STRATIGRAPHICAL CONFIRMATION 279 



cate the presence of a low ridge in Sharon time between these two anthra- 

 cite fields. The formation of such minor undulations to the westward 

 of the deep narrow trough of the basal Pottsville on the occasion of the 

 deformation of the old Mauch Chunk floor at the beginning of Pottsville 

 time would be normal and entirely comparable to the orogenic move- 

 ments which formed the deep basins to the eastward at the close of 

 Paleozoic time. 



PHYSICAL EVIDENCE OF THE POTTSVILLE TRANSGRESSION 



Although it is the writer's purpose to confine the scope of this paper 

 to the principal propositions that have originated in and have been 

 gradually worked out mainly through the study of the fossil floras, brief 

 reference should be made to some of the stratigraphic evidence in sup- 

 port of these propositions. The overlap and unconformity of the Bon 

 Air conglomerate beyond the limits of the earlier Pottsville terranes in 

 the outlying patches of Coal Measures in northern Alabama and along 

 the western margin of the coal field in Tennessee has long been recog- 

 nized. Farther north, in southern Kentucky, Mr M. R. Campbell* 

 notes the occurrence, to the northwest of the Bon Air area, of a river 

 channel which was filled by Rock Castle sediments. This river was 

 obliterated by the post-Bon Air transgression and before Corbin time, 

 which at latest is early as Sharon. f Reference has already been made 

 to the Allegheny valley, where Pottsville beds regarded as Lower Con- 

 noquenessing J rest on shales of Pocono (Lower Mississippian) age. 



The physical discordance between the Mississippian and the Potts- 

 ville is greatest near the northwest border of the coal field in Ohio and 

 Pennsylvania. Toward the south in Ohio the Sharon rests on the Max- 

 ville limestone. Farther north the Sharon is often wanting along the 

 flank of the Cincinnati axis, and the Lower Connoquenessing sandstone 

 rests in places on the Shenango shales or the Logan. In McKean 

 county, Pennsylvania, the Mississippian is eroded, so that Sharon or 

 higher beds rest on beds of Pocono age. The interval between the uplift 

 of the Mauch Chunk mud flats at the beginning of Pottsville time and 

 the supra-Sharon encroachment — that is, the period of erosion of the 

 Mississippian in northwestern Pennsylvania — is measured by the time 

 required for the erosion and redeposition of a great thickness of earthy 

 sediments exceeding a mile in depth in the southern portion of the 



*Atlas of the U. S. Geol. Survey, London folio. 



t Stratigraphical evidence confirmatory of these transgressions is coming to light in the quad- 

 rangles studied by the U. S. Geol. Survey. The data at hand have been excellently summarized 

 by J)r J. J. Stevenson in his paper on the Stratigraphy of the Pottsville, this volume, p. 37. 



X The Connoquenessing sandstones lie above the Sharon sandstone and shales. 



