Chemung and Catskill.—Stevenson. 25 
any such conception would be at variance with such evidence as 
from study of the stratigraphy. For the most part, the changes 
we have. That region was not above water at any time so as to 
make the Catskill deposit in a closed sea; no subaérial erosion 
took place there after the close of the Chemung, for the thickness 
of Montrose shales in the oil-wells and in northwest Pennsylvania, 
where they are Prof. White's Riceville shales, varies immaterially 
from their thickness in Somerset county, where they underlie the 
western edge of the Montrose or Honesdale sand-stone. 
But while making use of these variations in rate of subsid- 
ence as affording a convenient method of separating the Catskill 
and Chemung groups, we must not forget that in by far the 
greater part of the area, the conditions exhibited in the Catskill 
are but a continuation, and as it were an intensification of those 
existing in the Venango portion of the Chemung. The appear- 
ance of red rock with green and greenish gray sandstone begins 
in Pennsylvania very little above the Allegrippus conglomerate, 
and continues in irregularly increasing quantity to the top of the 
column, while in New York, red rock makes its first appearance 
in the Portage.* The amount of red between the conglomerates 
varies greatly, being seldom more, though often less, than ten per 
cent. along the eastern outcrop, while at some places in western 
Pennsylvania itis much greater. The Montrose shales are largely 
red along the easterly outcrop, but they show not a little varia- 
tion even there; while at the west, they are sometimes wholly red 
and at others without any red beds whatever. (rreenish gray and 
brown or reddish brown sandstones occur in large proportion in 
the Catskill itself. 
All observers agree that the passage of Chemung into Catskill 
is so gradual that, lithologically, no absolute line of separation 
ean be drawn in a great part of the Appalachian basin, The 
bond between Catskill and upper Chemung is even more intimate, 
as far as structure goes, than is that between the upper Chemung 
and the lower Chemung or Portage. As far as physical character- 
istics are to be depended on, the whole series is one, and the 
terms Catskill, Chemung, Portage might well be taken as names 
of epochal divisions of the Chemung period. 
The paleontological record confirms this conclusion drawn 
*James Hall in 28th Annual Report of the Regents on the State 
Museum. 1876, p. 15. 
