From England, Carboniferous rocks can be followed across northern and central France, into Germany, Bohemia, the Alps, Italy and Spain. In Russia this system occupies some 30,000 sq. m., and it extends northward at least as far as Spitsbergen. Carboniferous rocks are present in North and South Africa, and in India and Australasia; in China they cover thousands of square miles, and in the United States and British North America they occupy no less than 200,000 sq. m.; they are known also in South America.
The subjoined table expresses the typical subdivisions which can be recognized, with modifications, in the United Kingdom.
| Coal Measures. | Upper: Red and grey sandstones, marls and clays with occasional breccias, thin coals and limestones with Spirorbis, workable coals in the South Wales, Bristol, Somerset and Forest of Dean coalfields. Middle: Sandstones, marls, shales and the most important of the British coals. Lower: Flaggy hard sandstones (ganister), shales and thin coal seams. |
| Millstone Grit. | Grits (coarse and fine), shales, thin coal seams and occasional thin limestones. The fossil plants connect this group with the coal-measures; the marine fossils have, to some extent, a Carboniferous limestone aspect. |
| Carboniferous Limestone Series. | Upper black shales with thin limestones (Pendleside group) connecting this series with the Millstone grit above. The thick, main or scaur limestone (mountain limestone) of the centre and south of England, Wales and Ireland, which splits up in the Yorkshire dales (Yoredale group) into a succession of stout limestone beds between beds of sandstone and shale, and becomes increasingly detrital in character as it is traced northwards. Lower limestone shales of the south and centre of England with marine fossils, and the Calciferous Sandstone group of Scotland with marine, estuarine and terrestrial fossils. |
(See [Bernician], [Tuedian] and [Avonian].)
At an early period, owing to the immense commercial importance of the coal seams, it became the practice to distinguish a “productive” (flotzfuhrend, terrain houiller) and an “unproductive,” barren (flotzleerer) Lower Carboniferous; these two groups correspond in North America to the “Carboniferous” and “Sub-Carboniferous” respectively, or, as they are now sometimes styled, the “Pennsylvanian” and “Mississippian.” But it was soon discovered that the “productive” beds were not regularly restricted to the upper or younger division, and, as E. Kayser points out, the real state of the matter is more accurately represented by the subjoined tabular scheme.
| Continental Type of Deposit. | Marine Type of Formation. | ||
| Upper Carboniferous | Upper Productive Carboniferous | Younger Carboniferous limestone and the Fusulina limestone of Russia and Western North America | |
| Lower Carboniferous | Lower Productive Carboniferous | Culm (in part) | Lower Carboniferous limestone series |
While the continental type of deposit, with its coal beds, was the earliest to be formed in certain areas, and the marine series came on later, in other regions this order was reversed. It should be observed, however, that the repeated intercalation of marine deposits within the continental series and the frequent occurrence of thin coaly layers in the marine series makes any hard and fast distinction of this kind impossible.
The so-called “unproductive” or barren strata, that is, those without workable coals, are not always limestones; quite as often they are shales, red sandstones and red marls.
In subdividing the strata of the Carboniferous system and correlating the major divisions in different areas, just as in other great systems, use has to be made of the fossil contents of the rocks; stratigraphical units, based on lithology, are useless for this purpose. The groups of organisms utilized for zoning and correlation by different workers include brachiopods, pelecypods, cephalopods, corals, fishes and plants; and the results of the comparison of the faunas and floras of different areas where Carboniferous rocks occur are generalized in the table below.
The relative value of any group of animals or plants for the correlation of distant areas must vary greatly with the varying conditions of sedimentation and with the precise definition of the zonal species and with many other factors. It is found that the subdivisions in this system demanded by palaeobotanists do not always coincide with those acknowledged by palaeozoologists; nevertheless there is general agreement as to the main divisional lines.