GEOLOGY OF THE NEW YORK CITY AQUEDUCT 87 



river, and in addition a working shaft suitable for permanent use 

 has been started on each side of the river. These have thoroughly 

 explored the rock character to a depth of about 800 feet. It has 

 proven to be of constant type, a gneissoid granite, affected by 

 moderate amount of jointing, shear movements and occasional dike 

 intrusion. The two sides are alike, the rock in depth is com- 

 paratively free from water, nearly all coming from the adjacent 

 surface drainage. 



Persistent efforts have been made to use the drill in the river to 

 explore the rock channel, but with meager results. The difficulties 

 to be overcome in drilling in this tidal river to the necessary depth 

 are probably greater than have even been encountered in any 

 similar undertaking. The disturbance presented by the current, the 

 tide, the depth of water, the drift filling above the rock channel, 

 and the traffic in the river are a constant menace. The complex 

 character of drift filling in this gorge, especially the occasional 

 heavy bouldery structure, makes it necessary to reduce the size and 

 recase the holes repeatedly. But in this regard the work has 

 suffered less actual loss than by the menace of river traffic. 

 Several times after the greatest efforts had been put forth in 

 pushing the drills deep into the gorge a helpless or unmanageable or 

 carelessly guided steamer or scow has wrecked the work. In this 

 way some of the most critical locations have been lost together with 

 many months of labor. 



The results are shown on the accompanying drawings. 



It is worth noting that of those holes located far out in the river 

 channel only two have reached bed rock. Even these two have 

 penetrated the rock so little distance that there might be still some 

 doubt of permanent bed rock. The fact, however, that the rock 

 found is of the right type, i. e. like the walls of the gorge, leads to 

 the conclusion that the bottom was actually penetrated. Neither of 

 these holes are in the middle of the river, and, although the 

 maximum depth of 608 feet was reached by one of them, the central 

 portion of the buried channel proves to be still deeper. One hole 

 located near the middle was able to penetrate to a depth of 626 feet 

 without striking bed rock. But it was finally lost. The latest 

 results are from a boring that has reached a total depth^ (January 

 I, 1910) of 703 feet, the last 8 feet of which was believed by the 

 drillers may be in bed rock. All above is drift and silt. 



1 Subsequent exploration has proven that the bottom of the old channel 

 lies still deeper. This boring has been pushed to a depth of 751 feet with- 

 out yet touching bed rock (Oct. 8, 1910). 



