58 J. J. STEVENSON CARBONIFEROUS OF APPALACHIAN BASIN 



the Potts ville is 372 feet, an extraordinary thickness in comparison with 

 that in Elk and Cameron at the north and in counties at the south. 



Cambria and Indiana counties are south from Clearfield and Jeffer- 

 son. The former reaches to the crest of the Allegheny, where the Potts- 

 ville is exposed frequently. The thickness under the Viaduct axis is 

 estimated by Mr Piatt at not more than 250 feet, but he gives no meas- 

 urements in detail.* In eastern Indiana, under the Chestnut Hill arch, 

 he finds within Yellow Creek gap 70 feet of coarse sandstone, with 1 foot 

 of coal ver}^ near the bottom ; 4 or 5 miles farther south, in Black Lick 

 gap, near Heshbon, the succession is 



Feet 



Sandstone 20 



Shale 25 



Coarse massive sandstone , 25 



with the bottom not reached, the whole thickness being estimated as 

 somewhat more than 75 feet. At the mouth of this gap exposures are 

 very poor, and the thickness is given as not less than 60 nor more than 

 100 feet, f 



Along theConemaugh river, which separates Indiana from Westmore- 

 land county, the Pottsville shows noteworthy variations. Near Nineveh, 

 on the west side of Laurel hill, in Westmoreland countj^, the thickness 

 of sandstone is approximately 150 feet, only moderately coarse, with 

 layers of pea conglomerate. Some sandstone and iron ore underlie it, 

 adding in all about 25 feet to the thickness. No coals were seen, but 

 the exposure is not complete. On the easterly side of the gap made by 

 the river through Chestnut hill, the thickness is not more than 70 feet, 

 mostly fine sandstone and with a 1-foot coal bed at 7 feet from the bot- 

 tom, almost exactly the same as Mr Piatt's section at Yellow Creek gap ; 

 but on the west side of the mountain the sandstone has almost wholly 

 disappeared and the mass shows little aside from shales. It is unfortu- 

 nate that details respecting Clearfield county are wanting, for somewhere 

 within that county the Sharon-Olean conglomerate disappears, though, 

 as will be seen, the Sharon coals and shales persist, being apparently con- 

 tinuous with the Mauch Chunk shales below, so that they were placed 

 in the Lower Carboniferous by Stevenson in his study of this region. J 



Somerset county, south from Cambria and extending eastward to the 

 Allegheny crest, appears to offer few opportunities for measurement of 

 the Pottsville, though that formation is exposed to a greater or less ex- 

 tent at many localities. The only estimate offered by Mr Piatt is that 



* William G. Piatt : Cambria and Somerset Dist. I (H 2), 1877, p. 45. 

 f W. G. Piatt : Indiana County (H 4), 1878, pp. 125, 101, 187. 

 J J. J. Stevenson (K3), 1878, pp. 156, 172. 





