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• 1. Coal Measures, or Upper Carboniferous.
• 2. Mountain Limestone, or Lower Carboniferous.

The Coal Measures consist principally of limestone, sandstone, conglomerate, millstone grit, and shale. The limestone and sandstone exist in about equal proportions, and these constitute the chief part of the formation.

The Limestone is usually compact, subcrystalline, and occurs in strata of very variable thickness. Sometimes the layers are massive and many feet in thickness, but more frequently they are thin-bedded, and in some localities laminated or even foliated. The prevailing colors are light gray, blue, and brown. Occasionally they are dark gray and black, or, as in the Guadalupe Mountains, almost pure white. Veins of quartz sometimes traverse the strata from top to bottom, and in their western extension they are not unfrequently highly metamorphosed. In some localities they pass into nearly pure calcite, and in others they contain a great deal of chert.

The Sandstone is both massive and thin-bedded, and, in general lithological character, corresponds very closely with that of the Coal Measures of Illinois, Indiana, and Kentucky. It is both coarse and fine grained, more or less ferruginous, and exhibits every variety of hardness. Many of its layers are highly micaceous, and the rock often contains disseminated nodules of brown and black iron ore. In color it presents various shades of gray, yellow, brown, and black. The surface of the different layers are often beautifully ripple-marked. This rock sometimes passes into millstone grit and conglomerate, and, like the limestone, is not unfrequently traversed from top to bottom by tortuous veins of quartz.

The Millstone Grit generally occurs in thin layers, situated usually near the base of the sandstone, into which it passes by almost insensible gradations. It consists mostly of coarse quartz grains firmly cemented with silicious matter.

The Conglomerate of the Coal Measures has been observed only in Eastern Texas and the Choctaw Territory, where it sometimes presents a thickness of more than a hundred feet. The prevailing color is red, and it is composed altogether of well rounded pebbles of eruptive rocks, cemented with silicious and ferruginous paste.

The Shale occurs interstratified with the sandstone in beds of from a few inches to over a hundred feet in thickness. It is usually soft and of deep blue and black colors, and contains thin bands of black ferruginous limestone and sandstone interstratified. Wherever stone coal abounds it is highly bituminous. Selenite and iron pyrites are of frequent occurrence in it.