1911 Encyclopædia Britannica/Carboniferous System
CARBONIFEROUS SYSTEM, in geology, the whole of the great series of stratified rocks and associated volcanic rocks which occur above the Devonian or Old Red Sandstone and below the Permian or Triassic systems, belonging to the Carboniferous period. The name was first applied by W. D. Conybeare in 1821 to the coal-bearing strata of England and Wales, including the related grits and limestones immediately beneath them. The term is a relic of that early period in the history of stratigraphy when each group of strata was supposed to be distinguished by some peculiar lithological character. In this case the carbonaceous beds—coal-seams—naturally appealed most strongly to the imagination, and the name is a good one, notwithstanding the fact that coal-seams occupy but a small fraction of the total thickness of the Carboniferous system; and although subsequent investigations have demonstrated the existence of coal in other geological formations, in none of these does it play so prominent a part. The stratified rocks of this system include marine limestones, shales and sandstones; estuarine, lagoonal and fresh-water shales, sandstones and marls with beds of coal, oil-bearing rocks, gypsum and salt.
In many parts of the world there is no sharp line of demarcation between the Devonian and the Carboniferous rocks; neither can the fossil faunas and floras be clearly separated at any well-defined line; this is true in Britain, Belgium, Russia, Westphalia and parts of North America. Again, at the summit of the Carboniferous series, both the rocks and their fossil contents merge gradually into those of the succeeding Permian system, as in Russia, Bohemia, the Saar region and Texas. This has led certain geologists to classify the Devonian, Carboniferous and Permian into one grand system; E. Renevier in 1874 proposed to include these three into a single “Carbonique” system, later he retained only the two latter groups. There seems to be sufficient reason, however, to maintain each of these groups as a separate system and limit the term Carboniferous (carbonifèrien) in the manner indicated above. At the same time it must be remembered that there is in India, South Africa, the Urals, in Australasia and parts of North America an important series of rocks, with a “Permo-Carboniferous” fauna, which constitutes a passage formation between the Carboniferous, sensu stricto, and Jurassic rocks.
Stratigraphy.—No assemblage of stratified rocks has received such careful and detailed examination as the Carboniferous system; consequently our knowledge of the stratigraphical sequence in isolated local areas, where the coals have been exploited, is very full.
In Europe, the system is very completely developed in the British Isles, where was made the first successful attempt at a classification of its various members, although at a somewhat earlier date Omalius d’Halloy had recognized a terrain bituminifère or coal-bearing series in the Belgian region.
The area within which the Carboniferous rocks of Britain occur is sufficiently extensive to contain more than one type of the system, and thus to cast much light on the varied geographical conditions under which these rocks were accumulated. In prosecuting the study of this part of British geology it is soon discovered, and it is essential to bear in mind, that, during the Carboniferous period, the land whence the chief supplies of sediment were derived rose mainly to the north and north-west, as it seems to have done from very early geological time. While therefore the centre and south of England lay under clear water of moderate depth, the north of the country and the south of Scotland were covered by shallow water, which was continually receiving sand and mud from the adjacent northern land. Hence vertical sections of the Carboniferous formations of Britain differ greatly according to the districts in which they are taken.
The Coal-Measures and Millstone Grit are usually grouped together in the Upper Carboniferous, the Carboniferous Limestone series constituting the Lower Carboniferous.
In addition to the above broad subdivisions, Murchison and Sedgwick, when working upon the rocks of Devonshire and Cornwall, recognized, with the assistance of W. Lonsdale, another phase of sedimentation. This comprised dark shales, with grits and thin limestones and thin, impure coals, locally called “culm” (q.v.). These geologists appropriated the term “culm” for the whole of this facies in the west of England, and subsequently traced the same type on the European continent, where it is widely developed in the western centre.
Besides the considerable exposed area of Carboniferous rocks in Great Britain, there is as much or more that is covered by younger formations; this is true particularly of the eastern side of England and the south-eastern counties, where the coal-measures have already been found at Dover.
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.|
|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.
