678 



THE WONDERS OF GEOLOGY. Le«t. VII. 



some instances by an argillaceous, in others by a crystal- 

 line siliceous paste. Sandstones composed of the fine 

 detritus of similar materials are associated with the grit. 

 Waterworn fragments of shale, coal, red-sandstone, stems 

 of plants, &c. all bearing marks of transport by currents 

 and streams, are also often found imbedded therein. Beds 

 of coal are occasionally interpolated in this series, and in 

 some localities the lowermost strata consist of shales with 

 coal-plants, and contain nodules of ironstone similar to 

 those of the upper coal strata, and veins of lead and copper. 

 Satin-spar, and naphtha, petroleum, and other bitumi- 

 nous substances, occasionally occur in the shales of this 

 series. 



In Yorkshire the millstone grit becomes a very complex 

 deposit, containing several subordinate beds of coal ; and 

 is separated from the great inferior calcareous group 

 (known in the north of England as the Scar limestone), 

 not merely by shales and shaly limestones as in Derbyshire, 

 but by deposits, not less than 1000 feet thick, in which five 

 series of limestone strata, remarkable for their continuity 

 and unvarying thickness, alternate with great masses of 

 sandstone and shale, containing innumerable impressions of 

 coal-plants, and a few seams of coal.* 



14. Carboniferous or Mountain Limestone. f — The 

 third and lowermost series of the carboniferous system, is 

 an extensive assemblage of calcareous strata, composed 

 for the most part of subcrystalline grey limestone, disposed 

 in beds of considerable thickness ; the strata through a 

 depth of many hundred feet being separated only by very 

 thin clay partings. Shales, grits, and amygdaloidal rocks, are 

 intercalated in certain localities ; and the principal beds of 



* Professor Sedgwick, " Address to the Gcol. Soc." 1831. 



\ This calcareous rock is also called metalliferous limestone, from 

 its abounding in lead and other metallic ores : and < ncriniiai limn 

 from the prevalence of crinoideal remains in some of the beds (ante, 

 p. G40). 



