CARBONIFEROUS AGE. 295 



The Michigan Carboniferous area appears to have been an independent basin, at the 

 time of the formation of the rocks. There are four groups of strata, according to 

 Winchell : the first, or lowest, 173 feet of grits and sandstones, which he has called the 

 Marshall Group ; the second, 123 feet of shales and sandstones, called the Napoleon 

 Group; the third, 184 feet of shales and marlyte, with some limestone and gypsum, 

 called the Michigan Salt-group ; the fourth, the Carboniferous limestone, sixty-six feet 

 thick. This limestone is well exposed at Grand Rapids. 



In Ohio, the chief part of the Subcarboniferous is the Waverley sandstone, 640 feet 

 thick on the Ohio River, bearing evidences of shallow-water origin, and containing, 

 130 feet above its base, a black shale sixteen feet thick ; going northward, the middle 

 portion is a conglomerate. Above the sandstone, there is in some places a limestone 

 ten to twenty feet thick, of the age, according to Meek, of the Chester and St. Louis 

 groups. It was first found by E. B. Andrews. It is a magnesian limestone, and occurs 

 in Muskingum, Perry, Hocking, Vinton, Jackson, and Scioto counties. 



(b.) Appalachian region. — In Pennsylvania, two groups are recognized by H. D. 

 Rogers, the lower called by him the Vespertine series, and the upper the Umbral 

 series. It is probable that these divisions are equivalents of those in Tennessee. The 

 rocks of the lower group are, in the main, coarse grayish conglomerates and sand- 

 stones ; those of the upper group, soft shales, mostly of a red color. The lower group 

 is 2,000 feet thick near Pottsville. Through much of the anthracite coal basin, it con- 

 stitutes the encircling hills, as around the Wyoming basin, and in many places forms 

 a grayish-white band, over another of red, the latter due to the Catskill beds, — the 

 two thus making a red and white frame, as Lesley says, around the valleys or basins. 

 It thins rapidly to the westward, the rock retaining its whitish color and siliceous 

 character in Virginia. Sandstone beds alternate with the conglomerate ; and, in New 

 York, these finer layers abound in ripple-marks, and that oblique lamination (Fig. 61 e) 

 which is due to contrary currents. 



The shales of the upper group are soft, reddish, clayey beds, easily returning, on 

 exposure, to mud, the original condition of the material. They alternate with sand- 

 stone layers, especially in the lower part. At Towanda, Blossburg, Ralston, Lock- 

 haven, Portage Summit, etc., in upper Pennsylvania, the formation consists of two or 

 three thick strata of shale, separated by as many strata, 50 to 200 feet thick, of 

 greenish sandstone. (Lesley.) Some thin layers consist of an impure rough-looking 

 limestone. This red-shale formation is 3,000 feet thick at the Lehigh, Schuylkill, and 

 Susquehanna rivers; but on crossing the Coal-measures to the westward, it rapidly 

 diminishes. At Broad Top, it is less than 1,000 feet; at the Alleghany Mountain, 

 hardly 200; at Blairsville, 30 feet; and beyond, it is lost to view. (Lesley.) The soft 

 shales retain still the ripple-marks from the ancient waves, and rain-drop impressions 

 from the showers of the day. The Amphibian footprints described beyond are from 

 this formation. To the southwest, in Laurel Hill and Chestnut Ridge, there is some 

 impure limestone, along with red marly tes. 



In West Virginia, Monongalia County, the Chester limestone has been recognized 

 by Meek, six of its fossils being identical with Illinois species (Am. Jour. Sci., III. 

 ii. 217). On the Potomac, at Westernport (W. B. Rogers), there are about eighty feet 

 of impure limestone in the lower part of the formation, and 840 feet of overlying sand- 

 stone and shales. But, farther south and west, in Greenbrier Mountain, Pocahontas 

 County, the formation thickens to over 2,000 feet, and includes 822 feet of limestone. 



Seams of coal occur in the Subcarboniferous, at many places in Pennsylvania and 

 Virginia. In Montgomery County, Virginia, there is a layer of coal, two to two and 

 a half feet thick, resting on a bed of conglomerate ; and, thirty to forty feet higher, 

 there is another layer, six to nine feet thick, consisting of alternations of coal and 

 slate. These coal-beds occur in the Lower group, and are covered by the shales of 

 the Upper. In Pennsylvania, there is a coal-bed (and possibly two) in the same Lower 

 group, at Tipton, at the head of the Juniata, 600 feet below the Upper shales; but, so 

 far as known, it is a local deposit. (Lesley.) The Subcarboniferous coal deposits are 

 sometimes called false Coal-measures. 



