212 
GEOLOGY OF THE FOURTH DISTRICT* 
25. TULLY LIMESTONE. 
This rock marks in a most prominent manner the termination of the fossiliferous shales of 
the Hamilton group, and is succeeded by shales of a widely different character. It first ap¬ 
pears within the district on the western shore of Cayuga lake, extending for many miles, and 
is readily traced across the county of Seneca to the Seneca lake, where it is exposed upon 
both shores, and in the outlet of Crooked lake. 
It receives its name from the village of Tully, in Onondaga county, where it is better de¬ 
veloped than elsewhere, and better marked by its peculiar fossils. From this point westerly 
it becomes gradually thinner, until it is scarcely recognized. 
The rock is usually thick-bedded, but often divides by numerous irregular seams into small 
fragments. Sometimes, however, it is in courses of six inches or a foot thick, and quite close- 
grained and compact. In other situations I have noticed it, where the surface was completely 
checkered by seams ; and on breaking the mass, the whole crumbles into small angular frag¬ 
ments, much in the same manner as the shales containing pyrites. The rock is often an in¬ 
timate mixture of shaly and calcareous matter, the latter greatly predominating. Again it is 
almost purely calcareous, with shale in thin seams, separating the rock into wedge-form and 
irregular laminae. Its color, on first exposure, is blue, or often nearly black, but weathers 
to an ashen hue. From resisting the weather more firmly than the shales, it usually stands 
out in the face of the cliffs as a prominent band. It sometimes exhibits a tendency to a con¬ 
cretionary structure, but this is not usual. 
On the shores of Cayuga and Seneca lakes, its relations to the overlying and underlying 
rocks are distinctly seen along a distance of many miles. From where it first appears on the 
north, it soon dips to the level of the lake, and again rises to the southward, presenting several 
undulations, which continue the mass above the water much longer than otherwise. These 
undulations are recognized in the section, Plate VII; and they are even more distinctly marked 
on the western side of the lake, and upon Seneca lake. 
