Carboniferous System.

Carboniferous System. The strata included under this system rest upon those belonging to the Devonian and Old Red Sandstone system, and are overlaid by the Permian. The system derives its name from the amount of coal and carbonaceous matter which it contains. It is strongly developed in the British Islands, as in Wales, the midland and northern counties of England, the central lowlands of Scotland, and over a large part of Ireland. The following are the chief divisions of the system recognised in the British area, arranged in descending order:

• 1. COAL-MEASURES. { Red Sandstone Series.
  Upper Coal-bearing Series.
  Middle Coal-bearing Series.
  Lower Coal-bearing Series.
• 2. MILLSTONE Grit.
• 3. CARBONIFEROUS OR MOUNTAIN-LIMESTONE. { Yoredale Beds.
  Main or Scaur Limestone.
  Lower Limestone, Shale (England), and
  Calciferous Sandstones (Scotland).

The Carboniferous Limestone group is of very variable thickness, is widely distributed throughout the several districts in which the system is developed. In South Wales and the adjacent regions of England it is composed of a lower set of arenaceous and argillaceous strata, 200 to 500 feet thick, and an overlying massive gray or bluish limestone (500 feet thick) which is eminently of marine origin, being crowded with the debris of crinoids, corals, brachiopods, and other forms. Followed northward into the Pennine chain this limestone swells out to a thickness of 4000 feet, and is in that region overlaid by some 2000 feet of sandstones, shales, and limestones (Yoredale Beds). Passing into Northumberland and Scotland, the Carboniferous limestone series takes on quite a different character, the thick limestones of the more southern regions being represented by sandstones and shales, intercalated with which are seams of coal and ironstone, and only occasional beds of limestone. In Scotland the lower half of the Carboniferous system consists of two well-defined groups—viz. the Calciferous Sandstones, and overlying these the Carboniferous Limestone. These may be taken as approximately of contemporaneous origin with the Carboniferous limestone series of England. The Calciferous sandstones comprise two groups: the lower or Red Sandstone group consisting chiefly of red, gray or white, and yellow sandstones, with subordinate beds of shale; and the upper (Cement-stone group) including a considerable thickness of white and yellow sandstones, and dark shales with beds of impure cementstones, thin limestone, coal, ironstone, bituminous shale, occasional seams of gypsum, &c. With the lower or Red Sandstone group are associated a most extensive series of volcanic rocks (chiefly porphyrites, basalt-rocks, and tuffs) which form prominent ranges of high ground that sweep round the borders of the larger basins of Carboniferous strata. The hills that extend from the head-waters of the river Irvine to the shores of the Clyde, the Campsie Hills, the hill-ranges behind Burntisland, &c. in Fife, are examples. The Cement-stone group in the basin of the Firth of Forth is remarkable for its bituminous shales, now so largely worked for the production of mineral oils. Volcanic rocks are likewise associated with this group, but not so extensively as with the older Red Sandstone group. The Scottish Carboniferous limestone consists of a variable thickness of sandstones and dark shales, with which are intercalated seams of coal, fireclay, ironstone, and a few beds of limestone—the thickest of which rarely reaches 100 feet. The whole thickness of this series does not exceed 2700 feet as a maximum. Contemporaneous volcanic rocks occur in the series only in a few places.

In Ireland the Carboniferous system is represented chiefly by strata belonging to the mountain-limestone series. In the south and central areas the Welsh and English type appears—viz. massive limestones underlaid by thick shales, &c. Towards the north the Carboniferous limestone series approximates in character to that of the Lower Carboniferous of Scotland. Contemporaneous igneous rocks are associated with the limestone series in various parts of Ireland.

The Millstone Grit consists of a series of grits, sandstones, shales, &c., varying in England, from 300 feet up to 5000 feet in thickness. In Scotland the maximum thickness is 900 feet, and in some districts the series is hardly recognised.

The Coal-measures comprise a variable thickness of sandstones, and dark shales with seams of coal, fireclay, and ironstone. In South Wales this series attains a thickness of 8000 feet or thereabout; in Lancashire, 6600 feet; in the Midland districts, an average of about 3000 feet; in Northumberland and Durham, 3000 feet; in Scotland the maximum is 2100 feet. In Lancashire and Scotland the coal-bearing strata of this series are overlaid by a set of barren red sandstones.

The changes which the Carboniferous strata undergo as they are followed from England into Scotland show that the sea in which the thick mountain-limestone accumulated shallowed away towards the north; the Lower Carboniferous strata of Scotland pointing to estuarine or brackish and fresh-water conditions, interrupted now and again by longer or shorter spells when these shallow waters gave place to terrestrial conditions. The very subordinate position occupied by the thin limestones in the Scottish series sufficiently demonstrates that the low-lying tracts of Scotland were only occasionally invaded by the sea. Thus it has come to pass that many of the most important coal-seams of Scotland occur on the same geological horizon as the pure marine limestones of England and Ireland.

The Carboniferous system appears in many other parts of the world. In Europe the most notable coal-fields after those of Great Britain are the coal-fields of Liège and Mons (Belgium), and that of Saarbrück in the Rhenish Provinces. Various coal-fields also occur in France (St Etienne, &c.), in Hanover, Westphalia, Saxony, Bohemia, and Silesia. In Russia the strata belong chiefly to the lower part of the system, and are generally unproductive. A considerable coal-field, however, occurs along the northern shores of the Sea of Azov. Carboniferous rocks likewise occur in the Western Alps, but they are generally much altered, and disturbed, and barren. They are also known in the north of Spain, where they yield some coal. In China, coal-bearing strata are extensively developed, but they are worked in a very unscientific manner. Coal of Carboniferous age is worked in Japan, and in South Wales there is a considerable coal-field of the same age.

