GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 1 29 



New York City. An intrusive granite is described in United States 

 Geological Survey folio 162 in terms that correspond very closely 

 to the description of the Resei*voir granites of this bulletin. The 

 Wissahickon mica gneiss is doubtless the equivalent of the Man- 

 hattan schist of New York and the Octoraro schist of the Phila- 

 delphia region corresponds to the Hudson River slates and phyllites. 

 In that district as in the New York there is an obscure structural 

 relation which complicates the task of separating the chief mica 

 schist from the younger slate-phyllite formation. The case is 

 well stated by Doctor Bascom in United States Geological Survey 

 Folio 162. 



The problem of possible identity of the Manhattan-Inv/ood- 

 Lowerre series and the Hudson River- Wappinger-Poughquag. 

 This is the second big correlation problem presented by this area. 

 These two series of rocks (the question of whether they are different 

 is not raised at this point) are typically developed at the two diag- 

 onally opposite corners of the quadrangle. The Hudson River- 

 Wappinger-Poughquag series constitutes a very small border along 

 the north margin for a short distance and the Manhattan-Inwood 

 Lowerre series has a much larger development in the southeast quar- 

 ter of the quadrangle. Only one intermediate occurrence is found 

 in this quadrangle and that is the phyllite-limestone-quartzite 

 series which forms the dov^n- faulted block stretching for many miles 

 along Peekskill hollow. 



This intermediate strip has often been referred to in discussions of 

 this problem, and has been assumed by some to be the connecting 

 link between the slightly metamorphosed series of the north margin 

 and the very strongly metamorphosed series of the south side. 

 These members of the intermediate belt do, as a matter of fact, show 

 a somewhat more crystalline habit than the typical representatives 

 of the north margin but a much less metamorphosed habit than the 

 typical southern series. It is a rather simple matter to assume, there- 

 fore, that they form the true connecting link and that the very great 

 petrographic and structural differences of the two series are the 

 effects of a greater and greater metamorphism toward the south". The 

 striking thing, of course, is the fact that this down-faulted block, 

 although very near in position to representatives of the Manhattan- 

 Inwood series, maintains its similarity to the Hudson River- Wap- 

 pinger-Poughquag series so clearly as to leave no doubt of their 

 identical age, and this in spite of the fact that they are many miles 

 removed from each other in outcrop. On the contrary there is a 



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