DETAILED STUDY OF SECTIONS 



157 



the bed rock in the floors of those channels. They are considered in 

 order, proceeding from the mouth of Spuyten Duyvil creek eastward 

 about the island, so as to consider first Spuyten Duyvil creek, then the 

 Harlem, East, and Hudson (North) rivers in succession. The sketch 

 map of New York and vicinity shown in plate 35 will serve for location 

 of sections. Inasmuch as the problem, so far as it is petrographical, is 

 to locate the areas of limestone, it will be sufficient to refer to the harder 

 rock as gneiss without attempting to refer it to one horizon or another. 

 Spuyten Duyvil bridge, Harlem river. — This bridge is not founded on 

 the rock bed, but rests on piles. Wash borings were, however, put 

 down to the bed rock by Mr C. B. Brush, the depths varying from 94 to 

 124 feet below mean high water. No record has been preserved regard- 

 ing the kind of rock at the bottom, and it was probably not known. 

 Through the courtesy of the engineers of the New York Central rail- 

 road, the writer was allowed to examine the samples taken from the 

 bottom of these borings, but they afforded no clue regarding the char- 

 acter of the rock.* 



/HTM 



MANHATTAN 



8R0NX 



^r&lfWV* 



Figure I.— Sketch Map of Spuyten Day oil Creek. 

 Along the line of the swing-bridge. 



Both north and south of the creek at that point the gneiss rises 

 abruptly to form bluffs. In the railroad cut north of the creek local 

 northerly pitches of the beds were found, and the conclusion is drawn 

 that no southerly pitch sufficiently steep to carry the overlying bed of 

 limestone to the bed of the river across the strike is probable. 



The writer's view is that the stream here flows along a cross-fracture, 

 as stated by Stevens t and apparently concurred in by Gratacap.J 

 Under the causeway of the New York Central railroad across the northern 

 edge of this gorge piles were driven to a depth of 35 feet without meet- 

 ing rock.§ 



*See, however, "The Spuyten Duyvil swing-bridge, etcetera." Engineering News, vol. xliii, 

 1900, pp. 397-398, figure 1. 



fLoc. cit., p. 114. 



X L. P. Gratacap : Geology of the city of New York, 1901, p. 36. 



§ Information furnished by Mr F. L. Chase, engineer of bridges, New York Central and Hudson 

 River Railroad Company. 



