Oi XXIV.] THE CARBONIFEROUS GROUP. 465 



CHAPTER XXIV. 



THE COAL, OR CARBONIFEROUS GROUP. 



Carboniferous strata in the southwest of England — Superposition of Coal-measures 

 to Mountain Limestone — Departure from this type in North of England and 

 Scotland — Carboniferous series in Ireland — Section in South Wales — Under-clays 

 with Stigmaria — Carboniferous Flora — Ferns, Lepidodendra, Equisetaceas, Cala- 

 mites, Asterophyllites, Sigillarige, Stigmaria? — Coniferse — Sternbergia — Trigono- 

 carpon — Grade of Coniferse in the Vegetable Kingdom — Absence of 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 the accumulation of the Coal-measures 

 — Brackish-water and marine strata — Crustaceans of 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 esti- 

 mated by Professor Phillips 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 dis- 

 tinct 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 



l fnil mpncnrPQ i Strata of sna le, sandstone, and grit, with occasional seams 

 i. ooai-measures. -j of cQal fi>om 60Q tQ 12 ^ QQQ feet ^.^ 



I 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. 



3. Mountain or I A calcareous rock containing marine shells and corals ; 



Carboniferous -j devoid of coal ; thickness variable, sometimes 1500 

 Limestone. ( feet. 

 30 



