30 4 General Strata. 



Compact, when it consists of grains of nearly pure carbonate 

 of lime, so closely united that it will take a polish. Sandy, 

 when grains of quartz are aggregated with the grains of car- 

 bonate of lime, but so loosely as to be somewhat friable. 

 Varieties. — Dolomite, when it consists in part of magnesia, 

 and is friable. Verd-antiqite, when it is variegated in color 

 by the presence of serpentine, giving it more or less of a 

 clouded green. 



Names under the Transition Class. 



7. Argillite, is a slate rock of an aluminous character, 

 and nearly homogeneous, always consisting of tables or la- 

 minae whose direction forms a large angle with the general 

 direction of the rock. Subdivisions. — Clay Slate, when the 

 argillite is nearly destitute of all grittiness, and contains no 

 scales of mica or talc. Wacke Slate, when it is somewhat 

 gritty and contains glimmering scales of mica or talc. Va- 

 rieties. — Roof Slate, when the slate is susceptible of division 

 into pieces suitable for roofing houses and for cyphering slate. 

 Glazed Slate, when the natural cleavages are lined with a 

 black glazing. This variety contains anthracite coal and 

 marine organic relics. 



8. First Graywacke, is an aggregate of angular grains 

 of quartzose sand, united by an argillaceous cement, appar- 

 ently disintegrated clay slate, and is never above the calcife- 

 rous sandrock. Subdivisions. — Compact, when the grains 

 are so fine and united so compactly, as to be suitable for 

 quarrying. Rubble, when the grains, or a part of them, are 

 too large for quarrying. This division is often very hard, and 

 sometimes contains felspar, and has the appearance of coarse 

 granite ; though some of the largest pebbles are generally 

 rounded. It is often colored green with chlorite. Every 

 kind of first graywacke is almost horizontal — being a little 

 elevated at the edge next to the primitive rocks only. 



9. Sparry Limerock, consists of carbonate of lime, inter- 

 mediate in texture between granular and compact ; and is 

 traversed by veins of calcareous spar. Subdivisions. — Com- 

 pact, when the masses or blocks, between the veins of spar, 

 are sufficiently homogeneous and uniform to receive a pol- 

 ish. Slaty, when the rock is in slaty tables or laminae, with 

 transverse veins of calcareous spar. This rock is often cut 

 into very small irregular blocks by the spar, which gives it the 

 name of checkered rock. 



