GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 1 37 



4 or 5 feet thick and perhaps lo or 15 feet above the base of the 

 limestone. In the other case, the gneiss is farther removed from 

 the base and is of greater thickness. In both cases, this interbedded 

 gneiss shows injection effects and structure exactly similar to the 

 gneisses which lie below the Inwood. No way of explaining these 

 facts seems to be satisfying except that there is an actual transition 

 of the sediments and we are inclined, therefore, to accept the 

 extreme interpretation of the Manhattan-Inwood series and cor- 

 relate it with the Grenville. 



7 An additional item in the correlation of these sedimentary mem- 

 bers has been gathered from a detailed petrographic study of the 

 quality and source of their primary constituents. When this is 

 applied to the Hudson River formation on the north side of the 

 Highlands, especially in its coarser members, it is evident that the 

 material was originally derived from still older somewhat meta- 

 morphosed sediments. Fragments of sandstones, slates, graywackes, 

 phyllites, crystalline limestones and dolomites are present, and the 

 quality of the material supports the hypothesis that the fragments 

 were supplied by the disintegration of rocks of similar kind rather 

 than directly from a granitic or gneissic type of country rock. It 

 therefore appears that there must have been a series of somewhat 

 metamorphosed sediments within reach of weathering agents at the 

 time the Hudson River formation was being deposited. Remnants of 

 such an earlier series ought to appear as an older more meta- 

 morphosed and more obscure series. Such a requirement is fur- 

 nished by the Manhattan-Inwood-Lowerre series better than by any 

 other formation yet proposed as a supply ground for the materials 

 of the Hudson River formation. If this was not the supply, some- 

 thing else of like type must have been, and it seems much like beg- 

 ging the question to pass over a known thing for something of like 

 character that is purely imaginary. It is realized, of course, that 

 these arguments are not absolutely conclusive, for the fragments 

 found in the Hudson River do not tally exactly with the present 

 quality of the Manhattan-Inwood series ; but, if one assumes that the 

 higher and less profoundly metamorphosed portions of the series 

 was eroded to furnish this supply, the materials ought to be similar 

 to these fragments now found in the Hudson River beds. In 

 short, they should not be expected to have as elaborate meta- 

 morphism as those portions of the Manhattan which were buried 

 deeper and had opportunity for greater and longer continued modifi- 

 cation. 



