452 J. BAERELL ORIGIN OF THE MAUCH CHUNK SHALE 



It is seen from these details that the edge of tlie great Greenbrier 

 limestone of Virginia and farther west reaches into southwestern Penn- 

 sylvania only, thinning away to a knife-edge, and its place being appar- 

 ently taken near its margin by thick bedded gray to greenish sandstones, 

 noted as 80 feet thick in the Ebensburg folio. 



Eecent observations tend to show that over the western half of Penn- 

 sjdvania, as well as over the states of the central west, uplift and erosion 

 intervened to a greater or less extent between the latest deposits of the 

 Mississippian period and the earliest of the Pennsylvania,* the adjacent 

 formations being sharply distinct and different members of the Mauch 

 Chunk or equivalent formations coming into contact with the Pottsville 

 sandstone. The scantiness of the Mauch Chunk along the northern side 

 of the area appears, however, to have been due largely to the thinness of 

 the original deposition, since nowhere is it reported as more than 100 

 feet in thickness, and, as may be seen by tracing the formation eastward 

 along the north margin of the northern coal field, the red shales gradu- 

 ally disappear by transition into, and interfingering with, greenish shales 

 and flaggy sandstones of the same character as the Pocono. These facts 

 tend to confirm the idea of H. D. Eogers that the Umbral (Mauch 

 Chunk) shale was never deposited beyond these limits. Some erosion 

 of the upper strata appears to have also occurred, however, preceding 

 the earlier deposits of Pottsville conglomerate and sandstone. 



But such an erosion interval with resulting unconformity and strati- 

 graphic contrast between the two adjacent members is absent in the 

 region of the southern anthracite coal field, where there are from 500 to 

 600 feet of transition beds. These are described by A. D. W. Smith,f 

 who states "at times in these transition beds heavy conglomerates pre- 

 dominate, with but few sandstones and shales, or again the whole series 

 may be composed of coarse sandstones and shales, with the green and 

 reddish tinge running high in the formation, making it difficult, even 

 when a complete section is at hand, to decide where the line between the 

 two formations should be drawn." David White, from the investigation 

 of the fossil plants, concludes that half of the formation in the Pottsville 

 basin was deposited before deposition began in the western part of the 

 state. J 



These facts point to the conclusion that sedimentation was continuous 

 over the Pottsville trough of maximum subsidence. Consequently the 



♦ Chamberlin and Salisbury : Geology, vol. ii. 1906, p. 508. 



+ Second Geological Survey of Pennsylvania. Summary, final report, vol. ill, part 1, 

 1895. p. 1921. 



t The stratigraphic succession of the fossil floras of the Pottsville formation In the 

 southern anthracite coal field. 20th Ann. Rept. U. S. Geological Survey, part ii, 1900, 

 pp. 755-918. 



