426 J. J. Stevenson—Upper Devonian Rocks of Pennsylvania. 
gorge is cut down to the Devonian, but owing to the thick 
coat of debris, no exposures oceur and no section can be 
obtained south from the Conemaugh River. The fold declines 
north from that river, so that the gaps made within Cambria 
County by Chest and Black Lick Creeks reach barely to 
Formation X, and no gap in Clearfield County, south from that 
of the west branch of the Susquehanna, seems to expose any 
lower rock. These facts are gathered from the reports of 
Messrs. F. and W. G. Platt on Cambria, Somerset and Clearfield 
Counties, and from my own careful observations in Fayette 
and Westmoreland. 
Chestnut Ridge first shows the Devonian rocks near the 
National Road in Fayette County, but thence northward the 
axis diminishes in strength, a given stratum being fully 1,000 
feet lower at the Conemaugh than at the National Road. 
Between that road and the Youghiogheny River, several 
gorges are cut down to the Devonian, but no section can be 
obtained until the Youghiogheny River is reached. North 
from the river, owing to the decline of the axis in that direc- 
tion, the deepest gorges soon fail to reach the Devonian and 
no exposure exists between the Youghiogheny and the Cone- 
maugh. North from the Conemaugh the fold still decreases in 
strength, as is well shown by the fact that the Lower Coals 
creep constantly higher up its sides, so that the gaps made by 
Black Lick and other streams cannot do more than barely to 
reach Formation X, especially since the great Conglomerate of 
XII thickens very materially in that direction, as abundantly 
age from the report on Clearfield County by Mr. Franklin 
latt. 
There is no exposure whatever for more than fifty miles 
along the west slope of the Alleghany Mountain; no exposure 
ceurs in Negro Mountain or the Viaduct axis, so that no 
e 
thirty-five miles. . Surely under such circumstances one may 
hesitate before accepting any conclusion based on mere strati- 
graphy. 3 
But is lithology any better? At all exposures to which 
reference has been made, except those in Clearfield County, 
respecting which I have no knowled e, rocks more or less 
similar in appearance are found Ssaibilinacy below Formation 
X, which is believed to represent the gray Catskill of New York. 
As they are at the top of the Devonian, they are likely to be 
