174 
HELDERBERG DIVISION. 
§7. Schoharie grit (PL xx.). 
This rock is a brown decomposing sandstone, in consequence of a mixture of lime, which 
dissolves, and leaves a granular tender mass that may be broken in the hands : hence it is 
always soft upon the outside, from disintegration by the action of the weather. 
Thickness. In New-York, it attains a thickness of only four feet; and were it not that it 
is impossible to annex it to the inferior or next superior mass, it would be entitled only to 
the subordinate place of a layer. 
Extent. It is confined to the Helderberg range ; at least it does not reach Cherryvalley. 
It is about two feet thick at Leeds in Greene county, and appears on both sides of the 
church, resting on the Cauda-galli grit, which is elevated into a flat dome upon which 
the church stands (PI. XX. Sec. 5, 6). 
The general remarks upon the Oriskany sandstone, apply in part equally well to this 
rock. It succeeds a rock quite poor in fossils, a few mollusca only having been found in 
it as yet. Suddenly, however, a deposit is formed, which encloses a multitude of mollusca 
and a few Crustacea. Some parts of the rock are formed of the remains of animals ; and 
of these animals, it is quite doubtful whether any have been found in the inferior rocks. 
After four feet of rock had been deposited, not only the kind of material which for a short 
time had been in the process of accumulating, is changed, but the fauna is changed also ; 
so that after a comparatively brief space of time, its numerous species of living beings 
became extinct, and gave place to others. 
This rock, from its limited extent, is unimportant agriculturally ; neither does it, or the 
next mass below, furnish mineral bodies of importance. Its interest is principally for the 
palaeontologist. 
§8. Onondaga limestone ( Plates xx., xxi.). 
It is designed to include under this designation a dark colored limestone, which has been 
described in the Annual Reports under the names of Selenurus limerock, Seneca limestone , 
and Corniferous limestone. 
The Onondaga limestone is a gray and crystalline rock beneath, dark colored and some¬ 
what slialy above, through all that portion which received the appellation of Selenurus 
limerock. Lithological characters are not competent to distinguish this from any other 
gray or dark colored limestone. Disregarding the fossils, we may look for its connections 
in order to be satisfied of its identity. Above, it is succeeded by a black shale ; below, 
in the eastern part of the State, by the Schoharie grit and Cauda-galli sandstone ; in the 
middle and western part of the State, by the Oriskany sandstone and Manlius waterlimes 
and shales. One feature which is interesting, though not distinctive, is that it contains 
chert or hornstone, or, as it is usually called, flint. It occurs in layers and irregular 
masses, which are the most abundant in the superior portion. In the Helderberg, it is not 
so distinctly in layers ; but at Leeds in Greene county, it is made up of flinty layers in 
