WESTERN COUNTIES OF PENNSYLVANIA 25 



fossiliferous, often simply a fish-bone conglomerate. Dr O. St. John 

 regards the fishes as presenting the aspect of the lower Mississippian. 

 Doctor White states that the molluscan remains are related to the Kin- 

 derhook, but he is inclined to regard the total evidence as pointing 

 toward the lower Keokuk or upper Burlington. The Sharpsville sand- 

 stone underlies the Meadville shales; it contains in the lower part a 

 flinty limestone, non-fossiliferous in Crawford county, but fossiliferous 

 at Garland, in Warren county, and at Tidioute, in Venango, where it is 

 full of distorted shells, among which Spirijer disjunctus or a closely allied 

 form is most abundant. 



Below the Sharpsville are the Orangeville and the Oil Lake group, 

 the latter being equivalent to the Berea sandstone of Ohio. The total 

 thickness is approximately 440 feet, and the whole is referred to the 

 Pocono.^ 



Doctor Chance has summarized the variations of the " upper or gray 

 Pocono " as they appear along the northern border in Pennsylvania. 

 From the edge of the Allegheny plateau until one reaches western Clin- 

 ton county, the sandstones are fine grained, foliated and hard, gray or 

 greenish gray. In that county from 60 to 80 per cent is sandstone, the 

 rest is sandy shale ; but westward, along the lines followed by him, the 

 rocks become more shaly and less arenaceous, until more than half of 

 the hard sandstone below the Shenango has been replaced by olive and 

 gray shales. The thickness of the Pocono averages not far from 400 feet 

 in Clinton, Cameron, Elk, McKean, Warren, and Mercer counties, and 

 the series is regarded by him as equivalent to the Waverly of Ohio.f 



Southward from Mercer and Venango counties the anticlines do not 

 bring the Pocono to the surface, so that, in the southern counties, one is 

 dependent upon oil-well records, many of which have been tabulated by 

 Mr Carll. Here quotations are made only from his latest Pennsylvania 

 reports, which give the essential facts for Beaver, Butler, Allegheny, 

 Washington, and Greene counties, the last three being -west from West- 

 moreland and Fayette, in which are the Conemaugh and Youghiogheny 

 gaps, already referred to as offering good exposures of the Pocono. 



The Shenango sandstone is followed easily in the records, but appar- 

 ently the Meadville shales and limestone become irregular at no consid- 

 erable distance southward, and sometimes are replaced by sandstone. 

 In northern Mercer county the Shenango is 15 feet thick, with the upper 

 Meadville limestone at 25 feet below it; 30 miles southeast, at P]den- 

 burg, on the western border of Clarion county, a record shows that the 

 sandstone is 64 feet, resting on 283 feet of shale, extending downward 



*I. C. White : Geology of Erie and Crawford counties (Q, 4), 1881, pp., 77-96. 

 t H. M. Chance : G 4, p. 98. 



