GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 53 



Reservoir granite. ' The Reservoir granite is one of the most 

 widely distributed granites of the Highlands. It is this rock together 

 with the Canada Hill type which has done most of the granitization 

 of the Grenville. It is commonly found penetrating and invading 

 the older rocks, and is seldom found perfectly free from traces of 

 the gneiss. The best exposure is at the north end of Boyd Corners 

 reservoir where the granite can be seen to be an igneous rock 

 intruding the gneisses. A smaller exposure showing the rock type 

 fairly well is at the bridge across Conopus creek, east of Dennytown. 



It is a rather coarse-grained, gray, gneissoid granite. At first sight 

 it appears to be a true gneissoid granite, showing marked flowage 

 effects, and cut by numerous pegmatite and quartz stringers. The 

 pegmatite is much crumpled and distorted, but the quartz cuts 

 straight through all the other structures. On further examination 

 large included blocks of dark, banded gneiss are seen. They are 

 crumpled and distorted and contain within their margins all the 

 observed pegmatite. They are in reality in all stages of assimilation 

 and grade from almost angular pieces with distinct outlines to pieces 

 so completely dissolved in and penetrated by the granite that they 

 would be indistinguishable from it were it not for the pegmatites 

 which remain unaffected. In the partly dissolved pieces there is a 

 perfect transition from gneissoid granite to gneiss and the flow 

 structure of the granite is in every case parallel to the original 

 crumplings in the gneiss. The granite is therefore a syntectic. The 

 structure is not its own, but is imposed by the invaded gneiss. (See 

 discussion of the processes of petrogenesis, especially syntexis, 

 impregnation, and the developnient of internal structural habit, page 

 29.) 



The typical Reservoir granite as exposed at Boyd Corners reser- 

 voir has a rather coarse texture of white feldspar striated and 

 unstriated, and a very little quartz, and is strongly marked with 

 oriented streaks and patches of deep-brown mica. It is spotted with 

 the Same small purplish to dull-red garnet as the Canada Hill granite. 

 It has a tendency to break roughly along the mica planes, giving a 

 mottled appearance to the rock. It varies from this to a finer 

 grained, more definitely banded rock, produced by shearing. There 

 is, even at the reservoir, a slight tendency to the development of 

 the pink feldspar which becomes in the Mahopac granite a promi- 

 nent constituent of the rock. It differs from the Canada Hill granite 

 in its higher biotite content, and in the greater continuity of the 

 mica band?. But the mica content is not constant and some speci- 

 mens are difficult to distinguish from the Canada Hill. 



