428 DESCRIPTIONS OF ROCKS 
VARIETIES.—a. Litwminous shale, or Carbonaccous shale (Brand- 
schiefer of the Germans), impregnated with coaly material and yielding 
mineral oi] or related bituminous matters when heated. b. Alwm shale; 
impregnated with alum or pyrites, usually a crumbling rock. The 
alum proceeds from the alteration of pyrite or the allied pyrrhotite 
(p. 174). 
6. Argillyte, or Phyllyte-—An argillaceous slaty rock, 
like shale, but diifermg 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. Graytsh. 
e. Greenish ; f£. Ferruginous. g. Pyritiferous. h. Thick-laminated ; 
affording thick slabs, instead of slates. i. T/ick-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 feldspathic igneous rock, trachyte, of an ash- 
gray color, or of other light shades. ec. Pumiceous ; made of frag- 
ments of pumice. Pozzwolan 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 
