

68 LITHOLOGICAL GEOLOGY. 



argillaceous, gritty, ferruginous, concretionary, laminated, thick-bedded, thin-bedded 

 massive, shaly, Jiexible, and other kinds. Grindstones are made of an even-grained, 

 rather friable sandstone. Hard, siliceous sandstones, grit, and conglomerate, in re- 

 gions of nietamorphic rocks, are called quartzyte (p. 70). 



4. Sand-rock. — A rock made of sand of any kind, especially if not siliceous or 

 granitic. A calcareous sand-rock is one made of calcareous sand, as pulverized corals 

 or shells. 



5. Shale. — A soft, fragile, argillaceous rock, having an uneven, slaty structure. 

 Shales are gray to black in color, and sometimes dull greenish, purplish, reddish. 



Varieties. — a. Bituminous shale ; impregnated with petroleum, or with coaly ma- 

 terial yielding mineral oil or related bituminous matters when heated; called also 

 Carbonaceous shale ( Brandschiefer in German), b. Alum shale ; impregnated with alum 

 or pyrites, usually a crumbling rock. The alum proceeds from the alteration of pyrite 

 or the allied pyrrhotite (p. 59). 



6. Argillyte, or Clay -slate (Phyllyte). —An argillaceous slaty rock, like shale, but 

 differing in breaking usually into thin and even slates or slabs. Roofing and writing 

 slates are examples. It is sometimes thick-laminated. Moreover, unlike shale, it oc- 

 curs in regions of metamorphic rocks, and often graduates into hydromica and mica 

 slates. 



7. Tufa. — Tufa is a rock, not very hard, made from comminuted volcanic or other 

 igneous rock, more or less altered. Usually of a yellowish-brown, gray, or brown color; 

 sometimes red. The tufa made from those igneous rocks that contain iron-bearing min- 

 erals, such as doleiyte (trap), basalt, and the heavier lavas, is usually yellowish-brown 

 or brown in color (sometimes red), and often consists in part of palagonite, a result of 

 the alteration of the materials by means of heated water or vapor; and that made from 

 the feldspathic igneous rocks, trachyte, pumice, and the like, are of an ash-gray color, 

 or of other light shades. Pozzuolana is a light-colored tufa, found in Italy, near Rome, 

 and elsewhere, and used for making hydraulic cement ; Wacke, earthy, brownish, re- 

 sembling an earthy trap or doleryte, usually made up of trappean or dolerytic material 

 compacted into a rock that is rather soft. 



8. Sand. Gravel. — Sand is comminuted rock-material; but common sand is 

 mainly comminuted quartz, or quartz and feldspar, while gravel is the same mixed 

 with pebbles or stones. Sand often contains scales of mica, grains of magnetite and 

 garnet. 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 other- 

 wise comminuted. 



9. Clay. — Soft, impalpable, more or less plastic material, chiefly aluminous in com- 

 position, white, gray, yellow, red to brown and black in color. See p. 58. 



Varieties. — a. Kaolin; purest unctuous clay. b. Potter's clay ; plastic, free from 

 iron; mostly unctuous; usually containing some free silica, c. Ferruginous, Brick-clay; 

 containing iron in the state of oxide or carbonate, and consequently burning red, as 

 in making redbrick, d. Containing iron in the state of a silicate, and then failing to 

 turn red on being burnt, as the clay of which the Milwaukee brick are made. e. Alka- 

 line and Verifiable; containing 2 5 to 5 per cent, of potash or potash and soda, owing 

 to the presence of undecomposed feldspar, and then not refractory enough for pottery 

 or fire-brick, f. Marly; containing some carbonate of calcium, g. Black, Ampelite ; 

 from the presence of lignitic or coaly material, h. Alum bearing ; containing aluminous 

 sulphates, owing to the decomposition of iron sulphides present. 



10. Alluvium. Silt. Till. —Alluvium is the earthy deposit made by running 

 streams or lakes, especially during times of flood. It constitutes the flats adjoining, 

 and is usually in thin layers, varying in fineness or coarseness, being the result of suc- 

 cessive depositions. Silt is the same material deposited in bays or harbors, where it 

 forms the muddy bottoms and shores. Lasss is an earthy deposit, coarse or fine, fol- 

 lowing the courses of valleys, like alluvium, but without division into thin layers. 



Till is the unstratified sand, gravel, and stones, derived from glaciers. 

 Detritus (from the Latin for worn) is a general term applied to earth, sand, alluvium, 

 silt, gravel, because the material is derived to a great extent from the wear of rocks 



