The three regions just mentioned include the chief Carboniferous coal fields of North America.  Including a field in central Michigan evidently formed in an inclosed basin (Fig. 260), and one in Rhode Island, the total area of American coal fields has been reckoned at not less than two hundred thousand square miles.  We can hardly estimate the value of these great stores of fossil fuel to an industrial civilization.  The forests of the coal swamps accumulated in their woody tissues the energy which they received from the sun in light and heat, and it is this solar energy long stored in coal seams which now forms the world’s chief source of power in manufacturing.

The western area.  On the Great Plains beyond the Missouri River the Carboniferous strata pass under those of more recent systems.  Where they reappear, as about dissected mountain axes or on stripped plateaus, they consist wholly of marine deposits and are devoid of coal.  The rich coal fields of the West are of later date.

On the whole the Carboniferous seems to have been a time of subsidence in the West.  Throughout the period a sea covered the Great Basin and the plateaus of the Colorado River.  At the time of the greatest depression the sites of the central chains of the Rockies were probably islands, but early in the period they may have been connected with the broad lands to the south and east.  Thousands of feet of Carboniferous sediments were deposited where the Sierra Nevada Mountains now stand.

The Permian.  As the Carboniferous period drew toward its close the sea gradually withdrew from the eastern part of the continent.  Where the sea lingered in the deepest troughs, and where inclosed basins were cut off from it, the strata of the Permian were deposited.  Such are found in New Brunswick, in Pennsylvania and West Virginia, in Texas, and in Kansas.  In southwestern Kansas extensive Permian beds of rock salt and gypsum show that here lay great salt lakes in which these minerals were precipitated as their brines grew dense and dried away.

In the southern hemisphere the Permian deposits are so extraordinary that they deserve a brief notice, although we have so far omitted mention of the great events which characterized the evolution of other continents than our own.  The Permian fauna-flora of Australia, India, South Africa, and the southern part of South America are so similar that the inference is a reasonable one that these widely separated regions were then connected together, probably as extensions of a great antarctic continent.

Interbedded with the Permian strata of the first three countries named are extensive and thick deposits of a peculiar nature which are clearly ancient ground moraines.  Clays and sand, now hardened to firm rock, are inset with unsorted stones of all sizes, which often are faceted and scratched.  Moreover, these bowlder clays rest on rock pavements which are polished and scored with glacial markings.  Hence toward the close of the Paleozoic the southern lands of the eastern hemisphere were invaded by great glaciers or perhaps by ice sheets like that which now shrouds Greenland.