When the Devonian period closed the seas had retreated from most of North America, but in Waverlian time (early Mississip pian) the Gulf States and the western side of the Cincinnati uplift were inundated over small areas which later gradually ex panded to large size, and the Cordilleran sea submerged extensive areas in the Rocky Mountain region, connecting with the embay ment in the Central Interior. The northern Appalachian basin during Waverlian time extended over Ohio, Michigan and Penn sylvania the Central Interior sea from central Iowa southward and south-eastward to the Gulf, and south-westward to Texas; and the Acadian seas flooded a connected series of narrow troughs between mountain ranges. At the close of Waverlian time the seas had apparently withdrawn from the whole Cordilleran basin, and from practically the whole of the continent, so that the sedi mentary record to the Tennesseean is broken. Tennesseean time is characterized by general emergence with limited areas of sea in the Central Interior embayment and the southern Appalachian and Cordilleran troughs.
The marine life of the Waverlian was distinguished by a great abundance and rich variety of crinoids, and the Productus type of brachiopods flourished. Reef-building corals were rare but bryo zoans and cup corals were numerous. Sharks dominated the seas. Trilobites were almost gone. On land no record has been found of animals but low forms of plants were wide-spread. The marine life of the Tennesseean closely resembled that of the Waverlian, while the land record is better in that it reveals tracks of many amphibians. Petroleum and natural gas have been obtained from the Mississippian of Ohio, Pennsylvania, West Virginia and Ken tucky.
The Pennsylvanian is distinguished by the greatest known ac cumulations of coal, not only in America but in Eurasia as well, that have been laid down in all geologic history, though some of the later formations, especially those of the Cretaceous, also bear coal in much smaller extent and quantity. In the United States the Pennsylvanian formations of the Rocky Mountains bear no significant coal beds such as are found in the central and eastern parts of the country.
The Pennsylvanian-Permian was a period of quiet inundation, the seas transgressing the land from the south-west and spreading northward and eastward. Only local warping or uplift occurred in this period in most of America, except in the Pacific area from northern California to Alaska, though in Eurasia it was a period of extensive tectonic disturbance. In the Appalachian basin east of the Cincinnati uplift, and in the Interior Sea west of this axis into Nebraska, Kansas, Oklahoma and Texas the sea-level was oscillatory and deep-sea conditions alternated with littoral and marsh conditions, the coal being formed when the latter condi tions prevailed. The Pennsylvanian-Permian seas receded from
the North to the south-westward and finally vanished in Permian time, when the Appalachian revolution raised mountains probably 3 m. high in the eastern part of the United States, and the whole continent emerged to the same extent, approximately, as now.
The life, both plant and animal, of the Pennsylvanian-Permian was characterized by its relatively cosmopolitan character and distinguished by the change of dominance in biologic interest from the sea to the land. The vegetation, particularly of the humid, tepid marshes of the Pennsylvanian, was extremely luxu riant. The numbers, sizes and varieties of ferns and fernlike plants, horsetails, clubmosses, sigillarias or seal-trees, and lepi dodendrons or scale-trees has apparently never been surpassed in geologic history, and though many other low forms were abundant, it was these that gave the flora of the period its unique quality and furnished the parent material for the great coal beds. Through these bizarre forests a host of insects, some of them of great size and including some 500 species of cockroaches, swarmed, and grotesque amphibians and reptiles crawled from pool to pool and among the giant boles. The freshwater and marine life was likewise rich and varied with an abundance of sand-loving or mud-loving forms frequenting the shallow pools, lagoons and sea shores. With the desiccation and glaciation attendant upon Per mian time both plant and animal life declined in vigour and va riety, but made great advances in evolution toward adaptation to modern conditions.
The most important product, by far, of the Pennsylvanian formations, is coal. The important coal beds of Nova Scotia and New Brunswick; the great Appalachian coal-field including the anthracite beds of north-eastern Pennsylvania; the Michigan coal field; the great Indiana-Illinois-Kentucky coal-field; the Iowa Missouri-Texas field ; all these are of Pennsylvanian origin. Beds of iron ore in a few places, particularly Ohio and Pennsylvania, are associated with the coal, and fire-clay is often taken from the mines and pits where coal is extracted. Petroleum and natural gas have been found in some of the Pennsylvanian sandstones in Illinois, Kansas and Oklahoma.