224 TUNNELING 



the vertical platform, and the rock was drilled and blasted just 

 ahead of the shield. The progress of 4 ft. per day was made 

 in this material, of which there was about 65 ft. The rock 

 then became much more solid, with a roof that was self-sustain- 

 ing, and arrangements were made for removing the shield. 

 On Feb. 18 the work of removing the shield was begun, and 

 two days later everything was ready for the regular rock-tunnel 

 work in the heading, the shield having been taken apart and 

 removed in that time. 



At about the time that shield tunneling was being discon- 

 tinued at New York, it was being installed at Long Island. 

 An entire duplicate plant had been ordered for this side ; for, 

 although it had been originally intended to use one shield for 

 both headings, it was later deemed advisable to provide a shield 

 for each heading, so that there might be no delay, should soft 

 ground be met in both headings at the same time. In passing 

 through the soft seam at Ravenswood with the shield, no 

 especial difficulties were met. The material proved to be a 

 mass of soft-rock fragments, boulders and cinder-like stones im- 

 bedded in soft green chlorite. About a month was consumed 

 in passing through this seam, removing the shield, and prolong- 

 ing the cast-iron lining well into the rock on both sides of 

 the vein. With both tunnel headings now in rock, remarkably 

 rapid progress was made ; and as progress now had become of 

 great importance to the company, a liberal bonus, arranged on 

 a sliding scale, was given the foremen for work done over stated 

 amounts. Up to the time of the headings meeting, an average 

 progress of 69 ft. per week was made, while in rock, on both 

 the New York and Long Island sides. The record week of 

 the work was the one ending June 27, when at Ravenswood 

 95 ft. was driven, while on the New York side, the heading 

 was advanced 101 ft, making a total for the week of 196 ft. of 

 tunnel driven. Soon after the rock tunneling had been re- 

 sumed on the New York side, this heading reached Blackwell's 

 Island, and the troubles on this side were over. But at Ravens- 



