The Carboniferous is divisible into two sharply marked portions, the Lower, or Mississippian, and Upper, or Pennsylvanian, a distinction which is applicable in all the continents in which the strata of this period have been carefully studied.

In most parts of North America the Devonian passed so gradually into the Carboniferous that there is no definite line of division between them, but at Gaspe, and in Nova Scotia, New Brunswick and Maine, there was a time of upheaval and erosion toward the end of the Devonian, followed by a depression, in consequence of which there is an unconformity between the two systems. When the Carboniferous period began, most of New York, New England, and eastern Canada were above sea-level, but the Gulf of St. Lawrence covered western Newfoundland, most of New Brunswick, and part of Nova Scotia.

The Interior Sea expanded widely, probably covering nearly the whole of the Great Plains, and most of the old land areas of the West and Southwest, which had persisted through more or less of the Silurian and Devonian, were extensively submerged, probably including all of Mexico and the northern part of Central America. West of the Rocky Mountains the Carboniferous is much the most widely extended of any of the Palaeozoic systems, the sea reaching through British Columbia, on both sides of the Gold Range into southeastern Alaska.

Map of North America in the Lower Carboniferous.

Fig. 273. - Map of North America in the Lower Carboniferous. Black areas known exposures; white areas = land, or unknown; horizontal lines = sea.

In consequence of this great transgression, the Carboniferous strata rest unconformably, or sometimes in apparent conformity, upon all the older systems from the Pre-Cambrian to the Devonian. The Arctic coast of Alaska was submerged and several islands of the Arctic Sea, but the main portion of Alaska, which has great areas of Carboniferous rocks, appears not to have been invaded by the sea till Upper Carboniferous, or Pennsylvanian times. According to Girty none of the known Alaskan faunas, except from the north coast, " can be confidently referred to the Lower Carboniferous. The typical Mississippian is certainly absent as far as evidence has come to hand, and but one occurrence of a fauna definitely related to the Lower Carboniferous of California (Baird) has been found." (Girty.) The fossils of the Baird stage in California are entirely different from those of the Interior Sea, and if actually contemporaneous with the latter, must have been separated from them by some barrier. The Great Basin sea appears also to have been separated for a time from the Interior Sea, but communication was established before the end of the Lower Carboniferous. Over the Central States and the West, the Mississippian rocks are almost uniformly limestones, showing that this vast sea was clear and free from terrigenous sediments, but probably of moderate depth.

The northeastern portion of the Interior Sea was divided by the islands of the Cincinnati anticline into two bays, the eastern one of which covered most of Ohio, western Pennsylvania and Maryland; and the western bay occupied the southern peninsula of Michigan and had but a narrow communication with the first. The Appalachian valley trough has in its middle third a nearly or quite complete succession of the Devonian, but lacks the earliest , Mississippian, indicating a brief elevation of this region. In the South the eastern edge of the Interior Sea followed the line of the Appalachian fold probably into Virginia and there broke across the barrier and sent off some narrow sounds southward into the Appalachian Valley.

In the Acadian province, comprising Nova Scotia, New Brunswick and Newfoundland, the Lower Carboniferous is remarkably like that of Great Britain. In Nova Scotia, the Horton sandstone corresponds to the Calciferous of Scotland, and contains thin seams of coal; it is followed by the Windsor stage of marine limestone (= the British Scaur, or Mountain, Limestone) which contains beds of gypsum. The presence of gypsum shows not only that occasionally bodies of sea-water were shut off in closed lagoons, but also that an arid climate prevailed in the region.

In eastern Pennsylvania the Lower Carboniferous has a maximum thickness of 4000 feet, but thins rapidly southward and westward. The Pocono is a thick, hard sandstone, which caps many of the mountain ridges; it follows down the Appalachian line, thinning as it goes. The area of maximum sedimentation may have received largely continental deposits. Under different local names, the Pocono extends to eastern Kentucky. The Mauch Chunk shales form the remainder of the Lower Carboniferous in northeastern Pennsylvania, where the thickest portion not improbably represents a great delta, and the prevalence of sun cracks is indicative of an arid climate, such as probably prevailed in Nova Scotia. In Maryland, West Virginia, and Kentucky, part of this series is composed of a marine limestone {Greenbrier, Newman). The Lower Carboniferous of Virginia contains some workable beds of coal, the "false Coal Measures." Ohio was a region of slow sedimentation; here the Lower Carboniferous is formed by the Waverly series, which is divisible into seven stages, some of them carrying marine faunas, and is only 700 feet thick.

In the Michigan bay were deposited the sandstones, grits, and shales of the Marshall series, followed by shales with some limestone and gypsum, whence we may infer that the bay was for a time converted into a salt lake and that the climate was dry, as in Nova Scotia and Pennsylvania. The bay was soon again invaded by the sea, for a marine limestone overlies the gypsiferous beds.

Southwest of these more or less completely enclosed bays, the Interior Sea was clear and free from terrigenous material, so that in it were deposited great masses of limestone (1500 feet maximum thickness) formed from a most luxuriant growth of corals, brachi-opods and crinoids. In the Central States many different stages and substages may be distinguished in these limestones, and evidences are not wanting of fluctuating shore-lines. The Kin-derhook extends farther north than the Osage, while the St. Louis sea again extended northward. The Osage series is remarkable for the extraordinary abundance and variety of its crinoids, unequalled, perhaps, in the world. This produces a peculiar facies which is nearly confined to the Mississippi valley, but was extended to the southwest, into New Mexico. "But this condition appears not to have invaded other western parts of the Missis-sippian sea, where I believe, under uniform conditions, the Kin-derhook faunas persisted through Burlington and Keokuk [i.e. Osage] time without feeling, save in a subordinate degree, the influences which helped to differentiate the early Mississippian faunas of the Mississippi Valley." (Girty).