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|
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.
Breaks in the Stratigraphic Sequence.—The sequence of Carboniferous strata is not everywhere one of unbroken continuity. From central France eastward towards the Carpathians only later portions of the system are found. These generally rest upon crystalline rocks, but in places they contain evidence of the denuded surfaces of Lower Carboniferous, as in the basin of Charleroi, where the equivalent of the Millstone Grit contains fragments of chert which can only have come from the waste of the earlier limestones. This unconformity is generally found about the same horizon in the continental Culm areas, and it occurs again in the western part of the English Culm.
Tabular Statement of the Principal Subdivisions of the Carboniferous System.
|Coal Measures =
|Ouralien and Stephanien
(marine type) (continental type)
|Moscovien and Westphalien
(marine type) (continental type)
| Dinantien and Culm
(marine pelagic, (marine littoral)
in some areas)
In the eastern border of the Rhenish Schiefergebirge the Permian rests unconformably upon Lower Carboniferous rocks. In the United States, in Missouri, Pennsylvania, West Virginia, Kentucky, Ohio and elsewhere, there is an unconformable junction between the Lower and Upper Carboniferous, representing an interval of time during which the lower member was strongly eroded; it has even been proposed to regard the Mississippian (Lower Carboniferous) as a distinct geological period, mainly on account of this break in the succession.
Thickness of Carboniferous Rocks.—The great variety of conditions under which the sediments and limestones were formed naturally produced corresponding inequalities in the thickness. In the Eurasian land area the greatest thickness of Carboniferous rocks is in the west; in North America it is in the east. In Britain the Carboniferous limestone series is 2000–3500 ft. thick; in the Ural mountains it is over 4500 ft.; the Culm in Moravia is credited with the enormous thickness of over 42,000 ft. The Upper Carboniferous in Lancashire is from 12,000 to 13,000 ft.; elsewhere in Britain it is thinner. In western Germany this portion attains a thickness of 10,000 ft. In Pennsylvania the sandstone and shale, at its maximum, reaches 4400 ft., but even within the limits of the state this formation has thinned out to no more than 300 ft. in places. In Colorado the Lower Carboniferous is only 400–500 ft. thick; while the limestones of the Mississippi basin amount to 1500 ft. and in Virginia are 2000 ft. thick.
Life of the Carboniferous Period.—We have seen that in the Carboniferous rocks there are two phases of sedimentation, the one marine, the other continental; corresponding with these there are two distinct faunal facies.
(1) Fauna of the Marine Strata.—Numerically, the most important inhabitants of the clear Carboniferous seas were the crinoids, corals, Foraminifera and brachiopods. Each of these groups contributed at one place or another towards the upbuilding of great masses of limestone. For the first time in the earth’s history we find Foraminifera taking a prominent part in the marine faunas; the genus Fusulina was abundant in what is now Russia, China, Japan, North America; Valvulina had a wide range, as also had Endothyra and Archaediscus; Saccammina is a form well known in Britain and Belgium, and many others have been described; some Carboniferous genera are still extant. Radiolaria are found in cherts in the Culm of Devonshire and Cornwall, in Russia, Germany and elsewhere. Sponges are represented by spicules and anchor ropes. Corals, both reef-builders and others, flourished in the clearer waters; rugose forms are represented by Amplexoid, Zaphrentid and Cyathophyllid types, and by Lithostrotion and Phillipsastraea; common tabulate forms are Chaetetes, Chladochonus, Michelinia, &c. Amongst the echinoderms crinoids were the most numerous individually, dense submarine thickets of the long-stemmed kinds appear to have flourished in many places where their remains consolidated into thick beds of rock; prominent genera are Cyathocrinus, Woodocrinus, Actinocrinus; sea-urchins, Archaeocidaris, Palaeechinus, &c., were present; while the curious extinct Blastoids, which included the groups of Pentremitidae and Codasteridae, attained their maximum development.
