278 D. WHITE — DEPOSITION OF APPALACHIAN POTTSVILLE 



The Raleigh-Bon Air and Sharon conglomerates are at once evidence of 

 orogenic movement, on the one hand, and sea encroachment, on the 

 other. Stratigraphically each is transgressive, and I am disposed to 

 believe that the materials forming these beds, which are sometimes 

 thicker near their western outcrops, were in no small part obtained by 

 reworking of the coastal plain deposits. The disposition, form, and 

 structure of the little basins of Sharon coal along the northwest border 

 of the field are in strong evidence of such conditions. 



ARCHES OR BARRIERS WITHIN THE BASIN 



The study of the thickness and distribution of the sediments in the 

 region of northwestern encroachment suggests the existence of one or 

 more barriers or islands until very late Pottsville time. From (a) the 

 rapid thinning of the sections toward the northwest in the Northern 

 Anthracite region, (6) the thinness of the sediments in the small north- 

 eastern bituminous basins of Pennsylvania, (c) the thinning of the Potts- 

 ville on approaching the Allegheny front * in Clearfield and Cambria 

 counties, (d) the reported thinning of the Broad Top Pottsville in pass- 

 ing northwestward, and (e) the apparent absence of the Sharon sandstone 

 from this region, it would seem that a very low barrier extended from 

 the northeast through Bradford, Tioga, and Lycoming counties and along 

 the eastern margin of the Allegheny front, possibly passing to the west 

 of the Frostburg basin and dying out. The evidence contained in the 

 sections published by I. C. White, Stevenson, \V. G. Piatt, and Chance 

 in their reports for the western counties of Pennsylvania shows the pres- 

 ence of an area of Sharon sandstone extending from Ohio eastward 

 across the western tier of counties of the former state, while east of these, 

 in the Kittanning-Redbank region of the Allegheny valley, as is shown 

 by paleobotanical and stratigraphic studies, a barrier of Mississippian 

 continued in erosion until overswept by the waters of the greatly broad- 

 ened basin in Connoquenessing time. Westward of this barrier, which 

 probably passed near Pittsburg, an arm of the sea extended during 

 Sharon time northeastward through Ohio and into the western counties 

 of Pennsylvania. If the Pottsville measurements given in the state 

 reports from Indiana, western Clearfield, and northwestern Cambria 

 counties are correct, a shallow syncline of the Mauch Chunk floor may 

 have lain to the east of the Allegheny valley ; but on this matter addi- 

 tional evidence is needed. The decreased measurements reported on the 

 south side of the Northern Anthracite field and along the northwestern 

 border of the Eastern Middle Anthracite field, if correct, distinctly indi- 



* The thicker measurements of the interior of the coal field are based on the state reports. 



