[1] Trans. Geol. Soc. Lond., Second Series, vol. iii, p. 37.
[2] Edward Hull, Ternary Classification, Quart. Journ. Science, No. xxiii, 1869.
[3] King’s Monograph, pl. 2.
[4] Ramsay, Quart. Geol. Journ., 1855; and Lyell, Principles of Geology, vol. i, p. 223, 10th edit.
[5] Murchison’s Russia, vol. ii, pl. A, fig. 3.
CHAPTER XXIII.
THE COAL OR CARBONIFEROUS GROUP.
Principal Subdivisions of the Carboniferous Group. — Different Thickness of the sedimentary and calcareous Members in Scotland and the South of England. — Coal-measures. — Terrestrial Nature of the Growth of Coal. — Erect fossil Trees. — Uniting of many Coal-seams into one thick Bed. — Purity of the Coal explained. — Conversion of Coal into Anthracite. — Origin of Clay-ironstone. — Marine and brackish-water Strata in Coal. — Fossil Insects. — Batrachian Reptiles. — Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia Coal-measures with successive Growths of erect fossil Trees. — Similarity of American and European Coal. — Air-breathers of the American Coal. — Changes of Condition of Land and Sea indicated by the Carboniferous Strata of Nova Scotia.
Principal Subdivisions of the Carboniferous Group.—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 South Wales, for example, the thickness of the coal-bearing strata has been estimated at between 11,000 and 12,000 feet, while the various coal seams, about 80 in number, do not, according to Professor Phillips, exceed in the aggregate 120 feet.
The Carboniferous formation assumes various characters in different parts even of the British Islands. It usually comprises two very distinct members: first, the sedimentary beds, usually called the Coal-measures, of mixed fresh-water, terrestrial, and marine origin, often including seams of coal; second, that named in England the Mountain or Carboniferous Limestone, of purely marine origin, and made up chiefly of corals, shells, and encrinites, and resting on shales called the shales of the Mountain Limestone.
In the south-western part of our island, in Somersetshire and South Wales, the three divisions usually spoken of are: