64 NEW YORK STATE MUSEUM 



ciates, which were at that time exposed to erosion by folding and 

 uplift. 



It is an interesting fact, therefore, that the Hudson River forma- 

 tion shows similar derivation from something which preceded it. 

 Rock like the Hudson River graywacke could not have been made 

 from such a series as the Pre-cambrian basement on which it now 

 rests, represented by the gneisses and granites of the Highlands. It 

 could, of course, have been made from the simpler metamorphosed 

 sediments such as might be assumed to represent portions- of Gren- 

 ville or portions of the Manhattan-Inwood series. This is a case, 

 therefore, in which petrographic interpretation of the character of 

 the material constituting a rock has a decided bearing on its possible 

 relation to other formations in the region. 



(2) Phyllites and slates. The simpler slates require no further 

 attention in this discussion. The most modified of the Hudson 

 River formation representatives in this district are the phyllites of 

 Peekskill hollow. This rock is recrystallized, sheared, somewhat 

 crumpled, and perhaps granulated also in certain parts. It has 

 minute mica flakes abundantly developed which give the phyllitic 

 habit to the rock. As a matter of fact, however, quartz is much 

 more abundant in the rock than the hand specimen would suggest and 

 it has essentially a quartz-mica composition. In no place, however, 

 is it coarse grained. This is of particular significance in view of 

 the fact that the Manhattan schist formation with which this phyllite 

 is sometimes correlated and which occurs at Peekskill only about a 

 mile distant is very strongly foliated and very coarsely and com- 

 pletely recrystallized. 



The phyllite carries many pyrite crystals or pseudomorphs after 

 pyrite and occasional black carbonaceous looking material but other 

 identifiable constituents are rare (see accompanying photomicro- 

 graphs of this type, plates 37, 38, 39 and 4o). 



The Younger Igneous Rocks 



Peekskill granite. The Peekskill granite is a small irregular- 

 shaped mass about 3 miles long and 2 miles wide with its longer 

 axis running N of E, lying east of Peekskill. It is well exposed in 

 two quarries, one on the Crompond Road about 4 miles east of 

 Peekskill and the other a little east of the road running south from 

 Mohegan lake. The latter occurrence is the site of a quarry. The 

 product is known in the trade as Mohegan granite. 



It has an igneous contact with the gneisses and schists, cutting 

 them irregularly and sending pegmatites out into them. None of its 



