The workable coal-fields of North America, belonging to the Carboniferous system, are found in several distinct areas some of which were doubtless separate basins of accumulation, while others have become disconnected by denudation.

(1) In The Acadian Province

In The Acadian Province the coal measures occur in the island of Cape Breton, Nova Scotia, and New Brunswick; in Nova Scotia they are of immense thickness, 7000 feet, with 6000 feet of underlying conglomerate. The Coal Measures of Nova Scotia, the Upper Carboniferous, strongly resemble the type of development in central England. The immensely thick basal conglomerate is the Millstone Grit. A second basin of this province is near Worcester (Mass.), and a third extends through Rhode Island into southeastern Massachusetts The latter basins are metamorphic and yield a very hard anthracite.

(2) The Great Appalachian Field

The Great Appalachian Field has an area of more than 50,000 square miles. It covers most of central and western Pennsylvania, eastern Ohio, western Maryland and Virginia, and West Virginia, eastern Kentucky and Tennessee, to northern Alabama. In this field the measures are thinner than in Nova Scotia; the beds are thickest along the Appalachian shore-line, about 4000 feet in western Pennsylvania and 6000 in Alabama, thinning much to the westward.

(3) In Michigan

In Michigan the measures are only about 300 feet thick, and were doubtless laid down in an isolated basin.

(4) The Indiana-Illinois Field

The Indiana-Illinois Field, which extends into Kentucky, is from 600 to 1000 feet thick.

(5) The Iowa-Missouri Field

The Iowa-Missouri Field extends southward around the Palaeozoic island of southern Missouri into Arkansas and Texas. In Arkansas the Carboniferous system attains a greater thickness than anywhere else in North America, and all but an insignificant amount of this is Pennsylvanian.

The two latter fields are separated by a very narrow interval, and almost certainly were once continuous; the Indiana-Illinois field was probably also connected with the Appalachian area across western Kentucky and Tennessee.

As the coal measures are traced westward into Kansas, Nebraska, and adjoining states, we find them dipping beneath strata of a very much later date. When they once more return to the surface, as in the Rocky Mountain region, they appear under an entirely new aspect, being here altogether marine and containing no coal.

After the Pottsville age and during the formation of the Coal Measures, the Interior Sea was greatly restricted in the Mississippi valley by the broad, surrounding fringe of swamps and bogs, which the sea periodically invaded. The same succession of great swamps followed the Appalachian line into northeastern Pennsylvania and probably into southern New York. In eastern Pennsylvania the sea rarely came in during the formation of the coal measures, but once, at least, penetrated to Wilkes-Barre. Westward the Interior Sea probably did not extend to Nevada, as that of Pottsville time had done, but ended farther east along a line not yet determined. A shore-line in Colorado is indicated by the generally sandy and conglomeratic character of the Pennsyl-vanian rocks in that state, which have an eastern type of fauna. On that account, Girty has "tentatively assumed that the line of division between the Eastern and Western provinces passes through western Texas, central or eastern New Mexico, western Colorado, and so on upward, in a northwesterly direction, following nearly the trend of the Rocky Mountains".

The northwestern arm of the Interior Sea was shifted eastward from the position it had occupied in the Lower Carboniferous, and apparently joined the Arctic Ocean instead of the Pacific, submerging nearly the whole of Alaska, except a broad belt on the Pacific side, this belt of land continuing southward through British Columbia to Oregon. The area of the sea was somewhat diminished in southern and western Mexico and in Central America.