Claypole.] 500 {May 16, 
Tur NIAGARA Group oF New York ABSENT FROM Parry County, Pa. 
From the identifications here established it follows that nothing is left 
to represent the Niagara group in Perry county. If such a representa- 
tive existed it must lie on the top of the iron ore capping the Clinton 
group. But the green shale of the passage beds has yielded no fossils 
that can belong to a bed of that age. It holds, as above shown, & mingled 
fauna of the Clinton and Lower Helderberg ages. There is, consequently, 
no conclusion possible, except to infer the absence of the Niagara group 
from Perry county. 
The rapid thinning of the Niagara rocks in New York to the eastward 
prepares us for this conclusion. Two hundred and forty feet thick at 
Niagara Falls, it dwindles down to about one hundred and thirty feet in 
Wayne county, near Rochester. No other exposure occurs until we reach 
the slope of the Cincinnati anticline in Southwest Ohio, where it scarcely 
exceeds fifty feet. : 
Tun Uprrr Limit of Tan ONONDAGA GROUP. 
It is scarcely necessary to follow this subject further, as no doubt exists 
concerning the age of the mass of Limestone overlying these shales, The 
Lower Helderberg group in Perry county has a well-defined summit, 
being capped by the Oriskany sandstone, but an ill-defined base where it 
meets the Onondaga gray shales. Difference of opinion, consequently, 
may exist concerning the exact plane at which the separation should be 
made. <A short statement, therefore, of the facts and argument bearing 
on this point is appended. 
The Lower Helderberg rocks in Perry county as here defined, consist 
of the following : 
10’. White flint shales, ) Oriskany Sandstone. 
80’ Yellow flint shales. 
8’ Black cherty limestone. | Lower Helderberg 348’, 
150’ Lime shales. 
100’ Massive limestone. Onondaga Gray Shales. 
Regarding the age of all these beds, except the lowest, there is no room 
for doubt.* The Lime shales and the White flint shales both abound in 
the fossils that characterize the Lower Helderberg group in New York, 
The following partial list is sufficient to support this assertion. 
Fosstzs Common to tHe Limp SHALES OF PERRY County, PaA., AND 
ton Lower HeLpERBERG Rocks or New York: 
Discina discus, Hall. Merista levis, Vanuxem. 
+ Strophomena rugosa, Dalman. Mevista bella, Mall. 
Rensselaeria mutabilis, Hall. Megambonia aviculoidea, Hall. 
Rhynchonella nucleolata, Hall. Murchisonia minuta, Hall. 
Rhynchonella formosa, Hall. 
*This doubt is now removed by the note added below. 
+ This species and Spirifera macropleura, Con., abound in the White flint 
shale. 
