J8(> EKIE DIVISION. 



sumiiut. they are thick and more even, though a band of contorted sandstone appears two 

 hundred feet below the top of the mass. 



The new of the head of the gorge at Summit, is only one instance among many <>f the 

 wearing action of the streams. Upon this series and range of rocks from the Hudson to 

 Lake Brie, all the water courses cut through the Bhales and sandstones of this group. An 

 interesting fact is well worthy of notice in this place, namely, that as the New-York sedi- 

 mentary rocks are composed of hard and soft materials, the whole series seem to be cut 

 through from the Potsdam sandstone to the top of the Erie division. The aggregate amount 

 of the perpendicular falls of the streams which flow over the series, is not less than one and 

 a half miles, from the lop of the Catskill series, to the base of the Potsdam sandstone. 



§ 4. TULLY LIMESTONE. 



Towards the top of the series composing the Hamilton group, calcareous matter in- 

 creases ; and in the central counties, it is so far increased that a compact black limestone 

 has been deposited. In Albany and Schoharie counties, it is unknown ; neither has it been 

 noticed west of the Genesee valley, and yet beds of a black limestone occupy its place at 

 Moscow above the Moscow shales. These layers or strata are compact, black, bituminous, 

 and interlaminated with shale. They contain a lew fossils, the most interesting of which 

 is a microscopic orthoceratite ; and all the remains are extremely minute, but very nu- 

 merous. 



The thickness of the Tully limestone is from twelve to fifteen feet : hence the mass is 

 too inconsiderable to exert an influence upon the soil. The rock is exposed upon the west 

 shore of Cayuga lake, and the eastern shore of Seneca lake near Hathaway's landing; 

 also at Bellona in Ontario county, and the outlet of Crooked lake. At Bethel on Flint 

 creek, it forms a part of the banks ; while four miles northwest, it is only three feet thick. 

 Farther west, on Canandaigua lake, it is represented by a few inches only of impure cal- 

 careous rock. West of this lake, according to Mr. Hall, it is virtually absent, although 

 its place is indicated by bands of calcareous shale. 



