THE ALLEGHENY PLATEAU 21 



mostly of white and gray sandstones, sometimes holding rounded peb- 

 bles and frequently showing bands of greenish and reddish argillaceous 

 beds. The sandstones are tine grained and thin bedded.* Here the 

 lower beds are increasing in importance. 



Mr Sanders's section, in Blair county, south from Center, makes the 

 Pocono 1,241 feet thick, but he has included about 350 feet at the bottom 

 clearly belonging to the Catskill, so that the thickness is about 900 feet, 

 of which the upper two-thirds is a practically continuous gray, current- 

 bedded sandstone, broken only by two shale beds, one 8 and the other 

 20 feet thick. t No coal is reported from the Blair county Pocono ; the 

 Tipton coal of that county belongs to the Coal Measures, as originally 

 asserted by I. C. White, and not to the Pocono. Still farther south, in 

 Bedford county, and about 10 miles eastward from the face of the plateau, 

 Stevenson found that the Pocono can not exceed 920 feet, and suggested 

 that the interval may contain some Catskill rocks, so that the thickness 

 is approximately the same with that in Blair.J Mr O'Harra states that 

 a little south from the Pennsylvania line, in Maryland, and not far off 

 the strike from the Bedford locality, the thickness is but 258 feet, the 

 rock being a grayish green flaggy sandstone, with some shale and some 

 bands of conglomerate. He finds it but 80 feet thick in the gorge of the 

 Potomac, where it is a gray cross-bedded sandstone, with a few widely 

 scattered pebbles. § Doctor White's detailed section in Maryland, along 

 the Baltimore and Ohio railroad, where the Potomac emerges from the 

 Allegheny plateau, shows about 1,150 feet of gray, cross-bedded, mostl}^ 

 flaggy sandstones, apparently free from conglomerates and red shales. || 



The strip west from the front of the Allegheny plateau. — Returning to the 

 north outcrop, the first Pennsylvania survey found in Tioga county 

 (adjoining Bradford at the west) 300 feet of sandstones underlying the 

 shales and sandstones of the Mauch Chunk (Umbral), divided midway 

 by 5 feet of shale, and showing near the bottom a calcareous bed.^ 

 Doctor White estimates 573 feet as the extreme possible thickness for 

 Pocono near Blossburg, in this county, but the estimate is not given 

 with assurance, as exposures below the Mauch Chunk are wanting.'-'^* In 

 view of observations in counties on both sides of Tioga, the interval ap- 

 pears to be excessive. In Potter county (west from Tioga) Mr Ashburner 

 finds the extreme thickness for Mississippian not more than 400 feet, 



*E. v. d'lnvilliers : Geology of Center county (T4), 1884, p. 122. 



t R. H. Sanders : Geology of Blair county (T), 1881, pp. 13, 23. 



J J. J. Stevenson : Geology of Bedford and Fulton counties (T2), 1882, p. 70. 



§ C. C. O'Harra: Allegany county, pp. 109, 110. 



II L C. White : Notes on the Geology of West Virginia. Proc. Am. Phil. Soc, vol.'xix, p. 443. 



^H. D. Rogers : Geology of Pennsylvania, vol. ii, p, 520. 



**I. C. White: G5, 1881. 



