1210 TUNNELING 



rock to within 20 ft. of this point had been the regular hard 

 New York gneiss, with a dip towards Long Island of 10 from 

 the vertical, and a strike north and south at right angles to the 

 direction of the tunnel. Here it gradually began to soften, 

 becoming more and more micaceous until when about 20 ft. 

 beyond the water-bearing fissure the rock suddenly terminated, 

 running into a vein of soft material with the same dip and 

 strike as that of the rock. 



This new material proved to be a vein, principally of decom- 

 posed feldspar, gray in color, crumbling easily, and with no 

 perceptible grit. It still preserved a rock structure, and was 

 perfectly dry when undisturbed. But its exposed surfaces 

 were quickly acted upon by water, which it would absorb and 

 then wash away quite rapidly. The water-bearing fissure and 

 this soft vein were connected ; more water was also met at the 

 junction of the rock and the soft material, and later experience 

 proved that in passing through these soft veins water was 

 always to be found next to the rock a sort of water-course 

 on both sides of the soft vein. Had it not been for encounter- 

 ing this water, the tunnel might have been carried through the 

 soft material without employing compressed air, though the 

 prudence of attempting this might be questioned, for nothing 

 more insures the safety of both the men and the work than 

 compressed air in sub-aqueous tunneling. 



The finding of this soft material, so unexpected, was quite 

 a set-back to all concerned. However, it was decided to drive 

 a small timbered drift about 4 ft. wide by 6 ft. high to investi- 

 gate the ground ahead, and find how much of this material was 

 to be penetrated before solid rock was again met. This drift 

 was started and driven in for about 6 ft. Meanwhile a most 

 destructive action was going on between the water and the soft 

 material. The water running along the face of the rock had 

 washed out a cavity overhead in the soft ground. The walls 

 of this cavity were gradually breaking away, and the clay-like 

 substance falling down would close the outlet of the water into 



