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CHAPTER V.

2. MOUNTAIN LIMESTONE,OR LOWER CARBONIFEROUS.

The Mountain Limestone, or Inferior Carboniferous Group, rests immediately beneath the Coal Measures, and is composed principally of limestone, quartzose sandstone, and argillaceous shale. Of these the limestone greatly predominates, constituting fully five-sixths of the entire mass of the formation, and is traversed by the sandstone and shale in layers of variable thickness. It occurs both massive and thin bedded, the layers being in some localities over a hundred feet in thickness, while in others they do not exceed in this respect a few inches. It is usually hard and highly crystalline, and sometimes contains irregular masses of pure talc-spar. Occasionally it is homogeneous in texture, and breaks with a smooth conchoidal fracture. The predominating colors of this rock are light gray and blue, from which they pass into brown and black. In a few instances it is white. In some localities the seams are largely intermixed with chert, and are occasionally, more especially towards the west, traversed by veins of quartz, talc and fluor spar.

The Sandstone varies considerably in character. Most generally, however, it is hard, and composed of coarse quartzose grains. In some places it is soft, highly ferruginous, and rapidly disintegrating Its prevailing colors are light gray and yellow, from which it often graduates into red and brown. Occasionally it is white and saccharoidal. The different beds very seldom exceed a hundred and fifty or two hundred feet in thickness, and are usually well defined, there being an abrupt transition from the sandstone into the limestone.

The Shale sometimes occurs in seams seventy or eighty feet in thickness. It is mostly soft, of a dark blue or black color, and contains thin seams of dark limestone. In places it is highly ferruginous, and is often interspersed with nodules of brown iron ore. As in the preceding group, these rocks are generally harder and more crystalline towards the west than towards the east. In the former situation they are also sometimes found in a highly metamorphosed condition, and often emit a sulphurous odor when struck with a hammer.

EXTENT AND THICKNESS.

Commencing towards the east, we find the rocks of this group well developed near the western borders of Arkansas, where they exhibit by their exposed edges a general thickness (estimated approximately) of about two thousand feet. In Washington county, in that State, they form the