AMOUNT OF POTTSVILLE SEDIMENTS 277 



Appalachian region. It is probable that there was some encroachment 

 of the sea to the eastward also, particularly as the result of the extraor- 

 dinary subsidence under loading along the early axis. This, however > 

 may have been checked by the continuation of the orogenic movement. 

 The question as to whether the narrow axis of the early Pottsville was 

 a continuous basin or was composed of several basins lacks evidence of 

 a conclusive nature. The presence of a brachiopod fauna in the Middle 

 Pottsville (Horsepen) as far north as Sewell, on New river, seems to jus- 

 tify the belief in a free access to marine animals from the broad end of 

 the trough to the southward at this time. In Pennsylvania the paleon- 

 tological evidence as to communication of the early basin with the open 

 sea rests on the occurrence of Naiadites and Spirorbis in the Lower Lykens 

 group of the Anthracite region. At the time of Mercer Group deposition 

 the Pottsville sea was broad and open, so that marine faunas are found 

 in various regions of the trough. 



AMOUNT OF POTTSVILLE SEDIMENTATION 



An imperfect conception of the dimensions of -the basin at the close 

 (Raleigh-Bon Air) of Middle Pottsville time may be had from the con- 

 touring and coloring in the accompanying map ; but in estimating the 

 contents of the entire Pottsville basin one must bear in mind (1) that 

 the territory of the post-Bon Air encroachment lay entirely to the west of 

 the surviving Coal Measures area south of the Standing Stone, Tennes- 

 see, quadrangle, and is but partially preserved for observation north of 

 the latter until we reach the Ohio river; (2) that no allowance is made 

 for the supra-Sharon (Kanawha) beds, presumably 1,500 feet or more in 

 thickness, in Alabama ; and (3) that the basin is truncated at its present 

 eastern border, the deepest (though perhaps narrow) portion having 

 been removed. With these facts in view one may estimate the extent 

 of the orogenic movement to the eastward and the amount of erosion 

 of the old Blue Ridge region required to furnish the Pottsville sediments. 

 It is particularly remarkable that the region of greatest movement and 

 greatest sedimentation is in the extreme Southern Appalachian region, 

 where, if the measurements of the Coosa and Cohaba basins are correct, 

 sands, coals, and clays over a mile in thickness were laid down before 

 Sharon time. Even within the present confines of the coal fields, the 

 amount of the Pottsville sediments in the Southern Appalachian region 

 was several times that of all the existing rocks lying above the 1,000-foot 

 contour in the region to the eastward. 



From the facts set forth above, it will be seen that nearly all of the 

 present Appalachian Coal Measures area bears the marks, either paleon- 

 tological or stratigraphical, of the continued transgression of the sea. 



