﻿238 NEW YORK STATE MUSEUM 



4 The Lower East Side zone. On Delancey street east of the 

 Bowery, the line crosses the structure and at this point the whole 

 series of crystalline formations appears. Besides complicated struc- 

 ture there is also exceptionally deep alternation or decay of bed rock. 



5 The East river crossing — from the foot of Clinton street to 

 Bridge street, Brooklyn. 



1 Harlem river crossing 



Geologically the Harlem river between 155th and 200th streets 

 has the same relation to local formations for the whole distance. 

 It flows on the Inwood limestone bed which stands almost exactly on 

 edge, while the east river-bluff is formed by the underlying Fordham 

 gneiss, and the west, by a strong escarpment of Manhattan schist 

 which extends southward throughout the whole of Manhattan form- 

 ing the backbone of the island. 



At the selected crossing a short distance below High Bridge, near 

 167th street, the schist-limestone contact is in the river and appears 

 to be a low weak spot [see detail of record]. The limestone-gneiss 

 contact however is in the flat east of the river bank, near Sedgwick 

 avenue and seems to be more substantial. The structural detail and 

 relations are shown on the accompanying profile and cross section, 



[Pi. 35L 



It is observed by examination of the data secured by borings that 

 the limestone formation at this point is exceptionally heavily im- 

 pregnated with pegmatite dikes and stringers, and that interbedded 

 schist layers are large and numerous. 



The weakest spot found lies at the contact between schist and 

 limestone where there is probably some longitudinal displacement. 



A similar condition was found at the new Croton crossing 2000 

 feet farther north. On the whole bad decay does not extend very 

 deep — 150-200 feet. 



Several borings have been made and on them is based the only 

 judgment possible of the actual structure and physical condition of 

 rock. In most cases the evidence is easily interpreted for these 

 points. The most weakened spot, as well as the most difficult to 

 interpret in all its detail, is the limestone-schist contact. It is 

 judged that hole no. 17 cut through this contact zone. This boring 

 is located in the river 50 feet from the Speedway (west bank) on 

 the proposed tunnel line which crosses a short distance south of 

 High Bridge. It is known as hole no. 17/C38. Because of the 