Annelids (Spirorbis, Serpulites, &c.) are common fossils on certain horizons. The Bryozoa were also abundant in some regions (Polypora, Fenestella), including the remarkable form known as Archimedes.
Brachiopods occupied an important place; most typical were the Productids, some of which reached a great size and had very thick shells. Other common genera are Spirifer, Chonetes, Athyris, Rhynchonellids and Terebratulids, Discina and Crania. Some species had an almost world-wide range with only minor variations; such are Productus semireticulatus, P. cora, P. pustulosus; Orthotetes (Streptorhynchus) crenistria, Dielasma hastata, and many others.
Pelecypods among the true mollusca were increasing in numbers and importance (Aviculopecten, Posidonomya); Nucula, Carbonicola, Edmondia, Conocardium, Modiola. Gasteropods also were numerous (Murchisonia, Euomphalus, Naticopsis). The Pteropods were well represented by Conularia and Bellerophon. Amongst the Cephalopods, the most striking feature is the rise and development of the Goniatites (Glyphioceras, Gastrioceras, &c.); straight-shelled forms still lived on in some variety (Orthoceras, Actinoceras), along with numerous nautiloids.
Trilobites during this period sank to a very subordinate position, but Ostracods (Cythere, Kirkbya, Beyrichia) were abundant.
Many fish inhabited the Carboniferous seas and most of these were Elasmobranchs, sharks with crushing pavement teeth (Psammodus), adapted for grinding the shells of brachiopods, crustaceans, &c. Other sharks had piercing teeth (Cladoselache and Cladodus); some, the petalodonts, had peculiar cycloid cutting teeth. The Arthrodirans, so prominent during the Devonian period, disappeared before the close of the Carboniferous. Most of the sharks lived in the sea continuously, but the ganoids frequenting the coastal waters appear to have migrated inland. About 700 species of Carboniferous fish have been described largely from teeth, spines and dermal ossicles.
(2) Flora and Fauna of the Lagoonal or Continental Facies.—The strata deposited during this period are the earliest in which the remains of plants take a prominent place. The fossil plants which are found in the upper beds of the preceding Devonian system are so closely related to those in the Lower Carboniferous, that from a palaeobotanical standpoint the two form one indivisible period.
In the Lower Carboniferous the flora was composed of six great groups of plants, viz. the Equisetales (Horse-tails), the Lycopodiales (Club mosses), the Filicales (Ferns) and Cycadofilices, the Sphenophyllales and Cordaitales. These six groups were the dominant types throughout the period, but during Upper Carboniferous time three other groups arose, the Coniferales, the Cycadophyta, and the Ginkgoales (of which Ginkgo biloba is the only modern representative). Algae and fungi also were present, but there were no flowering plants. The true ferns, including tree ferns with a height of upwards of 60 ft., were associated with many plants possessing a fern-like habit (Cycadofilices) and others whose affinities have not yet been definitely determined. The fronds of some of these Carboniferous ferns are almost identical with those of living species. Probably many of the ferns were epiphytic. Pecopteris, Cyclopteris, Neuropteris, Alethopteris, Sphenopteris are common genera; Megaphyton and Caulopteris were tree ferns. Our modern diminutive “horse-tails” with scaly leaves were represented in the Carboniferous period by gigantic calamites, often with a diameter of 1 to 2 ft. and a height of 50 to 90 ft. The Carboniferous forerunners of the tiny club-moss were then great trees with dichotomously branching stems and crowded linear leaves, such as Lepidodendron (with its fruit cone called Lepidostrobus), Halonia, Lepidophloios and Sigillaria, the largest plants of the period, with trunks sometimes 5 ft. in diameter and 100 ft. high. The roots of several of these forms are known as Stigmaria. Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the lycopods. Cordaites, a tall plant (20-30 ft.) with yucca-like leaves, was related to the cycads and conifers; the catkin-like inflorescence, which bore yew-like berries, is called Cardiocarpus. Many large trees which have been looked upon as conifers on account of their wood structure may perhaps belong more properly to the Cordaitales. True coniferous trees (Walchia) do appear at the top of the coal measures.
