274 D. WHITE — DEPOSITION OF APPALACHIAN POTTSVILLE 



yond the border of the Raleigh, the Sharon and the supra-Sharon (next 

 to be described) constituting the entire bituminous area of Pennsyl- 

 vania and Ohio. Over most of this area the Sharon, when present, con- 

 stitutes the lowest Pottsville terrane. 



POST-SHARON TRANSGRESSION 



The remaining area of the coalfields, in which the Sharon is wholly or 

 largely absent,* is represented on the map by the very light tint. This area 

 includes a portion of the northern and the extreme northwestern coun- 

 ties of the northern bituminous regions as well as, probably, the Northern 

 Anthracite field. It is probable that the surface of the greater portion 

 of this region was at or above water level during Sharon Conglomerate 

 time.f It is the surviving area of the final northwest encroachment of 

 the Pottsville sea. 



Subsidence and Extension of the Basin under Loading 

 character and origin of the basin 



The study of the combined paleobotany and stratigraphy of the exist- 

 ing Pottsville areas in the Appalachian trough shows clearly the exist- 

 ence of a relatively narrow axial trough or series of basins at or beyond 

 the present eastern border of the coal region. In this trough the lowest 

 Pottsville was laid down, and from this axis the northwestward encroach- 

 ment progressed until the Sharon coal or higher beds were deposited in 

 direct contact with the eroded Mississippian to the north, or on the 

 flanks of the Cincinnati axis to the northwestward. 



There is little room for doubt that during the period of Sharon and 

 Kanawha sedimentation, if not earlier, the eastern Pottsville sea extended 

 across Tennessee and southern Kentucky into the Mississippi embay- 

 ment, which had already probably been reached westward from central 

 Alabama by the close (Bon Air) of the Middle Pottsville. J It is certain 



* As already stated, this, the lower member of the Pottsville in western Pennsylvania and north- 

 ern Ohio, is absent in the Allegheny and Red Bank valleys in the region of Armstrong county 

 and it is believed by the writer to be probably absent eastward to the Allegheny front and south- 

 westward beneath a portion of the Pittsburg coal area in northern West Virginia. 



fNear the northwest boundary of the coal field in Ohio the Sharon conglomerate is local only 

 and patchy in its occurrence. 



X As has already been stated, the thicknesses of the Pottsville given on the map are in numer- 

 ous cases subject to revision. They are, however, accurate for the most part, and the general 

 conditions of sedimentation set forth in the map and cross-sections are well founded. 



In drawing the contours in Tennessee trouble is experienced on account of the absence of the 

 supra-Sharon or "Beaver River" series except in the northeastern district of this portion of the 

 coal field. The rate of thinning toward the west is therefore based in part on the observed rate of 

 diminution in the'preserved areas and on the behavior of the beds to the north. On account 

 of the absence of information as to the approximate thickness of eroded uppermost Pottsville in 

 Alabama, the region south of the Briceville quadrangle is contoured as though these high forma- 

 tions faded out southward, thus allowing the thickness to fall back to that of the beds still remain- 

 ing in the Cahaba and Coosa basins. 



