GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 49 



to the delimitation of formations. These characteristics are dis- 

 cussed below. The units that deserve discussion on this basis of 

 unity of origin are described in some detail in the pages that follow. 



The Granville series. The Grenville beds are the dominant type 

 in a belt about 2 miles wide along the Hudson river from Fort Mont- 

 gomery to Cold Spring. There are good exposures along the New 

 York Central Railroad from Garrison to Manitou (Highland station) 

 and along the West Shore Railroad north and west of Fort Mont- 

 gomery, along the Garrison cut of the Catskill aqueduct, and on the 

 state road between Nelson Corners and McKeel Corners. 



The series is made up primarily of calcareous, micaceous and 

 quartzitic beds which weather easily. Consequently this formation 

 tends to form depressions, except where granitic intrusions have 

 hardened it. 



The Grenville is the oldest formation in the Highlands and there- 

 fore has suffered all the dynamic disturbances of the district, and 

 is cut by all the intrusives. As it was initially a series of impure 

 limestones, shales and sandstones the resulting product is of very 

 complex composition. It would be impracticable to discuss all the 

 varieties of metamorphic rocks produced, but the major processes 

 involved are treated elsewhere in this bulletin and some of the more 

 typical rock representatives are described below. 



In the outcrop the formation commonly shows as a series of banded, 

 crumpled and distorted rocks which weather characteristically to a 

 mottled black or a rusty brown color. The decomposing mica 

 (phlogopite) adds streaks and patches of a peculiar greenish yellow 

 so that the whole complex has an appearance markedly different 

 from that of any other formation of the quadrangle. The massive 

 limestones weather white, with the more resistant silicates standing 

 out in yellow to brown or greenish lumps above the surface. The 

 color of the weathered surface, and the graphite, serpentine or 

 tremolite always present are sufficient field criteria for determination 

 of this series. 



The disturbed condition of the beds along the Hudson river is 

 probably a local deformation, as the Grenville seen along Sand 

 Spring brook and Round hill is rather massive and blocky. 



Types of Grenville. Some of the characteristic types of the 

 Grenville are described more fully below. 

 Type a. Diopside-quartz rock. 



This is a dense, hard, light-colored, greenish gray rock. It is 

 a rather fine-grained aggregate of greenish pyroxene crystals in a 



