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 Pennsylvania. Although some of the minor changes are omitted, the section shows the rapid alternation of the strata: 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. Peat Bituminous Dismal Lignite Coal Anthracite Swamp Texas Penn. Penn. Moisture 78.89 14.67 1.30 2.74 Volatile matter 13.84 37.32 20.87 4.25 Fixed carbon 6.49 41.07 67.20 81.51 Ash 0.78 6.69 8.80 10.87 2. The vegetable remains associated with coal are those of land plants. 3. Coal accumulated in the presence of water; for it is only when thus protected from the air that vegetal matter is preserved.