180 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



two directions from an intermediate point, but the floor is marked by 

 sudden changes of level, particularly, however, where the reefs of gneiss 

 cross it at McCombs dam and at Third avenue, thus connecting ridges 

 of gneiss on the north with their extensions on the south. 



EAST RIVER 



Under East river limestone has been found at but three localities — 

 under the eastern channel at Blackwells island, in the western channel 

 on the Forty-second Street section, and in one of the drill holes beneath 

 the Manhattan pier of East River bridge number 3 .* The limestone east 

 of Blackwells island is probably enclosed between parallel fault walls, 

 and would appear to have been dropped down along them. These fault 

 walls are, however, not parallel to the gorge of the river, and the lime- 

 stone belt is, with much probability, soon cut off by faults which follow 

 the direction of the gorge, f The multiplied observations of gneiss, and 

 gneiss only, beneath bridge piers and in tunnels in many sections of East 

 river, the numerous reefs of the same rock in midchannel, and, more than 

 all, the nearly complete section of gneiss across the river at the Battery, 

 at Forty-second street, and the nearly complete section at Hell Gate — 

 these multiplied observations leave little room for doubt that the rock 

 bed of this river is mainly of the harder rock (see plate 35). 



HUDSON RIVER 



Regarding the bed rock beneath North river, comparatively little is 

 known. The great depth of this river in this vicinity and its thick floor 

 of drift and silt make it unlikely that very much will be learned about 

 its rock bed in the near future. There is no reason to suppose that lime" 

 stone may not underlie certain portions of it, though there is little reason 

 to assume that it does. The rock of the eastern shore is largely gneiss 

 and that of the western shore to the southward of the Palisade border 

 (south of the principal bend at Weehawken) is also either gneiss or 

 serpentine (see plate 35). The origin of the North River channel is 

 sufficiently accounted for, however, by its position along the contact of 

 the Newark beds with the crystallines. That this border is a fault border 

 seems to be abundantly proven, not only by its markedly rectilinear 

 extension, by the great scarp of basalt, and by the inferior position of 

 the newer terrane as revealed by surface development,^ but especially 

 by the new borings along the line of the proposed tunnels of the Penn- 

 sylvania Railroad Company (see figure 22 and page 176). 



♦Limestone is mentioned either in ledges or blocks at Corlears hook by some of the earlier 

 writers, but from recent borings it seems certain that boulders are referred to. 

 fSee ante, p. 167. 

 JCf. Bull. Geol. Soc. Am., vol. 13, 1902, p. 143. 