The animals preserved in the continental type of Carboniferous deposit naturally differ markedly from the fossil remains of the purely marine portions of the system. The inhabitants of the waters of this geographical phase include mollusca, which are supposed to have lived in brackish or fresh water, such as Anthracomya, Naiadites, Carbonicola, and many forms of Crustacea, e.g. (Bairdia Carbonia), phyllopods (Estheria), phyllocarids (Acanthocaris, Dithyrocaris), schizopods (Anthrapalaemon), Eurypterids (Eurypterus, Glyptoscorpius). Fishes were abundant, many of the smaller ganoids are beautifully preserved in an entire condition, other larger forms are represented by fin spines, teeth and bones; Ctenodus, Uronemus, Acanthodes, Cheirodus, Gyracanthus are characteristic genera.
Frequently a temporary return of marine conditions permitted the entombment of such salt water genera as Lingula, Orbiculoidea, Productus in the thin beds known as “marine bands.”
Remains of air-breathing insects, myriapods and arachnids show that these forms of life were both well developed and individually numerous. Among the insects we find the Orthoptera, Neuroptera, Hemiptera and Coleoptera represented; cockroaches were particularly abundant; crickets, beetles, locusts, walking-stick insects, mayflies and bugs are found, but there were neither flies, moths, butterflies nor bees, which is no more than we should expect from the conditions of plant life. Many insects, &c., have been obtained from the coalfields of Saarbrück and Commentry, and from the hollow trunks of fossil trees in Nova Scotia. Certain British coalfields have yielded good specimens: Archaeoptilus, from the Derbyshire coalfield, had a spread of wing extending to more than 14 in.; some specimens (Brodia) still exhibit traces of brilliant wing colours. In the Nova Scotian tree trunks land snails (Archaeozonites, Dendropupa) have been found.
In the later Carboniferous rocks the earliest amphibians make their appearance in considerable numbers; they were all Stegocephalians (Labyrinthodonts) with long bodies, a head covered with bony plates and weak or undeveloped limbs. The largest were about 7 or 8 ft. long, the smallest only a few inches. Some were probably fluviatile in habit (Loxomma, Anthracosaurus, Ophiderpeton); others may have been terrestrial (Dendrerpeton, Hylerpeton). Certain footprints in the coal measures of Kansas have been supposed to belong to lacertilian or dinosaurian forms.
The Physical Conditions during the Period.—In western Europe the advent of the Carboniferous period was accompanied by the production of a series of synclines which permitted the formation of organic limestones, free from the sediments which generally characterized the concluding phases of the preceding Devonian deposition. The old land area still existed to the north, but doubtless much reduced in height; against this land, detrital deposits still continued to be formed, as in Scotland; while over central Ireland and central and northern England the clearer waters of the sea furnished a suitable home for countless corals, brachiopods and foraminifera and great beds of sea lilies; sponges flourished in many parts of the sea, and their remains contributed largely to the formation of the beds of chert. This clearer water extended from Ireland across north-central England and through South Wales and Somerset into Belgium and Westphalia; but a narrow ridge of elevated older rocks ran across the centre of England towards Belgium at this time.
Traced eastward into north Germany, Thuringia and Silesia, the limestones pass into the detrital culm formations, which owe their existence to a southern uplifted massif, the complement of the synclines already mentioned. Sediments approaching to the culm type, with similar flora and fauna, were deposited in synclinal hollows in parts of France and Spain.
Thus western Europe in early Carboniferous time was occupied by a series of constricted, gulf-like seas; and on account of the steady progress of intermittent warping movements of the crust, we find that the areas of clearer water, in which the limestone-building organisms could exist, were repeatedly able to spread, thus forming those thin limestones found interbedded with shale and sandstone which occur typically in the Yoredale district of Yorkshire and in the region to the north, and also in the culm deposits of central Europe. The spread of these limestones was repeatedly checked by the steady influx of detritus from the land during the pauses in movements of depression. Looking eastward, towards central and northern Russia, we find a wider and much more open sea; but the continental type of deposit prevailed in the northern portion, and here, as in Scotland, we find coal-beds amongst the sediments (Moscow basin). Farther south in the Donetz basin the coals only appear at the close of the Lower Carboniferous.
