S58 THE CARBONIFEROUS GROUP. [Ch. XXIY 



CHAPTER XXIV. 



THE COAL, OR CARBONIFEROUS GROUP. 



Carboniferous strata in the soufhwest of England—Superposition of Coal-measurea 

 to Mountain limestone — Departure from this type in I^orth of England and 

 Scotland — Carboniferous series in Ireland — Sections in South Wales — Under- 

 clays with Stigmaria — Carboniferous Flora — Ferns, Lepidodendra, Equisetacese, 

 Calamites, Asterophyllites, Sigillariee, Stigmariae — Coniferee — Sternbcrgia— 

 Trigonocarpon — Grade of Coniferse m the Vegetable Kingdom — Absence ot 

 Angiosperms — Coal, how formed — Erect fossil trees — Parkfield Colliery — St. 

 Etienne Coal-field — Oblique trees or snags — Fossil forests in Nova Scotia — 

 Rain-prints— Purity of the Coal explained — Time required for ;Iie accumula- 

 tion of the Coal-measures — Brackish- water and marine strata — Crustaceans ot 

 the Coal — Origin of Clay-iron-stone. 



The next group which we meet with in the descending order is the 

 Carboniferous, commonly called " The Coal ;" because it contains many 

 beds of that mineral, in a more or less pure state, interstratified with 

 sandstones, shales, and limestones. The coal itself, even in Great Britain 

 and Belgium, where it is most abundant, constitutes but an insignificant 

 portion of the whole mass. In the north of England, for example, the 

 thickness of the coal-bearing strata has been estimated by Professor Phil- 

 lips at 3000 feet, while the various coal-seams, 20 or 30 in number, do 

 not in the aggregate exceed 60 feet. 



The carboniferous formation assumes various characters in different 

 parts even of the British Islands. It usually comprises two very distinct 

 members ; 1st, that usually called the Coal-measures, of mixed freshwater, 

 terrestrial, and marine origin, often including seams of coal ; 2dly, that 

 named in England the Mountain or Carboniferous Limestone, of purely 

 marine origin, and containing corals, shells, and encrinites. 



In the Southwestern part of our island, in Somersetshire and South 

 Wales, the three divisions usually spoken of by English geologists are : 



, „ , C Strata of shale, sandstone, and grit, with occasional seama 



1. Coal-measures | ^^ ^^^j^ ^^.^^ g^^ ^^ ^2,000 feet thick. 



C A coarse quartzose sandstone passing into a conglomerate, 



2. Millstone-grit < sometimes used for millstones, with beds of shale ; usually 



( devoid of coal ; occasionally above 600 feet thick. 



'^' o °k" ^T ^^ (. -^ calcareous rock containing marine shells and corals; de- 

 Oarboniterous > ^^j^ ^^ ^^^^ thickness variable, sometimes 900 feet, 

 limestone. ) ' 



The millstone-grit may be considered as one of the coal-sandstones 

 of coarser texture than usual, with some accompanying shales, in which 

 coal-plants are occasionally found. In the north of England some bands 



