KINDS OF ROCKS. 429 



of garnet, or of other hard minerals existing in the rocks of 

 the region. Occasionally magnetite or garnet is the chief 

 constituent. 



Volcanic sand, or Peperino, is sand of volcanic origin, 

 either the "cinders" or "ashes" (comminuted lava), 

 formed by the process of ejection, or lava rocks otherwise 

 comminuted. 



9. Green Sand. — An olive-green sand-rock, friable, or not 

 compacted, consisting largely of glauconite. See, for de- 

 scription and analysis, p. 307. 



10. Clay. — Soft, impalpable, more or less plastic material, 

 chiefly aluminous in composition, white, gray, yellow, red 

 to brown in color, and sometimes black. It has been made 

 chiefly from the feldspars, by decomposition. See Kaolinite. 



Varieties. — a. Kaolin, purest unctuous clay. b. Potter's clay, 

 plastic, free froni iron ; mostly unctuous ; usually containing some 

 free silica. Pipe-clay is similar, c. Fire-brick clay, the same ; but it 

 may contain some sand without injury, d. Ferruginous, ordinary 

 brick clay, containing iron in the state of oxide or carbonate, and con- 

 sequently burning red, as in making red brick, e. Containing iron in 

 the state of silicate, and then failing tc turn red on being burnt, as the 

 clay of which the Milwaukee brick are made. f. Alkaline and Vitri- 

 fiable, containing 2 5 to 5 per cent, of potash, or potash and soda, 

 owing to the presence of undecomposed feldspar, and then not refrac- 

 tory enough for pottery or fire-brick, g. Marly, containing some car- 

 bonate of calcium, h. Weak clay, containing too much sand for brick- 

 making, i. Alum-bearing, containing aluminous sulphates, owing to 

 the decomposition of iron sulphides present, and hence used for mak- 

 ing alum. 



The red pipestone of the North American Indians is an indurated 

 clayey rock from the Coteau de Prairies ; it has been named Catlinite; 

 and the gray is in part compact argillyte. 



11. Alluvium. Silt. Till. — Alluvium is the earthy deposit 

 made by running streams or lakes, especially during times 

 of flood. It constitutes the flats either side, and is usually in 

 thin layers, varying in fineness or coarseness, being the re- 

 sult of successive depositions. 



Silt is the same material deposited in bays and harbors, 

 where it forms the muddy bottoms and shores. 



Lmss is a fine earthy deposit, following the courses of 

 valleys or streams, like" alluvium, but without division into 

 thin layers. Occurs in elevated plains, along the broad parts 

 of large valleys, as the Mississippi, Ehine, Danube. 



Till is the unstratified sand, gravel, and stones, derived 

 from glaciers. 



