GEOLOGY OF THE NEW YORK CITY AQUEDUCT 263 



61 1-63 1 feet — 14 pieces same chloride foliated rock. Two 

 pieces of '* drillite " 



One piece of fresh rock — a gray gneiss of rather worn texture 



646-655.5 feet — 16 pieces of — a white and black and red 

 blotched rock — a garnetif erous gneiss. The rock is not a 

 common type but a similar variety is sometimes seen along 

 the margins of the granodiorite intrusions and belongs to the 

 Fordham gneiss series. 



Rock is fairly sound and for the size of core the saving is good. 

 (3 ^eet) 



2 Defection test. A deflection test on this hole indicates that the 

 drill has not departed more than 5° from the vertical. 



3 Behavior of drill. It has been possible to drive the casing down 

 after the drill witliout reducing the size and without enlarging the 

 rock hole to a final depth of 625 feet. 



About half of the water fed into the machine is lost — 10 gallons 

 per minute being fed and 5J/^ gallons recovered. 



The hole filled after each pull up as much as 100 feet with mat- 

 ter that either ran in from a crevice or was furnished by disinte- 

 gration of the walls or was simply the settling of matters held in 

 suspension during operation. These settlings or corings, as the 

 case may be, were of large amount (100 feet±) when the drill was 

 cutting far below the casing and small in amount (5 feet) when 

 the casing was driven down near to the bottom. This matter then 

 increases as the hole is deepened again below the casing. 



Cutting and progress are rapid and easy. 



4 Examination of the rock, (a) Hornblendic gneiss. A micro- 

 scopic examination of the green hornblendic gneiss shows that the 

 rock is not badly crushed and that the different original grains are 

 well interlocked. But the more easily affected mineral constituents 

 are very generally decayed and have become especially modified on 

 their surfaces where they interlock with other grains. The matter 

 developed is mostly chlorite — a mineral that is very soft and one 

 that in this case fails to furnish a very firm bond between the grains. 

 A disrupting force exceeding the strength of this soft mineral there- 

 fore, such as drilling with a small bit or forcing the drill, causes the 

 grains one by one to roll out or break apart and furnish the sus- 

 pended matter that seems to be so abundant in this hole. 



b The rock below 646 feet. This is a very unusual type of rock, 

 the petrographic cliaracter of which need not be taken up here. It 

 appears to be simply a contact variety, such as sometimes is devel- 



