154 W. H. HOBBS — CHANNELS SURROUNDING MANHATTAN ISLAND 



Newberry had earlier (1878)* offered a partial theory in assuming the 

 East river to be the old course of the Housatonic, and the Harlem and 

 Spuyten Duyvil Creek tributary streams. Such an explanation, how- 

 ever, fails to cover the exact location of these courses, which was the point 

 aimed at by Dana. Professor Kemp in his paper on the Geology of Man- 

 hattan island f accepts Dana's view, but adds that the course of the 

 Harlem river east of the McCombs dam requires another explanation, 

 and suggests that as its course below that point corresponds very closely 

 to the direction of the glacial scratches, the river may have been directed 

 by the glaciation. In a brief note on the Blackwells Island tunnel % he 

 has extended the theory to include the eastern channels, which was 

 merely suggested by Dana, and has quoted Dr F. J. H. Merrill in support 

 of this view. Later Merrill and Kemp have each elsewhere published 

 the same explanation. § Gratacap, || in his recent pamphlet on the Geology 

 of New York, refers to the view of Stevens that the Hudson river is under- 

 laid by limestone, and adds that it is an extremely unlikely supposition. 

 In the recently published New York city folio Willis IT apparently ascribes 

 the direction of the lower course of the Hudson to the location of weaker 

 beds along its channel, though he adds that this explanation may be 

 modified by the existence of faults. He says : 



"It [the Hudson river] had had a complex history, dating back to the Schooley 

 plain (upon which it probably began its course across the highlands), involving 

 adjustment to belts of weak rock, such as dolomite and arkose, and possibly in- 

 cluding effects of that faulting which has been described as traversing the Newark 

 rocks and of which there is some evidence in the physiographic relations of Staten 

 island. All that early history of the river awaits elucidation." 



Regarding the area east]of the Hudson, Willis says : 



"It probably happened that originally the rivers of the area had other courses 

 indifferently across the dolomite, schist, and gneiss, but such is the manner of 

 growth of streams and valleys that, though they were once so situated, the larger 

 ones must have become rearranged in precise adjustment to the lines of weak 

 rock. . . . Adjustment of valleys to belts of easily eroded rocks is a condition 

 which is reached only through prolonged competition among growing river sys- 

 tems, and when it is so perfectly accomplished as it is east of the Hudson, it 

 indicates that the streams have been long at work carving this intricate mosaic of 

 rock masses." 



* J. S. Newberry : Geological history of New York island and harbor. Pop. Sci. Month., vol. xiii, 

 1878, pp. 641-660. 



t Trans. New York Acad. Sci., vol. vii, 1887, p. 61. 



J J. F. Kemp : The geological section of the East river at Seventieth street, New York. Trans. 

 New York Acad. Sci., vol. xiv, 1895, pp. 273-276. 



§ F. J. H. Merrill: The geology of Greater New York. Trans. New York Acad. Sci., vol. xvi, 

 1897, p. 371. 



J. F. Kemp: Trans. Am. Soc. Civ. Eng., vol. xxxix, 1898, pp. 79-80. 



|| L. P. Gratacap : Op. cit., p. 39. 



lj New York city folio, p. 17, first and fourth columns. 



