66 LITHOLOGICAL GEOLOGY. 



Among the varieties there are — 



Bituminous shale. — Impregnated with petroleum, or with coaly material yielding 

 mineral oil or related bituminous matters when heated, or the odor of bitumen when 

 struck. Called also Carbonaceous shale {Brandschiefer in German). 



Coaly shale. — Containing coaly impressions or impregnations. 



Alum shale. — Impregnated with alum or pyrites — usually a crumbling rock. The 

 alum proceeds from the alteration of pyrite, or the allied pyrrhotite (page 59). 



(5.) Tufa. Pozzuolana. — Tufa is an earthy rock, not very 

 hard, made from comminuted volcanic rocks, or volcanic cinder, more 

 or less decomposed, and often forming beds of great extent. It is 

 usually of a yellowish-brown, gray, or brown color. 



The color varies with the nature of the material: basaltic rocks or lavas produce 

 brownish colors (the color is owing to the hydrous oxyd of iron present, derived from 

 the pyroxene or magnetic iron of the original rock, altered by the action of water) ; 

 feldspathic lavas produce light-grayish colors. Pumiceous tufa, which belongs to the 

 latter division, consists mainly of pumice in grains arid fragments, more or less altered. 



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

 for making hydraulic cement. 



Wacke. — An earthy, dark -brownish rock, resembling an earthy trap or doleryte, 

 and usually made up of trappean or dolerytic material compacted into a rock that is 

 rather soft. 



(6.) Sand. Gkavel. — Sand is comminuted rock of any kind; 

 but common sand is mainly comminuted quartz, or quartz and feld- 

 spar, while gravel is the same mixed with pebbles or stones. Occa- 

 sionally, sand contains scales of mica, and has a glistening lustre. Vol- 

 canic sand, or peperino, is sand of volcanic origin, either the " cinders " 

 or " ashes " (comminuted lava) formed by the process of ejection, or 

 from lava rocks Otherwise comminuted. 



(7.) Alluvium. Silt. Till. — Alluvium is the earthy deposit 

 made by running streams, especially during times of flood. It consti- 

 tutes the flats on either side of the stream, and is usually in thin layers, 

 varying in fineness or coarseness, being the result of successive depo- 

 sitions. Silt is the same material deposited in bays or harbors, where 

 it forms the muddy bottoms and shores. Till is an earthy deposit, 

 coarse or fine, following the courses of valleys or streams, like allu- 

 vium, but without division into thin layers, although in very thick 

 deposits. The till of the Alpine valleys is formed of pulverized rock 

 derived from glaciers. Detritus (from the Latin for worn) is a general 

 term applied to earth, sand, alluvium and the like. 



2. Metamorphic Rocks, not Calcareous. 



Metamorphic rocks are made from the sedimentary rocks above 

 enumerated, by some crystallizing process, and vary exceedingly in the 

 perfection of the crystallization they have undergone. Granite stands 



