474 



HELL-GATE, IMPROVEMENTS AT. 



angles to one another. The piers of rock were 

 about 15 feet square and 25 feet apart from cen- 

 ter to center. The roof of the cross-galleries, 

 which ran at right angles 

 to the lines of stratifica- 

 tion, was blasted down as 

 thin as it would be safe to 

 leave it. Considerable risk 

 was incurred in this part 

 of the work, from the dan- 

 ger of the rock's crumb- 

 ling, and from the uneven 

 and uncertain thickness of 

 the roof. The average 

 thickness was 18'8 feet, 

 and the minimum thick- 



a concretion of bowlders and shells formed 

 upon it. Had the excavations at any time 

 broken into a large seam, the mine would 

 have been flooded, and all the work and proba- 

 bly many lives would have been lost. Occa- 

 sionally small seams were met. One seam was 

 10 inches wide and 100 feet long ; another, from 

 1 to 4 inches wide and 400 feet long, extending 

 clear across the reef, carried 350 gallons of wa- 

 ter a minute. The latter was dealt with after 

 protecting the completed part of the work by 

 building across the gallery a door capable of 

 withstanding the pressure of the water. The 

 seams were all walled, as fast as they were 

 opened, with Portland cement. The total 

 length of the galleries was 21,670 feet. 



The galleries were excavated to depths vary- 

 ing with the uneven surface of the reef. The 

 roof was then drilled with holes for the recep- 

 tion of the explosive cartridges. The holes 

 were slanted upward at angles varying from 

 75 to 45, and were made from 8 to 10 feet 

 deep except where the existence of seams 

 open to the river made it impossible to obtain 

 the depth wanted and of sufficient capacity 



ness 10 feet. The exact thickness could not 

 be ascertained beforehand, for no soundings 

 could distinguish between the solid rock and 



THE CHARGES IN THE MINES. 



to receive a rigid two-and-a-half-inch cartridge 

 throughout their entire length. The holes were 

 charged with rack-a-rock as the principal ex- 

 plosive (see page 346). The mixing was done 

 in small batches on Great Mill Rock, in a lead- 

 lined trough, and the explosive was packed 

 at once into cartridge-cases 2J inches in di- 

 ameter and 24 inches long, made of copper 

 0-005 of an inch thick. To prevent the cor- 

 rosion of the copper by the chemical action of 

 the sulphureted water running through some 



