Subsequently the plant accumulation became
changed to coal, and the clay to shale. We find
records of these changes at the present day in
the alternating coal and shale seams.
Carboniferous rocks are widely distributed over the globe, underlying wide areas in North America, South America. Europe, China, Africa, and Australia. In classifying the rocks of these areas, the divisions of the first order are the same, but the smaller divisions are commonly local ones. The first order division consists in grouping the Carboniferous into:
{ 1 ) Sub-Carboniferous, known also as Lower Carboniferous, and ilississippian.
(2) Carboniferous proper. Coal Measures, or Pennsylvanian.
(3) Permian, Upper Carboniferous, or Dyas. The Permian is considered by some European geologists as a separate system, coequal with the Carboniferous: in the Mississippi Valley it is, however, inseparable from the upper non-coal- bearing fossiliferous limestones of the Jlississip- pian stage. In the United States the Carbonifer- ous sections along the Appalachians and in the Mississippi Valley arc divided as follows: Penn- sylvania — Sub-Carboniferous, ( 1 ) Pocouo sand- stone; (2) Mauch Chunk sandstone and shale; Carboniferous, (1) Pottsville conglomerate or millstone grit; (2) Lower productive measures, including coal, shale, and sandstones; (3) Lower barren measures, including sandstones and shales; (4) Upper productive measures, includ- ing coal, shale, and sandstones: Permian or Upper barren measures. Mississippi Valley — Sub-Carboniferous. (1) Kinderhook: (2) Osage, including (a) Burlington; (b) Keokuk; (c) Warsaw; (3) St. Louis group: (4) Chester or Kaskaskia group. Carboniferous, (1) Millstone grit; (2) Coal measures: Permian.
In the United States the Carboniferous is found underlying a number of areas. In Rhode Island there is a small one of highly metamor- ])hosed rocks, in which the coal-beds have been neaily changed to anthracite. A large area ex- tends from Pennsylvania southward to Alabama. and westward to Missouri and Arkansas and Texas. Along the Appalachians the prevailing rocks of this area are sandstones and shales, ■which contain many coal-seams and are much folded: but westward the folds die out, and limestones begin to predominate. Work.nble beds of coal are found in all the States of this area. In the Mississippi Valley the crinoidal limestones are important members. The Car- boniferous section shows a variable thickness, having a maximum of nearly 8000 feet in Penn- sylvania, and only 1200 to ITiOO in Illinois. Xear Pottsville. Pa., there are twenty-five coal- beds, whose aggregate thickness is 154 feet. In Alabama there are seventeen in one field. In the Western interior region, especially near the summits of the Rocky Mountains, there are Carboniferous strata consisting mostly of lime- stones, with no coal. There are also scattered areas in the Great Basin region, and along the Pacific Coast and in the Arctic region. The coal-beds are usually underlain by clay-lwds, in which are sometimes found the upright roots and trunks of trees that grew in the Carbonif- erous swamp.
In Europe, the coal measures of England are 3000 to 5000 feet thick, increasing even to 12,000 feet in South Wales. The Permian beds in Russia are of enormous extent, covering an area twice as large as France. Carboniferous rocks also occur in Germany, France, Belgium, and Austria. In China they extend over many thousand square miles as vast tablelands, and contain perhaps the richest coa! deposits of the world. They are also extensively developed in India, and in Australia and Africa. Both animal and vegetable remains are abun- dant in the Carboniferous, and are in many cases well preserved. The abundance of the latter is easily understood when we remember that coal has been formed by the accumulation of vege- table matter, and we consequently find the plant fossils in the coal itself, as well as the inclos- ing beds. There is a great uniformity of char- acter in the plant life, the same genera and often the same species occurring in v.idely separated regions. About 2000 species are kno-n which, with the exception of a few plants of doubtful relationship, may be referred to the following families: Equisetaceoe, lycopods, eonifera, ami ferns. Of the equisetace;e, the most abundant genus was Catamites (q.v. ), which included sev- eral species of large, tapering, reed-like plants that apparently flourished on the borders of the coal swamps. The lycopods were represented by the Sigillaria ( q.v. ) and the Lepidodendron (q.v.), closely related genera that are believed to have furnished a large part of the material for the formation of coal. The Sigillaria had gently tapering, fluted stems, which grew to a height of 50 feet or more, and had a diameter of 5 feet. The bases of the trunks, with their radiating roots, are often found in the clay that underlies the coal-seams, and for a long time they were supposed to be a distinct species. ( See Stigm.^eia. ) The Lepi<lodendron bore a great resemblance in structure and appearance to the club-moss of the present day, but it attained gigantic proportions. Conifers were probably abundant in Carboniferous times; they difl'ered widely, however, from existing conifers, and bore nut-like fruits, which have been frequently pre- .strved. (See Trigoxoc.rpus.) The genus Ctor- daites, which appears to have been very abun- dant, is classed by some botanists with the co- nifers ; by others, with the cycads. The fern family was represented by a large variety of species, some of which were tree-ferns. The most common forms were Sphenopteris, Cyelop- teris, Neuropteris, Odontopteris, and Pecopteris. The animal remains of the Carboniferous system are both numerous and well preserved, but they are found in greatest abundance in the Sub- Carboniferous limestones. Corals and crinoids are numerous, both as regards individuals and six'cies, and in places constitute great thicknesses of rock. Jlore than CoO sjiecies of crinoids have been described from the Sub-Carboniferous of America alone. .^mong the brachiopods, Pro- ductus. Spirifer, and Chonetes are most numer- ous, while molliwks are represented by ce])l!alo- pods, gastropods, and lamellibranchs, some species of which pass out of existence at the close of the period. Trilobites are present, but not in such variety as in the earlier Paleozoic times, and they diminish rapidly toward the end. Crustaceans are abundant, especially Beyrichia and Estheria, and they show a rapid development. Insects appear to have flourished in great numbers; the known varieties include spiders,
