160 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



western boundary of 

 the limestone beneath 

 the river to be along 

 a line of dislocation 

 diagonal to the bed- 

 ding. It is highly 

 probable that the east- 

 ern boundary is also 

 located on a parallel 

 fault, but the evidence 

 from this section is 

 not conclusive. 



Eighth Avenue bridge, 

 Harlem river. — This 

 . . bridge rests on piles. 

 g a All efforts to learn 

 s § the depth to which 

 s M they were carried 

 J £ have been futile. 

 s % Mc Combs Dam (Cen- 

 I & tral) bridge. — All piers 

 I Z of this bridge rest on 

 I. ® a reef of gneiss which 

 g % runs from one bank of 

 ! s Harlem river to the 

 other at depths vary- 

 ing from 24 feet unde r 

 the central pier to 3C 

 feet at the west bulk- 

 head line and 27 feet 

 at the east bulkhead 

 line. Mr Martin Gay, 

 assistant engineer of 

 the department of 

 bridges, New York 

 city, who has fur- 

 nished me with this 

 information, adds 

 that " on the line of 

 the bridge to the east- 

 ward the rock drops to an unknown depth in the swamp and comes to 

 the surface again at One hundred and sixty-first street " (see figures 



