SCHOHARIE GRIT. 
151 
20. SCHOHARIE GRIT. 
Grit Slate of Eaton. 
This rock is a very fine-grained siliceous limestone or calcareous sandstone ; when wea¬ 
thered, the calcareous matter disappears, leaving a porous siliceous mass of a brownish color, 
from the presence of hydrate of iron. In this condition it presents numerous casts of its 
peculiar fossils. It abounds in a species of Pleurorhyncus, Orthoceras and numerous forms of 
corals. The rock is well developed at Schoharie and in the Helderberg, but does not extend 
far westward. It is unknown in the Fourth District, but with the last and several others it 
has been enumerated in the order, that the numbering might be continuous, and a complete 
series of the rocks of the system be given in their order of succession, believing that such a 
course would facilitate the comparison of the groups in different parts of the State. There 
are formations at either extremity of the State which do not exist or are only partially deve¬ 
loped at the other, and if the practice of describing these in their respective order in the 
report of each District be followed, neither would present a complete series of the rocks of 
the system. This course will give the inhabitants of the western part of New-York a know¬ 
ledge of what exists elsewhere and which are wanting in that region, and also show them 
the points at which they are introduced in the middle and eastern parts of the State. 
21. ONONDAGA LIMESTONE. 
Included in the Corniferous Limerock. by Prof. Eaton. Grey Sparry Limestone of the 
Annual Reports. 
Throughout the greater part of the Fourth District the impure limestone terminating the 
Onondaga salt group is succeeded by the Onondaga limestone with usually the intervention 
of a few inches of sandstone before noticed, which in ordinary observations might be entirely 
overlooked. There is also sometimes a thin band of nonfossiliferous bluish grey limestone. 
The range of this formation is in an undulating line having a general east and west direction 
throughout the district, extending eastward to the Hudson river and westward far beyond the 
Niagara into Canada. Its northern outline is everywhere well marked, forming together 
with the next succeeding rock the second great limestone terrace, which rises to the south of 
the valley marking the range of the Onondaga salt group. 
