﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 87 



The commonest secondary mineral now filling these crevices is 

 chlorite, and, although it may completely fill the crevices it has little 

 binding strength. Any new disturbance or strain readily causes 

 separation along the same original lines. But in spite of the fact 

 that the core is broken into small pieces and shows so low percent- 

 age of recovery it is quite certain that the rock itself is not badly 

 decayed. An examination of one of the most doubtful looking 

 cores from the lower part of hole no. I showed under the micro- 

 scope little evidence of serious decay. This is believed to mean 

 that underground water circulation is not as abundant as the 

 fractured condition of the rock would lead one to expect. Further- 

 more, an examination of the cores in greater detail shows beyond 

 question that much of the fracturing is entirely fresh and must 

 have been done by the drill itself. It is certain that the low per- 

 centage of recovery is in part due to this cause. The small diam- 

 eter of the intermediate holes is contributory to the same results. 

 Some allowance must also be made for the difficulty of working 

 a machine from a raft on the lake. 



Comparison of the cores shows a decidedly higher percentage 

 of core recovery, and presumably therefore of rock solidity in all 

 of the other three holes — no. I, no. 5 and no. 15. 



Hole no. 2 — core recovered 14.8$ 

 " no. 1— " 34.6^ 



" no. 15— " 36.3^ 



" no. 5— " 38.9^ 



It therefore appears that the last three penetrate rock that is 

 more than twice as good in its capacity to stand drilling disturbance. 



A comparison of quality at different depths is believed to be still 

 more encouraging. The upper portions of all holes have poor 

 recovery and comparatively poor looking rock. But in depth there 

 is a marked improvement. 



In view of the fact that the tunnel will undoubtedly be located 

 somewhere below the -75-foot level, it is really only this lower sec- 

 tion that is of vital importance to the project. A tabulation and 

 comparison of core recovery from these lower portions is given 

 below. 



1 From total depth of hole 2 From depth -75' to bottom 



Hole no. 2 — = 14.8$ core recovery 25$ core recovery 



" no. 1— = 34.6* " ASfo 



« no. 15— =36.3^ " 66 fo 



" no. 5— =38.9^ " 42^ 



