DETAILED STUDY OF SECTIONS 



159 



It thus seems probable that the central pier of the Washington bridge 

 is located on a line of faulting which follows the east bank of the river. 



High bridge, Harlem river.— All but two of the seven piers of this bridge 

 which are in the river are on piles ; the other two, however, are on rock.* 

 Dana has fortunately preserved f a sectional view of this bridge, " re" 

 duced from a tracing obtained for me by Mr Benjamin S. Church at the 

 engineer's office in New York " (see figure 3). Mr Church was resident 

 engineer in charge of the Croton water works. From this interesting 

 section it appears that the walls of the Harlem gorge in this vicinity are 

 formed of gneiss, which plunges down beneath the silt and other river 

 deposits along a steep incline ; and, further, that in the middle of the 

 rock gorge there rises, pedestal like, a reef of marble which has furnished 

 a base for the three central piers of the bridge. 



KtWYORK ISLAND 



WESTCHESTER CO. 



Figure 3. — Section across Harlem River. 

 On line of High bridge. 



New New York aqueduct siphon under Harlem river. — The section of the 

 aqueduct in this vicinity has furnished geological information of the 

 first importance. The aqueduct crosses the river in an inverted siphon, 

 the lowest arm of which is about 300 feet below the surface of the river. 

 Vertical shafts descend through gneiss on either side of the river to this 

 depth, and a horizontal connecting arm penetrates approximately 800 

 feet of "hard limestone or marble" beneath the river (see figure 4). 

 The section shows the walls of this layer of limestone to be roughly par- 

 allel and to dip at very steep angles to the east. The first intention of 

 the engineers was to locate the horizontal arm of the siphon at a more 

 moderate depth (see figure 4), but on running the drift to the east a 

 " crevice " was encountered with crushed stone at the boundary of the 

 limestone. The report says : 



11 Under Harlem river the gneiss and limestone were found to meet on a con- 

 spicuous diagonal line without any visible solution of continuity, the particles of 

 the gray and of the white rock being so closely intermingled at the surface of 

 contact that the exact line of separation could not be traced." J 



The report and the section are therefore in accord in showing the 



*F. B. Tower: Illustrations of Croton aqueduct, New York, 1843, p. 110. 



tLoc. cit., pp. 435-436. 



t Report New York Aqueduct Commissioners, 1887 to 1895, p. 88 and sheet 32. 



XXII— Bull. Geol. Soc. Am., Vol. 16, 1904 



