132 NEW YORK STATE MUSEUM 



6 It is a very disconcerting structural fact that the crystalHne 

 series of the south stops abruptly on the Highlands mai^gin, and 

 although remnants of the Cambro-Ordovician series are found within 

 a mile to the north, nowhere does one find clear evidence of an over- 

 lapping of these two series such as would be expected if they are of 

 distinctly different age and separated by a long erosion interval. 

 This would be understood readily enough if there was clear-cut evi- 

 dence of great thrust faulting and very large displacements, but it 

 is a difficult thing to explain considering the somewhat .irregular 

 outlines that are represented. 



Principal points of dissimilarity and criteria indicating difference 

 of age. The criteria given above would seem sufficient to satisfy 

 almost anyone of the identity of the two formations. It is therefore 

 rather surprising to find so many points of discrepancy and to 

 find also that some of the objections are so difficult to explain away. 

 The chief points of this character may be enumerated as follows : 



1 There is a very striking difference in general physical appear- 

 ance and petrographic habit of the different members of the two 

 series, especially the upper member in each case. If one confines 

 the discussion to representatives found within the quadrangle, the 

 Hudson River-Wappinger-Poughquag series is strikingly less 

 metamorphosed than the Manhattan-Inwood-Lowerre series of the 

 south. Even if one considers fully the somewhat greater meta- 

 morphism of the down-faulted block of Peekskill hollow, the dis- 

 crepancy still remains very striking, and no student of petrography 

 would fail to discriminate between the phyllites of this locality and 

 the Manhattan schists of Peekskill only a mile away. 



2 There is a difference in thickness of certain of the members, 

 especially an entire lack of quartzite in most occurrences of the 

 Manhattan-Inwood series on the south side, as compared with the 

 600-foot thickness of the Poughquag of the other series on the north 

 side. This is not an impossible condition of course, but it is rather 

 surprising if the two are identical, especially in view of the fact that 

 the 600-foot bed of quartzite still continues in the down-faulted 

 block of Peekskill valley. Such a strongly developed member ought 

 to occur much more extensively than it does south of the Highlands 

 if the two series are the same. 



3 The entire lack of bedding in the Manhattan member of the 

 south side is in striking contrast with the rather frequently encoun- 

 tered strongly developed sandstone and graywacke beds of the Plud- 

 son River member of the north side. One would expect, even with 



