154 J. J. STEVENSON CAEBONIFEKOUS OF APPALACHIAN BASIN 



generalizing here. The interval between Upper Preeport and Lower 

 Kittanning coal beds varies little and apparently according to rule in 

 very considerable areas, while in the same areas the intervals between 

 intervening coal beds vary without any reference to the principal interval. 

 The coal beds of the Allegheny above the Brookville in no case extend 

 far into the interior of the basin and all are mere fringes around the 

 border. The coals mentioned in oil-well records can rarely be referred 

 to any definite horizon, and in most cases they represent carbonaceous 

 material drifted upon mud lumps or sand banks. 



Toward the' close of the Allegheny a small area in west central West 

 Virginia near the Ohio river received deposits of red mud, more or less 

 calcareous, accompanied often by greenish muds; and, somewhat earlier, 

 similar deposits were made in northeastern Kentucky. This is the be- 

 ginning of a condition which in gradually enlarging or contracting area 

 was to continue until the close of Carboniferous time, always predomi- 

 nating, however, within a small area in West Virginia and the adjoin- 

 ing part of Ohio. 



In many respects the Conemaugh is but the continuation of the Alle- 

 gheny; the variations in thickness are, geographically, very similar in 

 both. Along the easterly side the Conemaugh is approximately 500 feet 

 thick in Broad Top, at the northeast, while at Charleston, on the Kana- 

 wha, 250 miles southwest, the thickness has increased to 643 feet; but 

 in the intervening space for nearly 200 miles it averages not far from 

 600 feet. Along the northern border, in Pennsylvania, it is about 600 

 feet, and slowly decreases southwardly to 560 feet at the West Virginia 

 line, this being apparently about the average thickness in West Virginia 

 east from the line passing through Eitchie county. Midway in the basin 

 the sections show decrease southwestwardly, 600 in western Pennsylvania, 

 480 in Monroe, and 430 in Lawrence of Ohio, while along the western 

 outcrop in Ohio the thickness varies from 325 to 350 feet. The least 

 thickness reported, 275 feet, is on the extreme western outcrop, in Perry 

 county of Ohio, while farther east in that county it becomes 329. This 

 decrease westwardly across the basin is due to difiierential subsidence and 

 not to overlapping, for the notable members of the section persist to the 

 last. The rapid shortening of the section in Perry county of Ohio seems 

 to show that the shortline lay not far toward the west. 



Except in a very narrow strip along the southeasterly border in West 

 Virginia, the Conemaugh sandstones are more irregular than are those 

 of the Allegheny. One generally finds some sandstone of some sort in 

 the sandstone intervals, but shales predominate in l)y far the greater 

 part of the area. Certain sandstones appeaf with great regularity in oil- 



