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) 

 12 400 Inwood limestone — with several bad zones 



6400 Manhattan schist — probably good quality 



3 000 Fordham gneiss — probably good quality 



I 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 



Feet cent 



Yonkers gneiss 6200 (7.2) 



Fordham gneiss 7000 (8.1) 



Contact zones 4400 (S-i) 



Inwood limestone 12000 (13.9) 



Manhattan schist 50200 (58.5) 



Ravenswood grano-diorite . o (o) 



Unknown 6 000 (7.0) 



Total length 85 800 



Feet 



8000 



13000 



O 



21 400 



36200 







6000 



Per 

 cent 



(9 

 (15 



(25 

 (42 



(7 



4) 

 3) 



(0) 

 3) 

 6) 



(0) 

 0) 



84 600 81 200 



Feet 

 6000 



21 400 

 6800 



12400 

 6400 

 9800 



18400 



Per 



cent 



(7.3) 



(8.3) 

 (15.2) 



(7.8) 

 (12.0) 

 (22.6) 



Summary of quality 



Feet 



Good rock, ist grade 42400 



Probably fair, 2d grade. . . . 30800 



Probably poor, 3d grade 6 600 



Almost unknown 6 000 



Per 



cent 



(49-4) 



(35-9) 



{7-7) 



(7.0) 



Feet 

 33800 

 34800 

 lOOOO 



6000 



Per 



cent 



(40.0) 

 (41. I) 



(II. 8) 

 (7.1) 



Feet 

 39800 

 13 600 



9400 

 18400 



Per 

 cent 



(49.0) 



(16.7) 



(II. 6) 

 (22.7) 



85800 100. 84600 100. o 81200 100. o 



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 Catskill aque- 

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

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



