DETAILED STUDY OF SECTIONS 169 



hampered by the oozing into the tunnel of water and decomposed rock 

 in the form of a thick mud. In the construction of the new New York 

 aqueduct the same difficulty was encountered at the boundaries of the 

 limestone (see figure 14). 



Abandoned East River tunnel. — The only work that was done on this 

 tunnel was the excavation of a shaft in section 10 feet by 8 feet and 80 

 feet in depth to the grade of the proposed tunnel. The shaft was sunk 

 10 feet in the earth to the underlying gneiss, into which it was carried 70 

 feet. There occurred a very serious explosion when this stage of the 

 work had been reached, much damage to property being caused and a 

 number of persons seriously injured. The conditions are now favorable 

 to an early resumption of the work and the ultimate completion of the 

 tunnel. 



Man-o'-war reef. — The Man-o'-war reef forms a continuation of Black- 

 wells island to the south westward and has the form of an elongated prow. 

 Considerable work has been done by the engineers of the United States 

 army in removing the upper portions of this reef. Through the courtesy 

 of Captain Edgar Jadwin, of the Corps of Engineers, United States Army, 

 the writer secured a number of specimens from this reef. These speci- 

 mens represent respectively a granitic type of gneiss, a hornblende gneiss 

 of undoubtedly igneous origin, and a dense basalt like that so character- 

 istic of the Newark areas of the Atlantic border. It seems likely that this 

 latter rock may be from a portion of a narrow dike within the series of 

 crystallines. 



Section on center line of Forty- second street produced. — Through the courtesy 

 of Mr August Belmont and his chief engineer, Mr Allan A. Robbins, the 

 data from the series of core drillings made in the bottom of the East 

 river have been placed at my disposal. The revelations concerning the 

 topography of the river bottom and of the nature and depth of the bed 

 rock beneath are set forth in figure 15, which has been drawn from the 

 blue prints furnished by Mr Robbins. The line of the section passes 

 near the Man-o'-War reef, and those cores which were obtained near the 

 banks of the river and near the Man-o'-War reef are all hard gneiss. 

 Near the middle of the western channel drills (number 2 A) brought up 

 a core of compact dolomite. All others, save those already described 

 as from the vicinity of the reef and shores, were of decomposed rock 

 material, which in some instances the drillers were unable to separate 

 from the sand and silt immediately overlying. The samples from these 

 drillings have, however, been placed at the writer's disposal and sub- 

 jected to a careful microscopic examination. The material is throughout 

 partially disintegrated gneiss, consisting of feldspar, quartz, and biotite, 

 with smaller amounts of muscovite, magnetite, and garnet. Consider- 



