﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 221 



SUMMARY OF LINE C 

 Feet 



6000 Yonkers gneiss — good quality 

 17400 Fordham gneiss — good quality 



6800 Along contact between limestone and gneiss (questionable) 

 12400 In wood limestone — with several bad zones 



6400 Manhattan schist — probably good quality 



3 000 Fordham gneiss — probably good quality 



1 000 Fordham gneiss — unknown quality 



9 800 Ravenswood grano-diorite — mostly very good rock 

 18 400 Almost wholly unknown 



81 200 total 



Tabulated summary — Types of rock formations 



LINE A (WEST) LINE B (CENTRAL) LINE C (EAST) 



Per Per Per 



Feet cent Feet cent Feet cent 



Yonkers gneiss 6200 (7.2) 8000 (9.4) 6000 (7.3) 



Fordham gneiss 7000 (8.1) 13000 (15.3) 21400 (26.3) 



Contact zones 4400 (5.1) o (o) 6800 (8.3) 



Inwood limestone 12000 (13.9) 21400 (25.3) 12400 (15.2) 



Manhattan schist 50200 (58.5) 36200 (42.6) 6400 (7.8) 



Ravenswood grano-diorite .0(0) (0) 9800 (12.0) 



Unknown 6000 (7.0) 6000 (7.0) 18400 (22.6) 



Total length 85 800 84 600 81 200 



Summary of quality 



Feet 



Good rock, 1st grade 42 400 



Probably fair, 2d grade .... 30 800 

 Probably poor, 3d grade. . . . 6600 

 Almost unknown 6 000 



85800 



3 A 



LINE B 



LINE C 



Per 





Per 





Per 



cent 



Feet 



cent 



Feet 



cent 



(49-4) 



33 800 



(40.0) 



39 800 



(49.0) 



(35-9) 



34 800 



(41. I) 



13 600 



(16.7) 



(7-7) 



10 OOO 



(11. 8) 



9 400 



(11. 6) 



(7-0) 



6 00O 



(7.i) 



18 400 



(22.7) 



100. 



8460O 



100. 



8l 200 



100. 



Argument on choice of line 



In judging the quality of rock and its suitability for this con- 

 duit the factors of most weight are the same as those repeatedly 

 mentioned in connection with other portions of the Gatskill aque- 

 duct line. That is, in brief, that the harder crystalline rocks of the 

 ^Fordham gnedsis and) Manhattan schist types wherever known to be 



