PART II. CHAPTER XX. 243 



Carboniferous Group. 



pebbles of gneiss in the tertiary red sandstone of Auvergne, 

 point clearly to the rocks from which it -has been derived. The 

 red colouring matter may have been furnished by the decomposi- 

 tion of hornblende, or mica, which contain oxide of iron in large 

 quantity (see p. 102.). 



It is a general fact, and one not yet accounted for, that scarce- 

 ly any fossil remains are preserved in stratified rocks in which 

 this oxide of iron abounds ; and when we find fossils in the New 

 or Old Red sandstone in England, it is in the grey and usually 

 calcareous beds, that they occur. 



CHAPTER XX. 



THE COAL, OR CARBONIFEROUS GROUP. 



Carboniferous strata in the south-west of England Superposition of Coal- 

 measures to Mountain limestone Departure from this type in north of England 

 and Scotland Freshwater strata Intermixture of freshwater and marine beds 

 Sauroidal fish Fossil plants Ferns and Sigillarise Lepidodendra Calamites 

 Coniferse Stigmariae. 



THE next group which we meet with in the descending order 

 is the Carboniferous, commonly called "The Coal," because* 

 many beds of that mineral, in a more or less pure state, are in- 

 terstratified with sandstone, shale, and limestone, of which the 

 bulk of the formation is made up. The combustible coal itself, 

 even in Great Britain and Belgium, where it is most abundant, 

 constitutes but a small proportion of the whole mass. In the 

 north of England, for example, the thickness of the coal-bearing 

 strata has been estimated at 3000 feet, while the various coal- 

 seams, 20 or 30 in number, do not exceed 60 feet.* 



In the south-west of England, in Somersetshire, and in South 

 Wales, the Carboniferous series consists of, 



( Strata of shale, sandstone, and grit, with occasional 

 1st. Coal-measures, < seams of coal, sometimes exceeding 600 feet in 

 ;' J ; "~ Q thickness. 



f A coarse quartzose sandstone passing into a con- 

 2d. Millstone grit. < glomerate, sometimes used for millstones; de- 

 ( void of coal ; occasionally above 600 feet thick. 

 3d. Mountain or C A calcareous rock containing marine shells and 

 Carboniferous < corals, devoid of coal ; thickness variable ; some- 

 limestone, r times 900 feet. 



* Phillips; art. "Geology," Encyc. Britan. 



