The Mississippian series.  In the interior the Mississippian is composed chiefly of limestones, with some shales, which tell of a clear, warm, epicontinental sea swarming with crinoids, corals, and shells, and occasionally clouded with silt from the land.

In the eastern region, New York had been added by uplift to the Appalachian land which now was united to the northern area.  From eastern Pennsylvania southward there were laid in a subsiding trough, first, thick sandstones (the Pocono sandstone), and later still heavier shales,—­the two together reaching the thickness of four thousand feet and more.  We infer a renewed uplift of Appalachia similar to that of the later epochs of the Devonian, but as much less in amount as the volume of sediments is smaller.

THE PENNSYLVANIAN SERIES

The Mississippian was brought to an end by a quiet oscillation which lifted large areas slightly above the sea, and the Pennsylvanian began with a movement in the opposite direction.  The sea encroached on the new land, and spread far and wide a great basal conglomerate and coarse sandstones.  On this ancient beach deposit a group of strata rests which we must now interpret.  They consist of alternating shales and sandstones, with here and there a bed of limestone and an occasional seam of coal.  A stratum of fire clay commonly underlies a coal seam, and there occur also beds of iron ore.  We give a typical section of a very small portion of the series at a locality in Pennyslvania.  Although some of the minor changes are omitted, the section shows the rapid alternation of the strata: 

Feet
9 Sandstone and shale . . . . . . . . 25
8 Limestone . . . . . . . . . . . . . 18
7 Sandstone . . . . . . . . . . . . . 10
6 Coal  . . . . . . . . . . . . . . .  1-6
5 Shale . . . . . . . . . . . . . . .  0-2
4 Sandstone . . . . . . . . . . . . . 40
3 Limestone . . . . . . . . . . . . . 10
2 Coal  . . . . . . . . . . . . . . .  5-12
1 Fire clay . . . . . . . . . . . . .  3

This section shows more coal than is usual; on the whole, coal seams do not take up more than one foot in fifty of the Coal Measures.  They vary also in thickness more than is seen in the section, some exceptional seams reaching the thickness of fifty feet.

How coal was made.

1.  Coal is of vegetable origin.  Examined under the microscope even anthracite, or hard coal, is seen to contain carbonized vegetal tissues.  There are also all gradations connecting the hardest anthracite—­through semibituminous coal, bituminous or soft coal, lignite (an imperfect coal in which sometimes woody fibers may be seen little changed)—­with peat and decaying vegetable tissues.  Coal is compressed and mineralized vegetal matter.  Its varieties depend on the perfection to which the peculiar change called bituminization has been carried, and also, as shown in the table below, on the degree to which the volatile substances and water have escaped, and on the per cent of carbon remaining.