WESTERN COUNTIES OF PENNSYLVANIA 71 



Here, apparently at the place of the Upper Mercer limestone, is a lime- 

 stone bearing no resemblance to that bed either in structure or appear- 

 ance, but, according to Mr Piatt, almost exactly similar to the Tuscumbia 

 limestone or Silicious limestone as shown in the Chestnut Hill gaps. Its 

 appearance here, so far away from the nearest occurrence of any Mercer 

 limestone, shows it to be a purely local feature, for whose occurrence no 

 explanation is available. It disappears very quickly in all directions. 

 The Black shale of the section is near the place of the Tionesta coal bed. 



The Homewood sandstone is thicker toward the Indiana border, on 

 Mahoning creek, where it is 70 feet, and both Mercer coal beds are present 

 at about 2 miles above the last locality of the last section. The whole 

 series is exposed on the Allegheny, at the mouth of Mahoning creek, 

 about 12 miles from the Indiana border and 4 miles south from Clarion. 

 The section is practically the same as on Red Bank creek, but the Mer- 

 cer coal beds are not shown and only thin streaks of coaly matter occur 

 in the Sharon shales, which there are 38 feet thick. 



The most southerly exposure of any part of the Pottsville west from 

 Chestnut hill is on Cowanshannock creek, say 40 miles northwest from 

 the Conemaugh gap. There 63 feet are shown belonging to the Home- 

 wood sandstone in several massive layers, without pebbles and separated 

 by shale.* 



The Pottsville passes under cover in Armstrong, Butler, and Beaver 

 counties at a few miles south from their northern boundaries. Thence 

 southward in those counties, in Allegheny, Washington, and Greene, as 

 well as in Westmoreland and Fayette, west from Chestnut hill, informa- 

 tion can be obtained only from records of oil borings, which are not 

 wholly consistent; but those records are so numerous that it is possible 

 to trace the formations with close approximation. 



The record at Petrolia, in Butler county, is 



Feet 



Homewood sandstone . 66 



Shales and sandstones 145 



Sandstone , 148 



Here one is almost due west from the Red Bank Creek region of Clarion 

 and Armstrong. The succession is clearer on the western side of the 

 county, where one finds 



Feet 



Homewood 18 



Mercer group 110 



Upper Connoquenessing 65 



Shale 3 



Sandstone and shale 100 



* W. G. Piatt : Report of Progress in Armstrong County (H 5), 1880, pp. 88, 139, 143, 185, 194, 207, 

 215, 231. 



