GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 29 



11 Hudson River slates and phyllites. (Conformaljle above 



the Wappinger) 

 E Later intrusives belonging to the Cortlandt scries 



12 Norites and dioritic rocks of the principal Cortlandt area, 



13 Peekskill granite and Mohegan granite (of Cortlandt series 



age). 



PETROGRAPHIC GEOLOGY — ROCK FORMATIONS 

 This discussion is concerned chiefly with the following three 

 items: (i) petrogenesis, (2) products of the geologic processes 

 represented by groups of larger petrographic significance, (3) map- 

 pable formations and their petrography. 



Petrogenesis 



It has already been indicated that a confusion of types is char- 

 acteristic of the Highlands district. The whole range of rock types 

 capable of being developed from original complex sediments and 

 associated igneous intrusives with both an older and subsequent 

 dynamic history is shown here. And the complexity was not devel- 

 oped to its extreme in any one cycle. The history is bound up 

 with successive igneous invasions, some of them very acid and 

 others very basic so that an extreme range of composition is possible. 



Dominant types. The task of resolving this confusion into a 

 group of dominant types at first looks hopeless. But a critical 

 inspection with a microscope reveals the fact that there is, in most 

 cases, a recurrence of a peculiar habit or a critical character which, 

 when one has the clue, is recognizable. This suggests that there are 

 certain traceable relations which have a broad application in the 

 matter of relation and identity of formations. This is readily 

 recognized in the simpler Hudson River-Wappinger-Poughquag 

 series of Cambro-Ordovician age. It is less easily followed and 

 one's identifications become less certain in those formations of more 

 complicated history, such as the Manhattan-Inwood-Lowerre series. 

 The difficulty is so pronounced, in this case, that it is still a question 

 what the true age of these members may be. 



The difficulty of vv^orking with the Grenville is still greater because 

 of the excessive modification that this formation has suffered. The 

 limestone is the only member of the Grenville that maintains even 

 moderately well some decisive evidence of its former make-up, per- 

 haps because it is less susceptible to impregnation by igneous matters. 

 At any rate at many places the limestone can be readily found, but 



