14 A GEOLOGICAL HISTORY. 



Avenue (as is shown by the blue on the map.) 

 This rock begins, on the south side of, and under- 

 lies Governor's Island, which is its most southern 

 extent, passing through New York Island, and 

 running through the greater part of Westchester 

 County : it forms the rock, at the straits called 

 Hell-gate, and even at that place (where it may 

 be traced some 8 or 10 miles) underlying Long 

 Island. 



The Gneiss of New York Island is a peculiar 

 variety, and has more Mica than common, and 

 which is also in larger plates than usual for this 

 rock : some of this Gneiss, might with propriety, 

 be called coarse Mica Slate. It is not a good 

 building stone, on account of its splintering pro- 

 pensities, which makes it dress uneven and rough, 

 if it is struck with an intent to make a cross frac- 

 ture, it is sure to split the wrong way. On the 

 10th Avenue near 60th Street, it is associated with 

 Hornblende slate, and a granular Quartz rock, and 

 at the "Railroad Tunnel" at Yorkville, with 

 Hornblende. On the East River, above Bellevue, 

 are precipitous Gneiss rocks from 20 to 30 feet 

 high, forming bold bluffs, and overhanging the 

 water ; at this place, it is sometimes, charged with 

 Sulphuret of Iron in a state of decay, as may be 



