172 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



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rock is of the granitic type of gneiss , 

 which contains frequently small gar- 

 nets. The exception is from hole 

 number 2 near the center line, outer 

 side of the New York tower, the rock 

 from this core being a beautiful 

 white dolomite. Figure 18 is a 

 profile of the river along the line 

 of this bridge, which shows the lo- 

 cations of both towers and anchor- 

 ages. 



Figure 19 shows the location of 

 drill hole number 2 under the outer 

 edge of the New York tower. The 

 position of this drill hole between 

 gneiss in holes on either side of it 

 at the corners of the tower makes 

 it likely that this occurrence of 

 limestone is infaulted between 

 walls of gneiss — mortised into 

 gneiss — as was the case in Vosburg 

 hill, Toms hill, and elsewhere in 

 the Sheffield valley, or has been 

 pinched sharply in a fold. The 

 depth of the rock surface below 

 mean high water is, under the New 

 York anchorage, from 72 to 84 feet, 

 g> under the New York tower from 

 I 108 to 133 feet, under the Brook- 

 lyn tower 90 to 97 feet, and under 

 the Brooklyn anchorage 68 to 74 

 feet. 



Brooklyn bridge. — The New York 

 and Brooklyn piers were carried 

 down to the bed rock. In the re- 

 port of the chief engineer the follow- 

 ing statements are made in refer- 

 ence to the foundations beneath the 

 New York tower : 



ft°?!7&?5'?3.?.?$ "Theprojectingpeaksofbed rook which 



already made their appearance at 75 feet were blasted down for some distance 



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