GEOLOGY OF THE NEW YORK CITY AQUEDUCT 2'] 



These preserved cores, now aggregating many thousands of feet 

 have been of great service in determining the precise limits of 

 formations and consequently the geologic structure or cross section, 

 by which detailed estimates may be guided. 



Even these occasionally appeared to give insufficient data. The 

 peculiar behavior of certain holes, as, for example, one or more 

 at Foundry brook,^ led to the suspicion that the drill had swerved 

 from its course, following a particularly soft seam or zone, and 

 that the results secured by it without large corrections, were wholly 

 misleading. Tests proved that there had been a deflection. 



At this and many other places it later became very desirable to 

 form some quantitative as well as qualitative opinion of the condi- 

 tions existing in the underlying strata. The percentage of core 

 saved, the rate of progress of the drill, the behavior of the drill, the 

 condition of the core recovered, the loss of water in the hole — all 

 these of course were considered. 



For more definite evidence as to porosity and perviousness, a 

 series of carefully planned pressure tests^ were made. By shutting 

 off connection with the walls of the hole above a certain stratum 

 and forcing water in under pressure, it was possible to demonstrate 

 that certain strata or certain portions were practically impervious 

 in their natural bed, while others were much less so, and to get an 

 idea of their relative efficiency as water carriers. For the pressure 

 tunnels, especially, this test is a very suggestive line of investigation. 



1 At Foundry brook \sce discussion of this problem in pt 2], the remark- 

 able condition apparently shown was a reasonably substantial ledge of 

 granitic gneiss, 50 feet, followed below by 200 feet of apparently soft 

 sand and reported as such. No core could be recovered. So extensive 

 a zone or bed or layer or mass is hardly conceivable considering the 

 crystalline silicious character of the rock. It probably represents a steeply 

 dipping crush zone along fault movement where the increased underground 

 circulation has been unusually effective in producing decay. After enter- 

 ing this zone the drill swerved from its initial course and kept within tb.e 

 soft seam. 



2 The pressure test is made by means of a force pump, fitted with a 

 gage on which the pressure is recorded, connected by a pipe to the por- 

 tion of the hole to be tested, and so adjusted to a device for blockading 

 or damming the hole that the water pressure is confined to those portions 

 of the walls of the hole below the dam, or between two dams if an upper 

 and lower one are used. In this way any portion of a hole, or stratum or 

 several beds together may be tested and the amount of water absorbed 

 per unit of time per unit of pressure determined. This is, of course, 

 directly related to the porosity of the rock and is approximately inversely 

 proportional to its presumed value as an aqueduct carrier. 



