GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 1 33 



thorough metamorphism, that some of these sandstone beds would 

 maintain their identity and be developed as graywackes or quartz- 

 schists ; but as far as the writer is aware, there is nothing approach- 

 ing such a type on the south side. There are, to be sure, finer and 

 coarser crystallization effects, some of which are very strikingly 

 different, but the granular structural habit of sandstone or quartzite 

 or graywacke is distinctly absent. To this degree, therefore, the two 

 differ in a suggestive way in original quality as well as in meta- 

 morphism. 



4 It appears very plainly in the series of the south side, especially 

 in the Manhattan schist, that igneous injection, impregnation and 

 intrusion is a strikingly prominent and widely distributed feature. 

 Such an effect does not appear in the series of the north side within 

 the district under observation or along the Hudson river. Such a 

 difference might indicate different age, one older and one younger 

 than a certain igneous invasion, but of course, it is possible that 

 igneous invasion coming within reach of the members to the south 

 did not reach those on the north side. Some observers have gone 

 so far as to suggest that the difference in amount of metamorphism 

 may be due not so much to the difference of age as to influence of the 

 igneous invasion which caused extensive recrystallization of those 

 portions south of the Highlands coming within its reach. It is an 

 explanation well worth serious consideration and is along the line 

 of argument recently put forward so ably by Barrell. All that 

 one can say at this stage is that no sufficiently capable and new 

 igneous member is known to which to credit this amount of change. 



5 A striking difference of relation to the underlying gneiss series 

 is observable in a few places. This structural discrepancy is the 

 most insurmountable of the objections to the identity of these two 

 series. 



It is perfectly plain that the structures of the Cambro-Ordovician 

 series and of the gneisses as observed on the north margin of the 

 Highlands are entirely discordant. Undoubtedly after the gneisses 

 were developed a very long period of erosion ensued and the basal 

 quartzites represented by the Poughquag were laid down on that 

 eroded floor. Considering the history of the gneisses as outlined 

 elsewhere in this report, it is not surprising that their structures 

 should stand nearly vertical and that the quartzite bedding should lie 

 almost directly across this structure, as it does at certain places near 

 the north margin of this quadrangle. The granites of Storm King 

 and Breakneck do not break through into the overlying series. 



