80 STRUCTURAL GEOLOGY. 



myte, which is calcitic (a mixture of calcite and dolomite), is apt to crumble from 

 weathering, because the calcite is the most soluble, and becomes removed by filtrating 

 waters. 



As the kinds of fragmental materials which the rocks afford over the earth are either 

 sand, gravel, earth, or mud, and clay, the kinds of fragmental rocks are few. 



Fkagmental Rocks, not Calcareous. 



Conglomerate. — A consolidated gravel-bed, consisting of a mixture of pebbles, or 

 fragments of rocks, and finer material, a. If the pebbles are rounded, it is called pudding- 

 stone; h. if angular, breccia. Conglomerates are named, according to their constituents, 

 siliceous or qtiartzose, granitic, calcareous. 



Grit, Grit-kock. — A hard, gritty rock, consisting of coarse quartz sand and small 

 pebbles ; called also millstone grit, because used sometimes for millstones. 



Sandstone. — A rock made of sand ; a consolidated sand-bed. There are siliceous, 

 granitic, micaceous, feklspathic, calcareous sandstones, according to the character of the 

 material. They are thick-bedded or thin-bedded, according to the thickness of the beds ; 

 laminated when divisible into laminae or slabs ; shaly when splitting into thin pieces or 

 sheets like shale or an imperfect slate. There are also compact, friable, argillaceous, 

 gritty, ferruginous, concretionary, massive, flexible, and other kinds. Grindstones are 

 made of an even-grained, rather friable sandstone. Hard, siliceous sandstones, gi'it, and 

 conglomerate, in regions of metamorphic rocks, are called quartzyte (page 82). The 

 Arkansas Novaculite, or whetstone, is an exceedingly fine-grained sandstone microscopi- 

 cally porous through the loss by infiltrating waters of disseminated calcareous particles 

 (L. S. Griswold). 



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. 



Shale. — Consolidated mud or clay ; a soft, fragile, slaty, argillaceous rock. Shales 

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



Varieties. — a. Carbonctceous shale; black and impregnated with coaly material, 

 yielding mineral oil or related bituminous matters when heated (Brandschiefer in German). 

 b. Alum shale ; impregnated with alum or pyrites, usually a ciTimbling rock. The alum 

 proceeds from the alteration of pyrite, or an allied iron sulphide, in the rock. 



Argillyte, or Clay-slate {Phyllyte'). — A slaty rock, like shale, but differing in 

 breaking usually into thin and even slates or slabs. Hoofing and writing slates are exam- 

 ples. It Is sometimes thick-laminated. Moreover, unlike shale, it occurs in regions of 

 metamorphic rocks, and often graduates into hydromica and mica schists. It graduates 

 often into hard, thick-layered sandy beds, which used to be called gray ivacke. 



TtTFA. — Consists of comminuted volcanic sand and small fragments of lavas, more 

 or less altered. Usually of a gray, yellowish brown, or brown color, sometimes red. The 

 tufa made from those igneous rocks that contain pyroxene is usually yellowish brown oi- 

 brown in color (sometimes red) (often called ivacke) ; and that made from the feldspathifc 

 igneous rocks, trachyte, pumice, and the like, is of an ash-gray color, or of other light 

 shades. The finer deposits are often called ash-beds. Since volcanic ashes are often very 

 widely distributed by the winds, they make deposits beyond the limits of the volcano, 

 over the land, or lakes, or the sea-bottom ; and sometimes the deposits have great thick- 

 ness. Pozzuolana is a light-colored tufa, found in Italy, near Rome, and elsewhere, and 

 used for making hydraulic cement. 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. 



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

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



