Stevenson*] -Jo [Aug. 15, 



whole region north from the National road, the tenth bench is the most 

 widespread, the one which gives character to the country. At 8 miles and 

 a half from Washington, the general elevation of the country increases and 

 the road rises, so that from that point to the four-mile post the road oscil- 

 lates between the 1420 and the 1445 feet bench. But near that post it falls 

 to 1375 or 1380 feet and comes upon a fine terrace, which is of considerable 

 extent north and south from the pike. 



Between the second and third mile-posts, the road crosses an island of 

 the 1420 feet bench and at the second post it is again at 1375 feet. This is 

 the principal bench of the Chartiers Valley, being well shown on both 

 sides. The lower benches came out in their order from this horizon down 

 to the railroad depot at Washington, which is near the place of the sixth 

 bench. 



The observations from the river to Washington were made altogether 

 with the barometer, but under very favorable circumstances, for on repe- 

 tition the measurements showed insignificant variations ; and in both cases 

 the total change in the barometer during the passage was barely five one- 

 thousandths of an inch ; the altitudes of the stations terminating the line 

 having been well determined by railroad levels. 



Benches on West Side op Chestnut Ridge South from the 

 youghiogheny rlver. 



This slope shows for the most part a very regular face in the Redstone 

 region. Near the West Virginia line and beyond that southward as far as 

 the observations were carried, the rocks of the Lower Barren and Lower 

 Productive Coal Series have so far escaped erosion, that the benches below 

 the eighth are easily recognized. Northward from the middle of George's 

 township, in Fayette county, to the divide between Redstone and Dunbar 

 creeks, the lower rocks of the Coal Measures have been in great part re- 

 moved, and the massive Pottsville (Serai) conglomerate, resisting erosion, 

 has remained to give the mountain its present slope. The benches do not 

 exist where this rock forms the face of the ridge and traces of them are 

 very rare. It is noteworthy that the outcrop of this conglomerate, though 

 at a considerable distance from the plane of the axis, is not much below the 

 average elevation of the summit of the ridge. 



On the summit, at the National road, the surface is covered by a fine 

 reddish sand, almost free from clay, which is well shown at the Summit 

 hotel. The well on the opposite side of the road was digged in this sand 

 to the depth of 40 feet, and the people in the vicinity were surprised by 

 the occurrence of river snails and mussels, many of them being quite fresh 

 looking. This point is about 2400 feci above tide. A well marked bench 

 was seen on the Seaton road, 180 feet higher, which extends along that 

 road for nearly two miles south from the National road and is covered with 

 loose sand derived from the disintegration of the Pocono (Vespertine) 

 sandstone. A level bench covered by loose sand can be followed for sev- 

 eral miles along the crest of this ridge. 



