At the same time, the thousands of feet of marine and freshwater sediments, with their repeated alternations of limestones, sandstones, and shales, in which the seams of coal occur, prove a slow subsidence, with many changes in its rate, with halts when the land was at a stillstand, and with occasional movements upward.

When subsidence was most rapid and long continued the sea encroached far and wide upon the lowlands and covered the coal swamps with sands and muds and limy oozes.  When subsidence slackened or ceased the land gained on the sea.  Bays were barred, and lagoons as they gradually filled with mud became marshes.  River deltas pushed forward, burying with their silts the sunken peat beds of earlier centuries, and at the surface emerged in broad, swampy flats,—­like those of the deltas of the Mississippi and the Ganges,—­which soon were covered with luxuriant forests.  At times a gentle uplift brought to sea level great coastal plains, which for ages remained mantled with the jungle, their undeveloped drainage clogged with its debris, and were then again submerged.

Physical geography of the several regions.  The ACADIAN region lay on the eastern side of the northern land, where now are New Brunswick and Nova Scotia, and was an immense river delta.  Here river deposits rich in coal accumulated to a depth of sixteen thousand feet.  The area of this coal field is estimated at about thirty-six thousand square miles.

The Appalachian region skirts the Appalachian oldland on the west from the southern boundary of New York to northern Alabama, extending west into eastern Ohio.  The Cincinnati anticline was now a peninsula, and the broad gulf which had lain between it and Appalachia was transformed at the beginning of the Pennsylvanian into wide marshy plains, now sinking beneath the sea and now emerging from it.  This area subsided during the Carboniferous period to a depth of nearly ten thousand feet.

The central region lay west of the peninsula of the Cincinnati anticline, and extended from Indiana west into eastern Nebraska, and from central Iowa and Illinois southward about the ancient island in Missouri and Arkansas into Oklahoma and Texas.  On the north the subsidence in this area was comparatively slight, for the Carboniferous strata scarcely exceed two thousand feet in thickness.  But in Arkansas and Indian Territory the downward movement amounted to four and five miles, as is proved by shoal water deposits of that immense thickness.

The coal fields of Indiana, and Illinois are now separated by erosion from those lying west of the Mississippi River.  At the south the Appalachian land seems still to have stretched away to the west across Louisiana and Mississippi into Texas, and this westward extension formed the southern boundary of the coal marshes of the continent.