DETAILED STUDY OF SECTIONS 161 



5 and 6). Mr Gay was in all the caissons himself, and thus was able to 

 examine the rock personally. This reef crossing the Harlem is clearly 

 the continuation of the Ford ham Heights ridge, which was also once ex- 

 tended southward by a range of gneiss outcrops now largely removed.* 

 Soundings to rock east of Central bridge. — The data used in constructing 

 this profile (see figure 6) were furnished by the department of public 

 parks to Professor I. C. Russell, who, in his valuable paper on the Geology 



2 !2£l 220' *■***&) 



Figuee 5.— Section across the Harlem River. 

 Along the line of McCombs Dam (Central) bridge. 



of Hudson county, New Jersey ,t gives the depth of the underlying gneiss 

 at all points in the section. This section not only covers the bottom of 

 the river, but extends some 2,000 feet north of it into the lowland of 

 Crom wells creek. It is interesting in showing how faintly the present 

 channel is indicated in the configuration of the surface of the bed rock. 

 New York Central Railroad bridge across Croimoells creek. — This bridge, as 

 will be seen from the map, is almost a continuation to the eastward of the 



Figure 6. —Profile of Rock beneath Harlem River. 

 On line 400 feet east of Central bridge. 



McCombs Dam bridge, and data which it furnishes should therefore be 

 considered in relation to those of the last-mentioned section. The bridge 

 rests on piles which go down 120 feet, but do not reach to bed rock. J It 

 thus appears that the reef of gneiss which comes so near to the surface 

 of the Harlem river at McCombs dam plunges off to great depths but a 

 short distance farther to the east, the line of this declivity corresponding 

 in direction with the extension of Crom wells creek northward or parallel 



*See Viele's map and a paper by Ries in Transactions of the New York Academy of Science, 

 vol. x, 1891, pp. 113-114. 



f Lou. cit., p. 75. 



X Information furnished by Mr F. L. Chase, engineer in charge of bridges, New York Central and 

 Hudson River Railroad Company. 



