General Strata. 367 



in layers. Shelly, when it consists of shells and contains 

 hornstone in nodules or irregular masses. 



19. Third Graywacke, having the character of first and 

 second graywacke in general ; but differing in containing 

 much iron pyrites, fine grains of carbonate of lime, in larger 

 or smaller proportion, and in having the quartzose grains 

 mostly rounded. Subdivisions. — Pyritiferous slate, when 

 the rock has a slaty structure, and is in thin laminae, or in 

 blocks or thick layers. Pyritiferous grit, when the rock has 

 a silicious or gritty structure, containing a large proportion 

 of quartzose sand or pebbles. Varieties. — Red sandstone 

 and red wacke, when the grey rock passes into a dirty orange, 

 and thence into a red silicious sandrock. This has been 

 called old red sandstone ; but I do not believe that such a 

 general stratum is admissible. Conglomerate, (breccia,) 

 when the rock consists chiefly of rounded pebbles, of a light 

 red, greyish red, or rust-color. 



Names under the Superincumbent Class. 



20. Basalt, is a hornblende rock, not primitive, probably 

 of volcanic origin. Subdivisions. — Amygdaloid, when amor- 

 phous, of a compact texture, but containing cellules, empty 

 or filled. Greenstone trap, when of a columnar structure, 

 or in angular blocks, often coarse-grained. Variety. — Toad- 

 stone, when the amygdaloid has a warty appearance, and re- 

 sembles slag. 



Names under the Alluvial Class. 



21. Antediluvion, when the detritus is in layers, so situa- 

 ted that it must have been deposited from water, while stand- 

 ing over it at great depth, in nearly a quiescent state. Sub- 

 divisions. — Plastic clay, when it will not effervesce with acids ; 

 being destitute of carbonate of lime. Wlarly clay, when the 

 clay contains fine grains of carbonate of lime, sufficient to 

 effervesce strongly with acids. Bagshot sand and crag, 

 when it consists of quartzose sand, nearly pure, or combined 

 with a little loam, it is called bagshot sand ; when it passes 

 into a gravelly formation, often containing pudding-stone, 

 beds of clay, &c, it is called crag. Variety. — Hard-pan, 

 when the crag consists of gravel, strongly cemented by clay. 



22. Diluvion, consists of a confused mixture ol gravel, 

 sand, clay, loam, plants, shell-animals, &c, so situated, that 



