46 NEW YORK STATE MUSEUM 



c Contact and metamorphic products of complex origin 



d Deformation products (shear schists) 



e Igneous types 



/ Mixed types 



The chief representatives in these different classes need little 

 individual treatment because the mappable formations are discussed 

 in the section immediately following. But as a matter of petro- 

 graphic distinction, and as indicating the petrographic range of the 

 district, it may be useful to make a preliminary list, noting separately 

 the varietal types of each genetic class. 



a Sedimentary and organic rocks (little metamorphosed). This 

 class includes a large development of quartzites, represented by the 

 Poughquag formation, a larger development of limestones, repre- 

 sented by the Wappinger formation, and a still larger development 

 of slates and graywackes and more rarely of phyllites, represented 

 by the Hudson River formation. All these are somewhat affected 

 by metamorphism which has reorganized the original material more 

 or less and developed new structural quality. This is least apparent 

 in the quartzites and most prominent in the ]:)hyllites. Some of the 

 quartzites are sandstones, but in places of greater dynamic disti.r- 

 bance there is much tendency to recrystallization, the quality depend- 

 ing upon the original make-up of the rock and the dynamic history 

 of the particular spot from which it was derived. T;he slates are 

 much more variable and range from simple fine black slaty shales 

 to very siliceous and granular beds in which the material has been 

 considerably reorganized, but which have no slaty cleavage. The 

 slates are the most variable and show the dynamic influence more 

 than the other types in this class. 



The phyllites are more completely modified from their original 

 condition than any of the other rocks of this class. They represent 

 metamorphosed slates and are found only in the downfaulted block 

 bordering the west side of Peekskill Hollow creek valley and in 

 the continuation of the same block on the west side of the Hudson 

 at Tompkins cove. The most completely recrystallized limestones 

 and quartzites are associated with the phyllites in this valley. A 

 great variety of sedimentary representatives can be secured from 

 the series. They are all of undoubted Cambro-'Ordovician age. 

 Simpler types than these are not known since no simple unmodified 

 sediments except those of glacial and postglacial age occur within 

 the boundaries of the West Point quadrangle. 



