﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 1 67 



Several other borings encountered similar conditions, especially 

 those on the west flank of the valley within range of the belt in 

 which the fault seems to be located. 



Hole no. 9 reached the rock floor at a depth of 80 feet, and then 

 penetrated rock to a depth of 159.7 feet. All of the material is 

 badly decayed. Only 1 foot of core was recovered from the whole 

 boring and that is mostly quartz coming from a veinlet or peg- 

 matitic streak at 141 feet. Water under slight pressure was en- 

 countered in this hole also. But because of . the somewhat greater 

 elevation of the surface at this than at hole no. 2 there is not a 

 constant outflow. j 



Two other holes immediately 1 to the west show much better rock } 



condition — no. 1 showing 79$" core recovery. Also two on the east 

 side at greater distance [see accompanying profile] show good rock. | 



But one other no. 3 at a distance of over a thousand feet to the east 

 encountered another zone of decayed rock, the record being very 

 similar to no. 2 in that poorer conditions are shown at depth than 

 near the surface. Rock was found at a depth of 20.2 feet. From 

 20.2 to 116 feet the gneiss was quite hard, 55.3 feet of core being 

 recovered or 57.7$. But from 116 feet to the bottom 207.5 ^ eet tne 

 material was as bad as in hole no. 2, and no core was recovered. 



Several other tests were made on the borings with a view to de- 

 termining the character and extent of these features more definitely. 

 For example, if the interpretation given for the behavior of no. 2 

 and no. 3 is correct it ought to be possible to survey the holes and 

 determine a deflection from the vertical as the drill deviated from 

 its course to follow the softest streak. A survey conducted for this 

 purpose indicates just such a result. The accompanying sketch 

 shows the data plotted. The drill was deflected 4 36' at a depth 

 of 50 feet, 7 36' at 100 feet, 8° 2' at 150 feet and 9 40' at 

 198 feet. 



Pressure tests were made for porosity on some of the holes in 

 sound rock. Some of these data are given on the profile. 



Some of the rock of this valley, if very extensive, such as that 

 in borings no. 2, no. 3 and no. 9, would be very poor ground for 

 tunneling. The practical question involves especially the width of 

 these zones, are they a foot wide or are they a hundred? In an 

 attempt to help settle that question an inclined hole was proposed 

 that was to run at an angle low enough to crosscut these belts. 

 Accordingly hole no. 14 was bored inclined 40 ° 26' to the hori- 

 zontal and started on the solid granite gneiss. The results were not 



