of the Hudson River. 5 



to 800 ft. as a guess. More accurately than this it seems 

 impossible to determine the depths with the data in hand. 



Leaving in abeyance for the moment certain important 

 conclusions which follow from this unexpectedly great depth, 

 a few interesting features may be recorded regarding the 

 tunnel itself. The greater portion of the rock is granite from 

 shore to shore and for a mile or two on each side. It is a part 

 of the large Storm King batholith which is so important a fea- 

 ture of this portion of the Highlands. Where fresh it is a 

 pale green rock, moderately provided with dark silicates. It 

 contains the usual quartz, orthoclase, and plagioclase, together 

 with hornblende, less biotite and the accessories normally 

 found in granite. It is obviously under compressive strain, 

 since both in the shafts and in the tunnel the rock scales off, 

 with small explosions, a process called "popping" by the 

 workmen. In the shafts care became necessary to guard 

 against injury from falling pieces. In the tunnel, where the 

 popping tends to eat back into the roof and unduly enlarge 

 the cross-section of the tunnel, lagging and arch-like supports 

 of bent I-beams have been used to keep it from starting. 



At intervals shown in the plan, fig. 4, the tunnel cuts at small 

 angles the dark bands of diorite which were described from 

 the cuts of the West Shore E. K. in 1888 * Eleven of these 

 have been observed in the tunnel. They vary from a minimum 

 of about a foot in thickness up to a maximum of 25 or 30 ft. 

 The tunnel cuts them at such a slanting angle that it is difficult 

 to make accurate estimates. f These apparent dikes are much 

 richer in dark silicates than the granite and appear to be very 

 old intrusive masses. They are thoroughly granitoid in texture, 

 and consist of hornblende as the most abundant dark silicate, 

 of pale, green monoclinic pyroxene, and rare biotite. The 

 commonest feldspar is near labradorite, but orthoclase also 

 appears, and quartz is not entirely lacking. The description 

 of them as dikes is more likely to be true than to consider 

 them as basic schlieren or segregations in the granite magma 

 itself. 



The granite is cut by well-developed series of joints. The 

 master set runs at approximately the same course as the dikes, 

 and is thus parallel to the great structural features of this 

 portion of the Appalachians. The joints come into the tunnel 

 at angles of 15 to 20 degrees with the course. There is another 

 set nearly at right angles to the first. Still more rarely a set 

 crosses the axis of the tunnel nearly at right angles with its 



* J. F. Kemp, The Dikes of the Hudson River Highlands. Amer. Natural- 

 ist, Aug., 1888, 695-698. 



f The hearing of the tunnel in N. 56° 35' 14" E. referred to the true north. 

 The magnetic variation is 9° 40'. The dikes strike about fifteen to twenty 

 degrees nearer north. 



