156 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



with the general strike of the rocks, as it does also to the known occur- 

 rence of limestone at the bottom of the Harlem gorge. To the writer it 

 has seemed that there is a general tendency among geologists to over- 

 estimate the importance of limestone belts in conditioning the location 

 of valleys and low areas in the topography. The exceptional oppor- 

 tunity for testing this theory in the New York city area by reason of the 

 numerous artificial sections across the river channels presented itself to 

 him in the season of 1901, when considerable study was made of the 

 area. It is perhaps true that nowhere in the world could the general 

 problem be investigated under more favorable conditions. The inquiry 

 begun nearly four years ago has been prosecuted since as opportunity 

 has offered, until data gathered are about as comprehensive as they can 

 well be made now, though they are likely to be somewhat augmented in 

 the future. 



As already stated, the generally accepted hypothesis assumes that the 

 channels of the Hudson and East rivers and the sinuous course of the 

 Spuyten Duyvil and the Harlem, are alike underlaid by limestone, even 

 where gneiss forms the wall rock on either side. This latter difficulty 

 Merrill has sought to explain for the Spuyten Duyvil locality by assum- 

 ing a sudden and steep pitch to the southward of the northern gneiss 

 mass (Fordham gneiss) and the appearance of gneiss belonging to a 

 higher horizon superior to the limestone (Hudson schist) on the south of 

 the river. He says : 



" High cliffs of Fordham gneiss border the north shore of Spuyten Duyvil creek. 

 The course of the creek is at any given point approximately parallel to the strike 

 of the gneiss at that point, and the latter is everywhere seen to dip toward the 

 creek. This definite relation between the windings of the creek and the variations 

 in the strike of gneiss is due to the fact that the creek occupies the position of dolo- 

 mite beds which formerly overlay the gneiss, but which have been almost entirely 

 removed from view by solution and erosion. Several small outcrops of dolomite 

 can still be seen at low water on the south shore of the creek dipping to the south 

 along the cliffs of Hudson schist which line that shore" (loc. cit., p. 4, column 4). 



My own observations do not confirm the above statement in all par- 

 ticulars, especially the uniform steep southerly pitch of the beds. 



In the other localities where gneiss appears, forming both walls of the 

 river, an exceedingly sharp longitudinal syncline must be postulated in 

 order to bring in the limestone. 



DETAILED STUDY OF SECTIONS 



Order of presentation of results. — The data which have been secured 

 bearing directly on the configuration of the surface of rock beneath the 

 New York water fronts, refer both to the depth and to the character of 



