158 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



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Kings bridge and Dyckmans cut. — In the 

 vicinity of Kings bridge and along the line 

 of the ship canal the limestone is much in 

 evidence. The government work at Dyck- 

 mans cut consisted in the excavation of a 

 canal through a reef of white marble, and is 

 always referred to by the Corps of Engi- 

 neers, United States Army, as the unique 

 instance of excavating or dredging in lime- 

 stone in all their operations about New 

 . York island. 



Washington bridge, Harlem river.— This 

 bridge has three piers — one on the west of 

 the river, resting on gneiss, and two on the 

 east side (see figure 2). The east pier rests 

 on gneiss, while the middle pier is located in 

 the alluvial material at the very edge of the 

 water. This latter pier was sunk by caissons 

 to " an irregular rock, partly gneiss, partly 

 marble, with veins and pockets of very hard 

 quartz. 1 ' * Mr William R. Hutton, the chief 

 engineer in charge of the construction, has 

 obtained for the writer a very interesting 

 letter from Mr George Leighton, his assistant 

 engineer, regarding the rock found beneath 

 the middle pier. A portion of Mr Leigh ton's 

 letter follows : 



" The gneiss was nearly all of granular character, 

 and the upper portions of the ledge had disinte- 

 grated. I do not recall that it was more micaceous 

 than the ledge at and above Boscobel avenue, but 

 it was much inferior to that in structure and 

 density. The gneiss immediately overlying the 

 marble was much harder — better material. My 

 recollection is that the marble filled a fault in the 

 gneiss, in width something over half the width 

 of the caisson, and extending nearly parallel 

 with the sides of the caisson. There were quartz 

 deposits in the marble — one of considerable 

 size, which would strengthen the fault theory. 

 I carried a large specimen of the quartz to the 

 office." 



* William R. Hutton : The Washington bridge over the Harlem river at One hundred and eighty- 

 first street, New York city, a description of its construction, 1890, p. 21, plate xxxii. 



