EASTERN COUNTIES OF ALLEGHENY PLATEAU 51 



with the impure limestone and the lower or sandy limestone, but the 

 upper shales are very thin and the lower shales are wanting. 



A difficulty is encountered in reference to the upper shales, counted 

 here as part of the Mauch Chunk. The lower member of the Pottsville, 

 as shown in northwestern Pennsylvania, evidently disappears south- 

 wardly, so that the Sharon coal group is continuous with the Mauch 

 Chunk shale. Stevenson, in his Pennsylvania reports, included that 

 group in the Mauch Chunk, but afterward * in the light of I. C. White's 

 studies in the northern counties, transferred it to the Pottsville. The 

 Sharon is certainly present in the Conemaugh gap through Chestnut 

 hill, for an insignificant coal bed was seen there directly under the Potts- 

 ville; but the group is so thin that no rearrangement of the section is 

 necessary. Farther south, however, the Sharon becomes thicker and its 

 relations will be discussed in a later chapter. 



Southward from the Conemaugh the upper limestone quickly becomes 

 important. W. G.. Piatt does not appear to have found it on the west 

 flank of the Allegheny mountains, in Somerset county, or under the Via- 

 duct axis, though he found the silicious limestone on the Allegheny near 

 Ashtola ; but Stevenson found on the west side of Laurel hill, at 20 miles 

 south from the Conemaugh, 16 feet of limestone, separated b}^ about 50 

 feet of red shale from the silicious limestone below, and thence south he 

 reports frequent exposures to the Youghiogheny gap through Laurel hill, 

 where the two limestones are present on both sides of the anticline. Along 

 Chestnut hill the upper limestone increases rapidly south from the Cone- 

 maugh. The two beds of the Conemaugh gap were seeen in the Loyal- 

 hanna gap, both thin, silicious, and fossiliferous. On Jacobs creek, the 

 boundary between Westmoreland and Fayette, the upper limestone is 

 exposed for 40 feet, the top portion for 18 feet very pure and the lower 

 portion very argillaceous and with abundant fossils. The section does 

 not reach down to the silicious limestone. f Both limestones are present 

 in the Youghiogheny gap, where the silicious limestone is about 40 feet 

 thick. Stevenson has given recently a section on the National road in 

 Fayette county, which shows shales, 100 feet, and limestone and shale, 

 150 feet, the latter having at the base 35 feet of silicious limestone, with 35 

 feet of shales separating it from the main mass of limestone above. There 

 is a concealed space of 23 feet in the lower part of the upper limestone, 

 and the character of the slope suggests that it is filled with readily yield- 

 ing material, possibly very calcareous sandstone or very sandy limestone, 

 of which a little is shown just above it.J Dr I. C. White's studies carry the 



* Notes on the Lower Carboniferous groups, etcetera. Amer. Jour. Sci., vol. xxxiv, p. 37. 

 t J. J. Stevenson : Report on Fayette and Westmoreland district, part i, 1877 (K 2), p. 262. 

 X Notes on the Mauch Chunk of Pennsylvania. Amer. Geologist, vol. xxix, p, 244. 



