W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



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of the channel, the latter the greatest work of the 

 kind ever attempted. All of these obstructions 

 are of gneiss, and similar rock is to be found ex- 

 posed on the south shore of Wards island on either 

 side of the small bay which indents that shore. 

 Together these exposures of gneiss make almost a 

 complete section across the river. 



East River Gas Company's tunnel (Blachvells Island 

 tunnel). — There is considerable literature treating 

 of the construction of this tunnel. A shaft was 

 sunk on the New York side to a depth of 139 feet, 

 from which point a drift was carried on a descend- 

 ing grade of 0.5 to 100 beneath both channels of 

 the East river and Blackwells island to a vertical 

 ~ shaft on the Ravenswood, Long Island, side. The 

 c roof grades of the tunnel are on the New York side 

 t 104 feet below the level of mean low water and on 

 ~ the Ravenswood side 116.6 feet below the same 

 | datum plane. The writer is indebted to Mr J. 

 ^ Vipond Davies, one of the engineers, in charge of 

 c the construction, for much information of scien- 

 g tific value obtained during the construction of this 

 I tunnel. Mr Davies states that in sinking drills 

 & they were in no place able to penetrate more than 

 | 5 or 6 feet into what appeared to be hard gravel, 

 sand, and a great quantity of very large boulders 

 lying on the bed of the river. It is probable that 

 the strong tides of Hell Gate have scoured out all 

 the finer material, leaving only such as was too 

 large to be transported. It was thus found im- 

 possible by means of borings to determine in this 

 vicinity the depth of bed rock below the river sur- 

 face, but it was considered by the engineers in 

 their work that the line of the bottom of the river 

 was practically the surface of rock. The roof grade 

 of the tunnel is approximately 32 feet below the 

 deepest point of the river bed in the west channel 

 and about 65 feet in the east channel. The accom- 

 panying profile (figure 13) has been slightly modi- 

 fied from one published by Aims in the Journal of 

 the Association of Engineering Societies. The section is of special interest 

 to geologists, particularly as regards the composition and also the struct- 



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