given, as nearly as practicable, in accordance with 

 present common usage, which is, however, more or less varying and inconsistent. 



