168 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



o 



on this bridge is of scientific interest, chiefly be- 

 cause of the intersection of faults belonging in 

 two different series, as described in the last para- 

 graph. 



East River bridge number 4- (Blackwells Island 

 bridge). — This bridge is to connect Fifty-ninth 

 street, Manhattan, with a point between Rogers 

 and Charles streets, Queens. Diamond drills have 

 been put down along the proposed line of this 

 bridge beneath the piers on the Manhattan side 

 of the river, on both shores of Blackwells island, 

 and on the shore of Long Island City. The sur- 

 face of the rock is in all cases but a few feet be- 

 low the surface of the ground. The rock on the 

 east side of Manhattan island at the water's edge 

 slopes very abruptly toward the channel (nearly 

 45 degrees). On the west and east sides of Black- 

 wells island the rock surface is at about the level 

 of high water and is badly decomposed for a dis- 

 tance of from 2 to 5 feet below the surface. On the 

 Long Island shore the rock surface is also at about 

 the level of high water and is found more decom- 

 posed than upon the shores of Blackwells island, 

 it having been necessary to remove about 15 feet of 

 this material before suitable foundations could be 

 secured. Through the courtesy of Mr R. S. Buck, 

 former chief engineer in charge of this bridge, the 

 writer was enabled to examine cores from all the 

 drill borings and found them to represent a granitic 

 type of gneiss.* From Mr H. A. La Chicotte, since 

 engineer in charge, the section was obtained which 

 has been reproduced in figure 14. 



It is quite likely that the decomposition of the 

 gneiss at the shores along this section is to be 

 explained in the same way as is the decomposed 

 rock along the fissures in the Blackwells Island 

 tunnel ; namely, through the local fracturing of the 

 rock and the consequent introduction of water from 

 the river. During the construction of the Black- 

 wells Island tunnel the operations were greatly 



* Information regarding East River bridge number 4 was obtained from Mr R. S. Buck, former 

 chief engineer, and Mr H. A. La Chicotte, who succeeded him in charge of this bridge. 



