2l8 NEW YORK STATE MUSEUM 



that they would be found to extend considerably fartlier south at 

 depth than their surface outcrops. This angle of pitch is from 

 io° to 30°. 



In nearly all these cases, however, the obscurity of the actual 

 surface boundaries is as great a source of uncertainty as the effect 

 of dip and pitch, so that the boundaries as mapped may be con- 

 sidered sufficiently accurate for this comparative study of the lines. 



It is worth noting that the rock at the proposed depths of tunnels 

 would be, as a rule, more substantial than at the surface. But there 

 are several places on all of the lines where the exact condition is 

 unknown at the surface as well as at depth. The chief points of 

 this character will be noted in a later paragraph. 



Comparison of lines^ 



A comparison of the three lines submitted as the basis of ex- 

 amination — (a) the westerly one, (b) the central one, (c) the 

 easterly one [see accompanying map, pi. 32], as to rock formations 

 likely to be cut by them, furnishes the following figures : 



Line A. Going soiitlnvard from Hill Vieiv reservoir 



Feet 



6 200 Yonkers gneiss — good rock 

 I 400 Fordham gneiss 



1 400 Probably largely Inwood limestone with one weak zone 



(at YdiTL Cortlandt lake) 

 5 600 Fordham gneiss — good rock 



2 400 Near contact with limestone, probably in gneiss 



1 600 Crossing Harlem river — Inwood limestone 



4 OCX) Inwood limestone — probably fairly good rock 

 800 Inwood limestone — probably containing bad zone to 

 Speedway 

 16400 Manhattan schist (to 135th st.) 



2 000 Along contact between schist and limestone 



4200 Inwood limestone with one weak zone (to s. end of 

 Morningside Park) 



iThe statements of quality and extent of certain formations and zones 

 are capable of some modification as exploratory work progresses. Some of 

 these are noted in later sections of this report under special headings, such 

 as The Lower East Side, and The East River-Brooklyn, section. For the 

 present purpose, as showing the development of the geologic basis of the 

 project it seems preferable to leave the accompanying comparisons in their 

 original form as presented to the board. 



