hobbs.] STRUCTURAL GEOLOGICAL STUDIES. 
stone on 132d street, between Fifth and Sixth avenues, and on 1 
street, between Third and Fourth avenues. The " islands of gn< 
upon the island, which Gale so well describes, Stevens accounted foi 
by a network of longitudinal and transverse fractures, of which seven 
transverse fractures are mentioned, including one which separates 
the island from Westchester County, and five longitudinal ones. The 
three large longitudinal fractures follow the gorges of Hudson, Har- 
lem (between Washington and Fordham heights), and East rivers. The 
more important of the cross faults were located along Spuyten Duyvil 
Creek, through the gorge at Tubby Hook, and through the Manhat- 
tanville valley. He says: 
Our rocks are a portion of the mainland in Westchester County, from which we 
are cut off by a profound fault, a cross fracture in part, originally narrow, but widened 
by the abrading power of water and ice, gravel and bowlders, and, in part, by rup- 
ture of the strata longitudinally, with subsequent abrasion. 
Beginning on the North River, at Spuyten Duyvil Bridge, the first cross fracture 
extends SSE. 1,500 feet; then commences a longitudinal fracture, extending to Tubby 
Hook, the Spuyten Duyvil Creek flowing in it for 2,000 feet. Another cross fracture 
then commences, extending to the Harlem River, in which the creek flows 1,000 
feet, then it enters another longitudinal fracture, in which it flows NNE. 4,000 feet, 
where it meets with another cross fracture for 2,000 feet. Thence a longitudinal 
fracture extends as far southward as McComb's dam and bridge, running parallel 
with the North River; thence the fracture slightly inclines southeastward to Hurl- 
gate, on the East River. 
At Tubby Hook there is another cross fracture, extending from the North River 
into the second longitudinal one. At Manhattanville there is another, reaching 
across the entire island. 
Between the islands in the East River and the New York Island there is a longi- 
tudinal fracture through limestone and gneiss, extending from the mainland as far 
south as Avenue A. South of this point we infer a continuation of it to the southern 
extremity of the island. A similar longitudinal fracture begins at Dobbs Ferry and 
Hastings, through the same rocks as the eastern, and extends the whole length of 
the island, forming the channel of the North River, separating it from New Jersey. 
Stevens was the first to recognize clearly the prevailing southerly 
pitch of the rocks throughout the island as well as in the area of the 
Bronx. a 
Credner. — A paper by Credner, published in 1865/' adds little to 
what was alread}^ known of the structural geology of the island. 
Newberry. — A very graphic picture of the later phases in the physio- 
graphic development of New York Island was given by Newberry in 
1878/ 
The elevation of the land in Tertiary times, followed by the .sharp 
incision of the valleys, the subsequent submergence and flooding of 
these valleys, the abrasion and tilling accomplished in the Glacial 
a Stevens, R. P., Hist. Geol. New York Island: Annals Nat. Hist, New York, vol. 8, 1865, |>. L19. 
bCredner, Hermann, Geognostische Skizze der Umgegend von New York: Zeitschr. Deutsch. 
Gesell., Berlin, vol. 17, 1865, p. 390. 
c Newberry, J. S., Geol. hist. New York island and harbor: Pop. Sci. Monthly, vol. 13, 1878, pp. 
641-660. 
