KINDS OF ROCKS. 429 
of garn@® 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 Gecomposition. See Kaolinite. 
VARIETIES. — a. Kaolin, purest unctuous clay. b. Potter's clay, 
plastic, free from iron; mostly unctuous; usually containing some 
free silica. Pipc-clay is similar. c. Fire-brick clay, the same; but it 
may contain some sand without injury. d. Herruginous, 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 nipestone 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. 
Less 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, Rhine, Danube. 
Till is the unstratified sand, gravel, and stones, derived 
from glaciers. 
