428 DESCRIPTIONS OF ROCKS-- 



Varieties. — a. Bituminous shale, or Carbonaceous shale (Brand* 

 schiefer of the Germans), impregnated with coaly material and yielding 

 mineral oil or related bituminous matters when heated, b. Alum shale; 

 impregnated with alum or pyrites, usually a crumbling rock. The 

 alum proceeds from the alteration of pyrite or the allied pyrrhotite 

 IP. 174;. 



6. Argillyte, or Pliyllyte. — An argillaceous slaty rock, 

 like shale, but differing in breaking usually into thin and 

 even slates or slabs. Roofing and writing slates are exam- 

 ples. It is sometimes thick-laminated. Unlike shale, it 

 occurs in regions of metamorphic rocks, and often graduates 

 into hydromica, chloritic, and mica schists, and also, on 

 the other hand, into shale. Often called Clay-slate. 



Varieties.— a. Bluish-black, b. Tile-red. c. Purplish, d. Grayish. 

 e. Greenish ; f . Ferruginous, g. Pyritiferous. h. Thick-laminated ; 

 affording thick slabs, instead of slates, i. Thick-bedded ; a massive 

 rock, affording thick blocks or masses, j. Staurolitic. k. Ottrelitic. 



Extensive quarries of slate exist in Vermont at Waterford, Thet- 

 ford, and Guilford, in the eastern slate range of the State ; in North- 

 field in the central range, and in Castleton and elsewhere in the 

 western range, the last of Lower Silurian age if not the others. There 

 are excellent quarries also in Maine and Pennsylvania. The rock fur- 

 nishes also thick slabs for various economical purposes. A trial as 

 to water absorption, and a close examination as to the presence of 

 pyrite, is required before deciding that a slate rock is fit for use, how- 

 ever even its fissile structure. Kinds with a glossy surface are most 

 likely to be impervious to moisture. 



7. Tufa. — A sand-rock or conglomerate made from com- 

 minuted volcanic or other igneous rocks, more or less altered. 

 Usually of a yellowish-brown, gray, or brown color, some- 

 times red. 



Varieties. — a. Dolerytic or basaltic; tufa made from those igneous 

 rocks that contain iron-bearing minerals, such as doleryte (trap), basalt, 

 and the heavier lavas ; it is usually yellowish-brown or brown in 

 color, sometimes red ; and often consists in part of palagonite (p. 312). 

 b. Trachytic; made of the f eldspathic igneous rock, trachyte, of an ash- 

 gray color, or of other light shades, c. Pumiceous ; made of frag- 

 ments of pumice. Pozzwlana is a light-colored tufa, found in Italy, 

 near Rome, and elsewhere, and used for making hydraulic cement, 

 Wacke is an earthy brownish rock, resembling an earthy trap or dole- 

 ryte, 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 usually comminuted quartz, or quartz 

 and feldspar, while gravel is the same mixed with pebbles 

 and stones. Sand often contains grains of magnetite, or 



