GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK I35 



It is true of course, that there is an elaborate deformation of the 

 members south of the Highlands and that the principal deformation 

 lines of different periods are parallel so that the later folding and 

 faulting follow the general structure of the earlier periods. This 

 tends to confuse the situation very much and, in places where a great 

 deal of shearing took place, one might expect to find such a general 

 parallelism of structure that most contacts would appear to be con- 

 formable. But one can not believe that the minor internal structure 

 would be as consistent as it is on that hypothesis. 



If one were to assume a later granite invasion involving the basal 

 member of the Cambro-Ordovician series so as to form an injection 

 gneiss out of the Poughquag quartzite, then one might conclude that 

 the Fordham gneiss of New York City, with its apparent conformity 

 with the Inwood limestone, is simply the injected Poughquag and not 

 a Precambrian type at all. A comparison, however, of the gneisses 

 of the Highlands and of New York City, especially those portions 

 referred to as the Grenville in the Highlands, leaves no doubt but 

 that the banded gneisses of New York City are precisely the same 

 as the injection gneisses of the Highlands and that there is no room 

 for serious consideration of the Poughquag injection theory. In 

 other words, the gneisses of the two areas have similar history and 

 there is no doubt that in major character they are essentially alike 

 and should be regarded as identical. 



This leaves the problem where it was before, with this structural 

 dilemma. If the Hudson River- Wappinger and the Manhattan- 

 Inwood series are the same, they must be unconformable with the 

 gneisses in all the different occurrences and in that case the apparent 

 conformity of the southerly series must be a deformation effect. 

 But if, as appears to be the case, the southerly series is conformable, 

 whereas the northerly one is strictly unconformable, then the two 

 series can not be correlated and there must be a very great differ- 

 ence in age. 



If one may assume for the moment that the southerly series is 

 older and that it represents some of the most ancient sediments 

 encountered in the district, then it would not be surprising to find 

 that the basal members which were probably quartzose f ragmentals in 

 composition had yielded much more readily than the limestones to 

 impregnation, injection and absorption by the invading granites. 

 These lower members thus appear to have been completely trans- 

 formed into gneisses and actually represent a part of the injected 

 Grenville series. The}'- contain numerous smaller beds of limestones 

 which clearly indicate that in part the series is sedimentary, but they 



