A GEOLOGICAL HISTORY. 15 



seen at Hell-gate ferry. The dip of this rock 

 varies from the extreme of horizontal to vertical. 



5th Hornblende Slate. — (See plate l,ftg. 1 <^2. 

 marked 4.) This rock, is associated with the 

 Gneiss in many parts of the island ; at Spuyten- 

 duyvel bluff, at the north end of the island, a large 

 range is seen, which has been opened as a quarry. 

 At Manhattan ville, as you go north from the village, 

 there is also a large bluff of this rock. The struc- 

 ture of this rock is lammellar, with black flat and 

 long crystals of Hornblende, and grains of Quartz 

 disseminated through it. 



6th Quartz Rock. — (See plate ^-,fig- 1 <fy 2, mark- 

 ed 5.) There are on the 10th Avenue near 60th 

 Street, veins of various thickness of gray, granular 

 Quartz, which, when broken out in hand specimens, 

 is so friable, as to crumble into sand ; this is asso- 

 ciated with the Gneiss and Hornblende slate, in a 

 ledge of rock east of the Serpentine. 



7th Primitive Limestone (See plate l,fig* 1 § 

 2, marked 7.) of Kingsbridge is well known ; it 

 is a Dolomite,* and has all the varieties of white, 

 gray and light blue, granular, coarse marble ; it 

 begins at the south end of Mr. Dyckman's farm, 



* This Dolomite I examined some 16 years ago, and found it to contain 

 about 28 per cent of Carbonate Magnesia, from which I manufactured 

 good Epsom Salts (Sulphate of Magnesia.) 



