KINDS OF ROCKS. 65 



Feldspar — hard, usually light-colored. G. = 2-5-2-8. Either cleavably crystalline or 

 cryptocrystalline. 



Hornblende and Pyroxene — hard, usually dark-green to black; heavy. G. = 2 - 8-3 , 't. 

 Often tough. 



Mica — slaty, glistening with mica scales, not very hard, not greasy to the touch. 

 G. =2-5-2-8. 



Hydrous mica — often slaty, somewhat glistening, a greenish, grayish, or brownish 

 color, not very hard ; a, greasy feel. G. = 2 - 4-2-7. 



Chlorite — often slaty, soft, an olive-green color: a little greasy to the touch. G. = 

 2-7-3-2. 



Serpentine — massive ; rather soft ; dark or light green ; but little greasy to the touch. 

 G. =2-4-2-6. 



Carbonate of lime — moderately soft, effervescing readily with acids. G. =2.5-2.8. 

 Usually massive ; white to black. 



Carbonate of lime and magnesia, or Dolomite — like the preceding ; but not effervescing 

 readily unless the acid is heated. 



In the names of rocks, the termination ite is here changed to yte, as done in the 

 author's System of Mineralogy (1868), in order to distinguish them from the names of 

 minerals. Granite is excepted. 



1. Fragmental Rocks, exclusive of Limestones. 



(1.) Conglomerate. — A rock made up of pebbles or fragments 

 of rocks of any kind, (a) If the pebbles are rounded, the conglom- 

 erate is a pudding-stone ; (b) if angular, a breccia. 



Conglomerates are named, according to their constituents, siliceous 

 or quartzose, granitic, calcareous, porphyritic, pumiceous, etc., using 

 these terms as already explained. The cementing ingredient may be 

 calcareous, siliceous, ferruginous, and occasionally of other kinds. 



(2.) Grit, Grit-Rock. — A hard, gritty rock, consisting of sand 

 and small pebbles, called also millstpne grit and grindstone grit, be- 

 cause used sometimes for grindstones. Also applied to a hard, gritty 

 sandstone. 



(3.) Sandstone. — A rock made from sand agglutinated. There 

 are siliceous, granitic, micaceous sandstones, according to the char- 

 acter of the material. There are also compact, friable, argillaceous 

 (containing clay), ferruginous (containing iron), concretionary, marly 

 (containing some carbonate of lime), flexible and other kinds of sand- 

 stone. 



4. Sand-rock. — A rock made of sand of any kind, especially if not 

 siliceous or granitic. If the sand is calcareous, it is called a calcareous 

 sand-rock, as beds of pulverized corals or shells ; if basaltic, it is a 

 basaltic sand-rock ; and so on. 



(4.) Shale. — A soft, fragile rock, made from clay (hence an argil- 

 laceous rock, argilla being the Latin for clay), having an uneven slaty 

 structure. Shales are gray to black in color, and sometimes of dull 

 greenish, purplish, reddish, and other shades. 