In North America, the crustal movements at the beginning of the period are less evident than in Europe, but a marked parallelism exists; for in the east, in the Appalachian tract, we find detrital sediments prevailing, while the open sea, with great deposits of limestone, lay out towards the west in the direction of that similar open sea which lay towards the east of Europe and extended through Asia.
The close of the early Carboniferous period was marked by an augmentation of the orogenic movements. The gentler synclines and anticlines of the earlier part of the period became accentuated, giving rise to pronounced mountain ridges, right across Europe.
This movement commenced in the central and western part of the continent and continued throughout the whole Carboniferous period. The mountains then formed have been called the “Palaeozoic Alps” by E. Kayser, the “Hercynian Mountains” by M. Bertrand. The most western range extended from Ireland through Wales and the south of England to the central plateau of France; this was the “Armorican range” of E. Suess. The eastern part of the chain passed from South France through the Vosges, the Black Forest, Thuringia, Harz, the Fichtelgebirge, Bohemia, the Sudetes, and possibly farther east; this constitutes the “Varischen Alps” of Suess.
The sea had gained somewhat at the beginning of the Carboniferous period in western Europe, but the effect of these movements, combined with the rapid formation of detrital deposits from the rising land areas, was to drive the sea steadily from the north towards the south, until the open sea (with limestones) was relegated to what is now the Mediterranean and to Russia and thence eastward. Similar events were meanwhile happening in North America, for the seas were steadily filled with sediments which drove them from the north-east towards the south-west, and doubtless those movements which at the close of this period uplifted the Appalachian mountains were already operative in the same direction.
The folding of the Ural mountains began in the earlier part of this period and was continued, after its close, into the Permian; and there are traces of uplifts in central Asia and Armenia.
None of these movements appears to have affected the southern hemisphere.
The net result of the erogenic movements was, that at the close of the period there existed a great northern continental mass, embracing Europe, North Asia and North America; and a great southern continental mass, including South America, Africa, Australia and India. Between these land masses lay a great Mediterranean sea—the “Tethys” of Suess.
The conditions under which the beds of coal were formed will be found described under that head; it will be sufficient to notice here that some coal seams were undoubtedly formed by jungle or swamp-like growths on the site of the deposit, and it is equally true that others were formed by the transport and deposition of vegetable detritus. The main point to observe in this connexion is that large tracts of land in many parts of the world were at a critical level as regards the sea, a condition highly favourable to frequent extensive incursions of marine waters over the low-lying areas in a period of extreme crustal instability.
Vulcanicity.—In intimate relationship with the mountain-building orogenic crustal movements was the prevalence of volcanic activity during the earlier part of this period. In the Lower Carboniferous rocks of Scotland intercalated volcanic rocks are strikingly abundant, and now form an important feature in the geology of the southern portion of that country. Of these rocks Sir Archibald Geikie says: “Two great phases or types of volcanic action during Carboniferous time may be recognized—(1) Plateaus, where the volcanic materials discharged copiously from many scattered openings now form broad tablelands or ranges of hills, sometimes many hundreds of square miles in extent and 1500 ft. or more in thickness; (2) Puys, where the ejections were often confined to the discharge of a small amount of fragmentary materials from a single independent vent.” The plateau type was most extensively developed during the formation of the Calciferous Sandstone; the puy type was of somewhat later date. Basic lavas, with andesites, trachytes, tuffs and agglomerates are the most common Scottish rocks of this period. Similar eruptions, but on a much smaller scale, took place in other parts of Great Britain.
