CHAPTER XXIV.
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 — Section in South Wales — Underclays with Stigmaria — Carboniferous Flora — Ferns, Lepidodendra, Calamites, Asterophyllites, Sigillariæ,Stigmariæ — Coniferæ — Endogens — Absence of Exogens — Coal, how formed — Erect fossil trees — Parkfield Colliery — St. Etienne, Coal-field — Oblique trees or snags — Fossil forests in Nova Scotia — Brackish water and marine strata — 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 at 3000 feet, while the various coal-seams, 20 or 30 in number, do not in the aggregate exceed 60 feet.[308-A]
The carboniferous formation 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 south-western part of our island, in Somersetshire and South Wales, the three divisions usually spoken of by English geologists are:
| 1. Coal-measures | { | Strata of shale, sandstone, and grit, with occasional seams of coal, from 600 to 12,000 feet thick. | |
| 2. Millstone grit | { | A coarse quartzose sandstone passing into a conglomerate, sometimes used for millstones, with beds of shale; usually devoid of coal; occasionally above 600 feet thick. | |
| 3. Mountain or Carboniferous limestone | } | A calcareous rock containing marine shells and corals; devoid of coal; thickness variable, sometimes 900 feet. |
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 of limestone, with pectens, oysters, and other marine shells, occur in this grit, just as in the regular coal-measures, and even a few seams of coal. I shall treat, therefore, of the whole group, as consisting of two divisions only, the Coal-measures and Mountain Limestone. The latter is found in the southern British coal-fields, at the base of the system, or immediately in contact with the subjacent Old Red Sandstone; but as we proceed northwards to Yorkshire and Northumberland it begins to alternate with true coal-measures, the two deposits forming together a series of strata about 1000 feet in thickness. To this mixed formation succeeds the great mass of genuine mountain limestone.[309-A] Farther north, in the Fifeshire coal-field in Scotland, we observe a still wider departure from the type of the south of England, or a more complete intercalation of dense masses of marine limestones with sandstones, and shales containing coal.