24 The American Geologist. January, 1892 
feet thick in Fulton county, is wanting. In northern Pennsyl- 
vania, the decrease in thickness is abrupt for a few miles, but the 
final disappearance of rocks of the Catskill type is in Warren 
county, just as in New York it is in Allegany county east from 
the Genesee river.* In southwestern Pennsylvania the Catskill 
is wanting, because the rocks have thinned out; whether the dis- 
zippearance in northwest Pennsylvania is due only to thinning or 
to interlocking with rocks of different color, cannot be determined 
in our present state of knowledge. 
The upper sub-divisions of the Chemung, when followed west- 
ward, are found to vary much after the same manner throughout. 
The abrupt changes observed in the Catskill had no predecessors 
in the Chemung, except in southwestern Virginia, where the 
whole series, Chemung and Catskill, as well as most of the under- 
lying Hamilton and much of the overlying Pocono have disap- 
peared. The Chemung section thus grouped 
Shales 
| Sandstone 
VENANGO- Shales and sandstone 
| Sandstone 
Shales and flags 
can be recognized not merely along the eastern outcrop from New 
York to far beyond New river in Virginia, but also in western 
Pennsylvania many miles beyond the western limit of the Catskill 
beds. 
It is sufficiently clear that, at the close of the time embraced in 
the Chemung group, a physical change occurred, which, though 
not observable along the eastern outcrop, becomes very distinct 
within 100 miles westward or northwestward. During the whole 
of the Chemung period, the subsidence was less and less rapid 
toward the west and northwest, though doubtless keeping pace 
there as at the east with accumulation of deposits, which, in that 
direction, became less in quantity and finer in grain, as the rocks 
at the west and northwest were not such as to yield much coarse 
material. But at the close of the Chemung, the subsidence be- 
came still less rapid toward the west and northwest, so that 
the area in which Catskill was deposited became narrower toward 
the south.+ It is altogether unnecessary to resort to the concep- 
tion of elevation in western Pennsylvania or Virginia; indeed 
+Because of the southwestward trend of the Appalachian land area. 