In North America the system attains a great development. The Carboniferous strata here are divided into two groups: the Lower or sub-Carboniferous (corresponding to the Carboniferous limestone of Britain) and the Carboniferous, comprising the millstone grit and the coal-measures. According to Professor Dana, the coal-bearing area of the North American continent is approximately as follows:

It is worthy of note that Carboniferous strata have likewise been detected in the Arctic regions, as in McIville Island, the northern coast-lands of Baffin Bay, in Bear Island, and Spitzbergen.

Economic Products.—The most important of these are of course the coals, ironstones, and limestones. The individual coal-seams vary in thickness from less than an inch up to many feet or even yards; these last, however, consist generally of several seams which have come together, owing to the thinning-out of the beds that separate them elsewhere. The total thickness of workable coal naturally varies in the different coal-fields—in some of which we may have a greater thickness of strata or a larger number of seams. In South Wales—the largest coal-field in Britain—the total thickness of coal exceeds 200 feet; in Lancashire the workable coals give an aggregate of 100 feet; in Northumberland and Durham the aggregate is 47 feet; in Lanarkshire the united thickness of workable coal-seams (true coal-measures) varies from 30 to 70 feet or more, and 15 feet in the limestone series; in Midlothian there is an aggregate in the coal-measures proper of 43 feet, and in the limestone series of 68 feet. The coals vary much in character (see COAL), some being used for ordinary household purposes, others for steam, &c., others for the production of gas, mineral oil, &c. In the American coal-measures (with a maximum thickness of 3000 feet) the maximum of included coal is 120 feet. In Britain the bedded iron ores are principally clay-ironstones, and the carbonaceous variety called blackband (see IRON ORES). Hematite also occurs massively in large 'pockets' in the thick limestones (Cumberland and Northumberland). Ores of zinc, lead, and antimony are likewise met with in the limestones, as in Derbyshire, Cumberland, Yorkshire, Wales, &c. The fireclays are largely worked in many places for the manufacture of bricks, &c. The sandstones likewise are held in great repute for building purposes, some, however, being much more durable than others. The thin-bedded varieties again are largely used for flags or paving-stones.

Other kinds of sandstones are employed for millstones, grindstones, crushers, &c., and some of the harder kinds for road-metal. The limestones are in extensive request for mortar, cement, agricultural purposes, &c., some of the black and coloured or highly fossiliferous varieties being in considerable demand for ornamental mason-work. Amongst the shales the bituminous kinds are the most important, but from others sulphur and sulphuric acid are extracted, while some are employed in the preparation of alum and sulphate of iron. Locally certain minerals occur in sufficient quantity to be of economic importance, such, for example, as fluor-spar, used for ornamental purposes, and Baryta (q.v.), or heavy-spar.

Life of the Period.—Among the most characteristic and abundant fossils of the Carboniferous system are, as might have been expected, the plants, most of which belong to the flowerless or cryptogamic division. Ferns were especially numerous, such as Pecopteris, Sphenopteris, and Neuropteris (q.v.), and the arborescent forms, such as Caulopteris (q.v.). Besides these there were great lycopodiaceous trees such as Lepidodendron (q.v.), tall Equisetaceæ such as Calamites (q.v.), and the remarkable sigillarioid trees, as to the affinities of which there is still much doubt (see SIGILLARIA). Large coniferous trees (Araucarioxylon) are also met with now and again. Amongst these some botanists rank Cordaites, a form of not infrequent occurrence. Others, however, are of opinion that this plant had affinities with the lycopods and even with the cycads. The contemporaneous land-fauna has left but few traces. Here and there, however, specimens of true air-breathers are met with. Amongst these are amphibians, such as Archegosaurus (q.v.), millepedes, and insects of various kinds, such as extinct forms of May-flies, cockroaches, crickets, locusts, &c., and others which cannot with certainty be referred to any existing type, for they seem to have affinities with various widely separated modern forms. Scorpions abounded, and these ancient forms closely approximate in character to the living type. Land-snails have also been found, not differing much apparently from the living Pupa. The fauna of the seas and lagoons is much better represented than that of the land. The minute foraminifera swarmed in places—their shells occasionally forming beds of limestone. Corals were likewise extremely abundant, their remains crowding many of the limestones. Common forms are Lithostrotion, Clisiophyllum, Zaphrentis, Favosites, &c. Other great limestone-formers were the crinoids (such as Platycrinus, &c.), the columns and detached plates of which are among the most abundant fossils in the limestones and calcareous shales of the system. The peculiar armless crinoids, Pentremites, are likewise characteristic Carboniferous fossils. Crustaceans were represented by a few Trilobites (q.v.), the last survivors of that great family, and by ancestral forms of our ostracods, phyllopods, and king-crabs, as well as by the extinct group of eurypterids. Polyzoa, such as Fenestella, are very common, as are also brachiopods; among the more frequently occurring forms being Productus, Spirifer, Terebratula (q.v.), &c. Lamellibranchs are fairly represented, especially by the Aviculoid group—some of the forms having apparently frequented estuaries or lagoons. Gasteropods and pteropods also occur, and are locally abundant, but they are not so common fossils as the brachiopods. Cephalopods are not infrequent, especially Orthoceras (q.v.), of which over 150 species are known from this system. Other notable forms are Nautilus (q.v.) and Goniatites (q.v.). Fishes are tolerably common. They come under the two groups of Ganoids and Placoids. The former are represented by such forms as Rhizodus and Megalichthys; and the latter by numerous large and often finely sculptured fin-spines, and hard palatal crushing teeth. These large sharks preyed upon the smaller ganoids, whose teeth and scales are common in the Coprolites (q.v.), or fossil droppings of the former.