GEOLOGY OF THE NEW YORK CITY AQUEDUCT 223 



25.3;?), and the chief advantage of Hne A over line C lies in its 

 much smaller amount of " unknown " ground (6000 feet vs. 18,400 

 feet or y.o^ vs. 22.6%). On these grounds line A is the least ob- 

 jectionable of the three lines proposed. 



But it is also clear from an examination of the field, as is shown 

 on the accompanying map [pi. 32], that it is possible to avoid 

 some of these objectionable features or certain parts of them and 

 materially improve the figures by shifting the line to a sort of com- 

 promise position between line A and line B. This compromise 

 line, or the trial lines from which the final tunnel line may result, 

 should follow as closely as possible the gneiss and schist ridges 

 and should avoid the limestone areas and known weak zones wher- 

 ever possible. 



Depth of tunnel 



The rock formations in general at the required depths are no 

 more objectionable on ^Manhattan island or in The Bronx than at 

 other localities on the Southern aqueduct. There are weak places 

 and crush zones to be crossed and some of them can not be avoided 

 by any possible manipulation of the line, but these most question- 

 able spots constitute but a small proportion of the whole distance. 

 The depth most suitable must depend chiefly upon the depth neces- 

 sary at the worst spots. 



Comparative cost of construction if lines are shifted 

 The question is best answered by reference to the geological map. 

 It will be noted especially that the belts of the different rock forma- 

 tions are usually narrow, and that they run nearly parallel to the 

 average direction of the lines. Therefore a shift of line to no great 

 distance would at many points place it within an entirely different 

 formation. It is also notable that all of the lines run along or near 

 the contacts between formations for long distances. At such points 

 a very small shift would wholly change the type of rock and rock 

 quality. Some shifting is desirable. 



In general it may be assumed that the limestone belts would be 

 easiest and cheapest to penetrate wherever they are fairly substan- 

 tial, but they undoubtedly also contain the greater proportion of 

 weak and troublesome ground and must be considered least desir- 

 able from the standpoint of maintenance and durability. The 

 gneisses are probably most expensive to penetrate and the schists, 

 medium. Both are more expensive than limestone but both are 

 more likely to prove acceptable for other reasons. 



