PENNSYLVANIAN. 433 



It will be seen that the eastwaid-increasing amplitude of the folds results in erosion of the 

 Pennsylvanian from a number of the anticlines within the plateau region. The most promi- 

 nent of these are known as the Deer Park, Laurel Hill, and Chestnut Ridge anticlines. The 

 interveniag folds have been described as First, Second, and Third bitiiminoiis basins, etc., or 

 synonymously designated by geographic terms."* 



A radius from the border of the plateau coal field to the southwest corner of Pennsylvania 

 spans a section extending from a point at or near the base of the upper Pottsville to the top 

 of the Pennsylvanian as at present somewhat conventionally drawn. The middle and lower 

 Pottsville are not encountered, as the entire upper Carboniferous area of Pennsylvania, out- 

 side of the anthracite fields, together with the whole of the Ohio Pennsylvanian, falls in the 

 region of extension and overlap of the later Pottsville sea. The lower divisions are, however, 

 present at the type locality, Pottsville, in the southern anthracite field. 



The terranes of the several sections throughout Ohio and Pennsylvania have received careful 

 consideration and correlation by E. V. d'lnvilliers^^^'' and J. J. Stevenson,"^^ who give full 

 details or citations. The original data relating to the bituminous area wiU for the most part 

 be found in the "Geology of Pennsylvania," by H. D. Rogers, 1858; in the county reports of 

 the Second Geological Survey of Pennsylvania; in the reports of the Ohio Survey (especially 

 volume 6 and Bulletin 9) ; in the following folios issued by the United States Geological Survey: 

 Warren, Foxburg-Clarion, Beaver, Kittanning, Rural VaUey, Elders Ridge, Indiana, Latrobe, 

 Ebensburg, Johnstown, Amity, Sewickley, BrownsviUe-ConnellsviUe, Rogersville, Waynesburg, 

 Masontown-Uniontown, and Huntington ; in United States Geological Survey Bulletins 65, 279, 

 304, 318, and 349; and in the report of the Topographic and Geological Survey Commission of 

 Pennsylvania for 1906-1908. 



Pottsville group. — The Pottsville group, earlier known as "Conglomerate series," "Potts- 

 ville conglomerate" ("XII"), and "New River series," forms the lowest division of the 

 Pennsylvanian. As the basal and conglomeratic stage of the "Serai," the last of the ideal 

 time divisions proposed by the brothers Rogers'"* for the Paleozoic in America, it was earlier 

 called the "Serai conglomerate." It has sometimes been roughly correlated with the Mill- 

 stone grit of Great Britain, though on the paleobotanic evidence it appears that the upper 

 Pottsville is to be correlated with the middle Coal Measm-es of the United Kingdom. 



The Pottsville as a whole is the most natural of the Appalachian upper Carboniferous 

 subdivisions, for it represents the first or main epoch of sea transgression in the cycle of 

 Peimsylvanian sedimentation. In his characterization of the Pottsville I. C. White ""^^ points 

 out the strong contrast which it presents to the red and greenish softer sediments below it, 

 and to the softer, generally more argillaceous, terranes above it. 



"The series as a whole possesses a large amount of hard white or grayish sandstone, 

 much of which is conglomeratic. The sandstones are harder, more compact, and siliceous; 

 than any of the lower coal series above. Bowlders of this sandstone i^ake a smooth polish 

 when rolled along river beds, but this is not the case with most sandrocks above this." 



On account not only of the highly variable thickness of the different component terranes 

 but also of the absence of one or more of them in different regions it is difficult to give a 

 generalized section of much value. The following regional section in Mercer County, Pa., 

 near the Ohio line, prepared by I. C. White,'"* is fairly typical of the upper Pottsville in Ohio, 

 Maryland, and western Pennsylvania; and though of unusual thickness (301 feet) it is espe- 

 cially important on account of its location at or near the type localities for most of its members. 

 The bracketed matter is added by the present writer. 



" For description of structure, anticlines, and basins, see Summary Pinal Kept. Second Geol. Survey Pennsyl- 

 vania, vol. 3; Bull. U. S. Geol. Survey No. 65; Ohio Geol. Survey, Kept. V; West Virginia Geol. Survey, vols. 2 

 and 2A; Maryland Geol. Survey, vol. 5, p. 260. For the anthracite regions see Second Geol. Survey Pennsyl- 

 vania, mine maps in Atlas of anthracite*region, and Reports of progress A and AA; also Rogers, H. D., Geology of 

 Pennsylvania, 2 vols., 1858. 

 48011°— 12 28 



