Carboniferous System

Carboniferous System, a great series of Palaeozoic rocks named from the occurrence of coal (q.v.) in its upper portion, reaching sometimes a thickness of 20,000 feet. It generally passes conformably downwards into the underlying Old Red Sandstone, and in Bohemia, at Autun in France, and elsewhere, it passes conformably upward into Permian rocks. Carboniferous rocks seem mostly to have accumulated in the sea not far from land, or in lagoon swamps that have been compared to the mangrove swamps of the present day. The close of the Devonian epoch would seem to have been marked by great though gradual geographical changes, so that an open sea extended from the west of Ireland to Westphalia, undergoing during the earlier part of the Carboniferous epoch continuous depression, but shallowing towards land to the north of Derbyshire. Subsequently, during the latter part of the epoch, though depression must have continued, at least intermittently, the "lagoon type" of shallower water conditions seems to have extended southward over most of the area occupied previously by the "marine type." In the open sea a very pure limestone, sometimes foraminiferal, sometimes crinoidal, and sometimes coralline, known as the Carboniferous, or, from the scenery it now often forms, as Mountain Limestone, accumulated to a depth in some places exceeding 6,000 feet. The lagoon type, on the other hand, is represented by thousands of feet of sandstone and grit, with occasional conglomerate and shale, with seams of coal (q.v.) resting on beds of fire-clay, and with beds of clay-ironstone (q.v.) nodules. False-bedding (q.v.), ripple-mark, and suncracks tell of the shallow water origin of the sandstones, and the coal-seams mark successive forest-growths during considerable pauses in the sinking of the area. Volcanic activity during the earlier part of the epoch is marked by intercalated rocks in Derbyshire, the Isle of Man, and especially in the south of Scotland, where some sheets reach a thickness of 1,500 feet. In Russia, China, and western North America, Carboniferous rocks cover large areas horizontally, as does the Carboniferous Limestone in Ireland; but in England the limestone forms the axial Pennine anticlinal from Northumberland to Derbyshire, and elsewhere the system is mainly preserved in synclinal basins or "coal-fields," once united but now detached. The limestones contain a rich marine fauna, 1,500 species having been described. They are largely composed of foraminifera, such as Fusulina; abound in corals, such as Lithostrotion; in crinoids, such as Platycrinus; in polyzoans, especially Fenestella; in brachiopods, especially Productus and Spirifer; and in pelecypods; and contain the blastoid Pentremites, numerous gastropods, pteropods, and cephalopods, the last of the trilobites and numerous fish, some of large size, represented by spines and teeth like those of rays or sharks. The flora of the shales and coal includes Calamites (q.v.), Lepidodendron (q.v.), and Sigillaria (q.v.), reaching the size of trees; ferns, such as Anthracosia, characterising the higher beds; and, apparently from higher ground, some little known conifers. Mussels, probably fresh-water, such as Anthracosia, scorpions, millepedes, a great variety of insects belonging to a primitive type (Palaeodictyoptera), especially from Commentry in France, and snails, such as Pupa and Zonites, and large salamander-like labyrinthodonts (q.v.), such as Archegosaurus, the earliest of their class, occur in the same beds with this flora, though an occasional band contains marine shells. The system may be subdivided as follows: -

Upper - Coal-Measure series. (3,000 feet in Scotland; 12,000 feet in South Wales.)

• Upper: 150 to 500 feet.
• Middle: With Pennant Grit. 8,000 to 4,000 feet.
• Lower: With Gannister (a sileceous fire-clay), 450 to 2,000 feet.

Middle. - Millstone Grit.

• 300 to 5,500 feet.

Lower. - Carboniferous Limestone series.

• Yoredale Shales and Grits. 300 to 4,500 feet.
• Thick or Scaur Limestone. 500 to 3,500 feet.
• Limestone Shale or Tuaedian, with Calciferous Sandstone of Scotland. 100 to 1,000 feet.

The divisions, as will be seen, vary exceedingly in thickness. In the north a few coal-seams occur in the Limestone and Millstone Grit; but in the south the latter is known as Farewell Rock, no coal occurring in or below it. From its barrenness it is called Moor Rock in the north. In South Wales there are about eighty coal-seams with a total thickness of 120 feet; in Staffordshire 30 feet are worked as one seam. It is probable that the highest beds of the Coal Measures, present at Autun, and in Bohemia, are absent in Britain. In addition to coal and iron (haematite, as at Ulverston, from the Limestone, and clay-ironstone from the Coal Measures) the system yields muoh valuable flagstone, especially the Yorkshire flags; the Craigleith or Calciferous sandstone (q.v.) for building; various marbles, grey, black, and encrinital; millstones, grindstones, and honestones; ores of lead, copper, and zinc in veins in the Limestone; and, by distillation of the often bituminous shales, paraffin, alum, and copperas.