KINDS OF ROCKS. 73 



Chlorite — often slaty, soft, an olive-green color ; but 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 effer- 

 vescing readily with cold acid. 



An easy effervescence with dilute muriatic acid indicates the presence of car- 

 bonate of lime. 



1. Fragmental Rocks, exclusive of Limestones. 



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

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

 merate 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-ingre- 

 dient 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 millstone grit and grindstone grit, because used 

 sometimes for grindstones. Also applied to a hard, gritty sand- 

 stone, as the paving-stone of the Hudson Eiver. 



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

 are siliceous, granitic, porphyritic, basaltic, or calcareous sandstones, 

 according to the nature of the material. But the calcareous is 

 called calcareous sand-rock rather than sandstone. There are also 

 compact sandstone, friable sandstone, ferruginous sandstone, concretionary 

 sandstone. 



Micaceous sandstone. — A sandstone glistening with scales of mica. 



Argillaceous sandstone. — Containing much clay with the sand; also called 

 ehaly sandstone, when thin-laminated in structure. 



Marly sandstone. — Containing carbonate of lime, so as to effervesce when 

 treated with dilute acid. 



Flexible sandstone. — See Itacolumite, $ 88. 



(4.) Shale. — A soft, fragile rock, made from clay, and having an 

 uneven slaty structure as explained on page 71. Shales are gray to 

 black in color, and sometimes of dull greenish, purplish, reddish, 

 and other shades. 



Among the varieties there are — 



Bituminous shale. — impregnated with bitumen, or yielding the odor of bitumen 

 when struck. 



