DETAILED STUDY OF SECTIONS 



163 



Park Avenue Railroad bridge. — The approx- 

 imate surface of the rock in this section is 

 brought out in the sketch profile which has 

 been furnished by Mr Alfred P. Boiler, con- 

 sulting engineer, who was the engineer in 

 charge of construction (see figure 9). Gneiss 

 is found near the present grade beneath the 

 piers on both banks of the river. The north 

 bank of the river is formed by a nearly ver- 

 tical wall of gneiss, which was followed to a 

 depth of 70 feet by the piles beneath one of 

 the piers. The strike of this wall is, accord- 

 ing to Mr Boiler, about at right angles to the 

 line of the bridge. This wall would appear 

 to represent a line of dislocation. Beneath 

 the central pier of the bridge a drill was put 

 down 104 feet to either a ledge or a boulder 

 of limestone. Mr Boiler says : 



11 We found some pieces of marble, which is 

 probably the same as the Tuckahoe marble in 

 Westchester county." 



Third Avenue bridge, Harlem river. — In con- 

 structing this bridge the caisson of the south- 

 west pier, which is 70 feet from the Manhat- 

 tan shore, rests on a ledge of gneiss at a depth 

 of 52 feet. The bottom of the caisson was 

 24 feet by 110 feet, and an unbroken area of 

 gneiss extended the entire width of the cais- 

 son and was uncovered for 30 feet of its 

 length.* The other piers of the bridge rested 

 on boulders (see figure 10). 



Second Avenue bridge. — The north and south 

 abutments and the four piers of this bridge 

 rest alike on the detrital material of the river. 

 Borings were sunk beneath the piers to depths 

 ranging from 40 to 48? feet without encount- 

 ering rock of any kind. The profile of figure 

 11 has been kindly furnished by Mr J. J. R. 

 Croes, who was chief engineer in charge of 

 construction. 



J 



* Information furnished by Mr Edward A. Byrne, assistant city engineer, New York. 



