74 LITHOLOGICAL GEOLOGY. 



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 pyrites. 



(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. 

 Pumiceoti8 tufa, which belongs to the latter division, consists mainly of pumice 

 in grains and fragments, more or less altered. 



Pozzuolana is a kind of light-colored tufa, found in Italy, near Rome and 

 elsewhere, and used for making an hydraulic cement. 



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

 lerite, and usually made up of trappean or doleritic material compacted into a 

 rock which is rather soft. 



(6.) Sand. G-ravel. — Sand is comminuted rock of any kind ; but 

 common sand is mainly comminuted quartz, or quartz and feldspar, 

 while gravel is the same mixed with pebbles or stones. Occasionally 

 sand contains scales of mica and has a glistening lustre. Volcanic 

 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 constitutes the 

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

 ing in fineness or coarseness, being the result of successive deposi- 

 tions. 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 by the action of 

 glaciers. Detritus (from the Latin for worn) is a general term ap- 

 plied to earth, sand, alluvium, and the like. 



2. Metamorphic or Crystalline Rocks, not Calcareous. 



82. Metamorphic rocks are made from the sedimentary rocks 

 above enumerated by some crystallizing process, and vary exceed- 

 ingly in the perfection of the crystallization they have undergone. 

 Granite stands at one end of the series, and hard sandstones called 



