366 General Strata. 



somewhat rounded, and all the varieties of this rock, in some 

 localities, form the floor of salt mines and salt springs. Sub- 

 divisions. — Marl slate, when the rock is soft, slaty, and con- 

 tains minute grains of carbonate of lime. Sandy, when it is 

 in solid blocks or layers, consisting of red or bluish-grey 

 quartzose sand. Varieties. — Grey-band, the uppermost lay- 

 ers of biuish-grey sandrock. Conglomerate, (breccia,) con- 

 sisting chiefly of rounded pebbles, red, grey, or rust-color, as 

 under the superincumbent rocks at Mount Holyoke, the Pal- 

 is des, on the Hudson river, &c. 



15. Ferriferous Rock, is a soft, slaty, argillaceous, or a 

 hard, sandy, silicious rock, embracing red argillaceous iron 

 ore. Subdivisions. — Slaty, consists of green, or bluish-green, 

 smooth soft slate, generally immediately under the layer of 

 red argillaceous iron ore. Sandy, consists of a grey, or rus- 

 ty-grey aggregate of quartzose sandrock, in compact blocks 

 or layers, overlying or embracing red argillaceous iron ore. 

 Variety. — Conglomerate, consists of rounded pebbles, cem- 

 ented together by carbonate or oxide of iron, or adhering 

 without cement. 



16. Lias, consists of rounded grains of quartzose sand, clay 

 slate, and sometimes partly of other aluminous compounds, 

 of a dark or light grey color, aggregated with fine grains of 

 carbonate of lime. Subdivisions. — Calciferous slate, when 

 it is of a slaty texture, and the argillaceous and calcareous 

 constituents predominate. Calciferous grit, when it is in 

 blocks or thick layers, and the quartzose sand or sharp grit, 

 predominates. Varieties. — Conchoidal, when the slaty kind 

 is separated into small divisions, somewhat of a lenticular 

 form, by natural conchoidal cleavages. Shell grit, when 

 the gritty variety consists, in part, of petrifactions of quartzose 

 sand. 



17. Geodiferous Limerock, consists of carbonate of 

 lime, combined with a small proportion of argillite or quartz, 

 in a compact state, mostly foetid, and always containing nu- 

 merous geodes. Subdivisions. — Swinestone, when it con- 

 tains very little or no quartzose sand, is irregular in structure, 

 foetid, and abounds in geodes. Sandy, when it contains 

 quartzose sand, is stratified, scarcely foetid, and contains but 

 few geodes. 



18. Cornitiferous Limerock, consists of carbonate of 

 lime, embracing hornstone. Subdivisions. — Compact, when 

 the rock is close-grained ; and it generally contains hornstone 



