History of the Geology of New York Island. 115 



Island, there is a longitudinal fracture through limestone and 

 gneiss, extending from the main land 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 frac- 

 ture begins at Dobb's 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. 



The cause and reason of these longitudinal fractures will be 

 explained as we progress in our report. 



The main mass of the rocks of the island is gneiss. In the 

 gneiss lie veins and beds of granite, anthophyllite, and horn- 

 blende. 



Granite. — When this rock occurs in veins, their strike is 

 generally coincident with that of the gneiss, viz. N.N.E. and 

 S.S.W. Their angle of dip is also coincident with that of the 

 gneiss. The granite is interlaminated with the stratification of 

 the gneiss. These veins are much more numerous upon the 

 western than on the eastern side of the island. They vary in 

 thickness from a few inches to sixteen feet. Their extent, N. 

 and S., is limited not by fault or fracture, but by an insensible 

 fading out into gneiss. In the process of grading streets and 

 of levelling blocks of lots, they are often completely removed, 

 ; ,thus showing that they have not been injected from beneath ; 

 and I am led to the conclusion that they are of metamorphic 

 origin, similar to the gneiss itself. 



When granite occurs in massive beds, as at the corner of 

 E. 53d st. and 4th avenue, it lies across the strata ; and when 

 the metamorphism is perfect, it can be distinctly separated 

 from the gneiss ; but following the mass along the fresh exca- 

 vations, it is seen to insensibly blend into gneiss, similar to the 

 veins. When the active forces of the ancient " Ice Period" 

 acted upon the softer gneiss, removing it from around the 

 granite, on its northern aspect, it gives it the form of large 

 rounded bosses and sometimes ridges. 