Granites, porphyries and porphyrites belonging to this period occur in the Saxon Erzgebirge, the Harz, Thüringerwald, Vosges, Brittany, Cornwall and Christiania. Porphyrites and tuffs are known in the French Carboniferous. In China, at the close of the period, there were enormous eruptions of melaphyre, porphyrite and quartz-porphyry. In North America, the principal region of volcanic activity lay in the west; great thicknesses of igneous rocks occur in the Lower Carboniferous rocks of British Columbia, and from the middle of the period until near its close volcanoes were active from Alaska to California. Igneous rocks of this period are found also in Australasia.
Climate.—That the vegetation during this period was unusually exuberant there can be no doubt, and that a general uniformity of climatic conditions prevailed is shown not only by the wide distribution of coal measures, but by the uniformity of plant types over the whole earth. It is well, however, to guard against an over-estimation of this exuberance; it must be borne in mind that the physiographic conditions were peculiarly favourable to the preservation of plant remains, conditions that do not appear to have obtained so completely in any other period. The climate, we may assume from the distribution of land and water, was generally moist, and it was probably mild if not warm; conditions favourable to the growth of certain types of plants. But there is no good evidence for an excess of carbon dioxide in the atmosphere—an assumption founded on the luxuriance of the vegetation, coupled with the fact that vulcanicity was active and wide-ranging. Carbon dioxide may have been present in the air in greater abundance in earlier periods than it is at present, but there is no reason to suppose that the percentage was appreciably higher in the Carboniferous period than it is now.
The occurrence of red deposits in western Australia, Scotland, the Ural mountains, in Michigan, Montana and Nova Scotia, &c., associated in some instances with the formation of gypsum and salt, clearly points to the existence of areas of excessive evaporation, such as are found in land-locked waters in regions where something like desert conditions prevail. The xerophytic structures found in some of the plants might seem to corroborate this view; but similar structures are assumed by many plants when dwelling in brackish marshes and morasses.
The abundance of corals in some of the Carboniferous seas and possibly also the large size of some of the Productids and foraminifera may be taken as evidence of warm or temperate waters.
In spite of the bulk of the evidence being in favour of geniality of climate, it is necessary to observe that certain deposits have been recognized as glacial; in the culm of the Frankenwald, in the coal basins of central France, and in central England, certain conglomeratic beds have been assigned, somewhat doubtfully, to this origin. They have also been regarded as the result of torrential action. Glacial deposits certainly do exist in the Permo-carboniferous formations, which are described under that head, but in the true Carboniferous system glaciation may be taken as not proven. The foreign boulders of granite, gneiss, &c., found in the coal-measures of some districts, are quite as likely to have been dropped by rafts of vegetation as to have been carried by floating icebergs. Economic Products.—Foremost among the useful products of the Carboniferous rocks is the coal (q.v.) itself; but associated with the coal seams in Great Britain, North America and elsewhere, are very important beds of ironstone, fire-clay, terra-cotta clay, and occasionally oil shale and alum shale. Oil and gas are of importance in the Lower Carboniferous Pocono sandstone of West Virginia and in the Berea grit of Ohio, where brine also occurs.
In the Carboniferous Limestone series, the purer kinds of limestone are used for the manufacture of lime, bleaching powder and similar products, also as a flux in the smelting of iron; some of the less pure varieties are used in making cement. The beds of chert are utilized in the pottery industry, and some of the harder and more crystalline limestones are beautiful marbles, capable of taking a high polish.
The sandstones are used for building, and for millstones and grindstones. Within the Carboniferous rocks, but due to the action of various agencies long after their deposition, are important ore formations; such are the Rio Tinto ores of Spain, the lead and zinc ores and some haematite of the Pennine and Mendip hills and other British localities, and many ore regions in the United States.
References.—For a good general account of the Carboniferous system, see A. Geikie, Text Book of Geology, vol. ii. (4th ed., 1903); and for the American development see T. C. Chamberlin and R. D. Salisbury, Geology, vol. ii. (1906). These two works give abundant references to the literature of the subject. See also, Recent Additions to Geological Literature, published annually by the Geological Society of London since 1893; and Neues Jahrbuch für Mineralogie (Stuttgart). (J. A. H.)