GEOLOGY OF THE NEW YORK CITY AQUEDUCT 233 



parallel to the strike. Such movements are capable of cutting out 

 the intermediate limestone entirely from between the schist and 

 gneiss. How much of such modification exists, in the almost total 

 lack of data bearing upon the question, it is impossible to say. The 

 intermediate belt is indicated on the accompanying map [pi. 34] , as a 

 limestone area. At one point at least the limestone does occur in 

 the older borings, i. e. on the southeastern margin of the Man- 

 hattan pier of the Manhattan bridge (bridge no. 3), at the foot of 

 Pike street. 



On the Brooklyn side no formations of this series except the 

 Fordham and its associated igneous masses, such as the Ravens- 

 wood granodiorite, have been identified within the area under study. 

 Limestone is reported (Hobbs reference to Veatch) near Newtown 

 creek, a little beyond the eastern margin of the present map. 



Structure of the East river area 

 Manhattan side. In all of the area south of 59th street, 

 structural features are even more obscure than the areal geology. 



There is no reasonable doubt but that weak zones will be found 

 as frequently in the Manhattan schist portion of this area as on the 

 line north of 59th street, but they can not be indicated as closely. 

 No cross fault of large consequence can be identified, but there is 

 some evidence of a minor zone that should be encountered on Fifth 

 avenue, in the vicinity of 32d street. The Pennsylvania tunnels and 

 the subway both cross this line and so far as known there were no 

 serious weaknesses developed. There is nowhere any evidence of 

 an important depression like the Manhattanville valley. 



It is confidently believed that the problems on this southerly por- 

 tion of Manhattan are involved chiefly with the longitudinal struc- 

 tures produced by folding and faulting and subsequent disintegration 

 along such zones. 



Crossing of East river 



From 59th street to the East river there seems to be no reason 

 for a preference between the two lines P and Q.^ On the Brooklyn 

 side likewise there is no known geological reason for preference. 

 Such basis for choice as is now known relates to the East river 

 channel alone. Since this is at the same time the most difficult sec- 

 tion of the line to explore and probably the most uncertain section 

 to estimate as to condition and consequent depth of tunnel, it would 

 be especially useful to be able to make a decisive selection of cross- 

 ings at once. 



^ For location of these lines see map, pi. 32. 



