174 HELDERBERG DIVISION. 



§7. Schoharie grit (PI. xx.). 



This rock is a brown decomposing sandstone, in consequence of a mixture of lime, which 

 • lives, and leaves a granular lender mass thai may be broken in (lie hands : hence it is 

 always soft apon the outside, from disintegration by the action of the weather. 



Thickness. In New-York, it attains a thickness of only four feel ; and were it not that it 

 is impossible to annex it to the inferior or next superior mass, it would he entitled only to 

 the subordinate place of a layer. 



Extent. Ii is confined to the Helderberg range ; at least it does not reach Cherryvalley. 

 It is about two feet thick at Leeds in Greene counn, 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 i'rw niollusca only having been found in 

 it as yet. Suddenly, however, a deposit is formed, which encloses a multitude of mollusca 

 and a lew 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 !>eing9 

 ime 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 limcrock, Seneca limestone, 

 and Corniferous limestone. 



The Onondaga limestone is a gray and crystalline rock beneath, dark colored and some- 

 what Bhaly above, through all that portion which received the appellation of Selenurus 

 limcrock. 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 



