GEOLOGY OF THE WEST POENT QUADRANGLE, NEW YORK 55 



but much less so than in the Reservoir granite. The cleavage is 

 therefore not as distinct as in the Reservoir granite. 



The Mahopac granite shows in thin section a combination of the 

 characteristics of the Reservoir and Storm King granites. It has 

 the marked perthitic growth which is the most constant feature of 

 the Storm King, and also the intensel}^ pleochroic biotite. The 

 granulated edges of the feldspar, the later simultaneous introduction 

 or development of the quartz and biotite, and the accessory mus- 

 covite show resemblance to the Reservoir granite. These characters 

 are considered critical and the rock is therefore judged to be essen- 

 tially a facies of the Reservoir type. It may even be that this is a 

 more fundamental or less abnormal type than the Reservoir, but in 

 any case it seems to belong to the same invasion. The peculiar 

 zoisite inclusions of the Reservoir granite are lacking, possibly 

 because of the lower lime content of the country rock absorbed by 

 the Mahopac magma. This is also shown by the feldspars which 

 are chiefly perthite and microcline. 



Megascopically the granite appears to be an intermediate type. It 

 is a light pink gneissoid granite which looks very much like the 

 Yonkers gneiss of the Tarrytown quadrangle. It is a more distinct 

 field unit than the Reservoir and not so distinct as the Storm King 

 granite. 



This granite is therefore petrographically a transition variety 

 between the Reservoir and Storm King granites rather than a clear- 

 cut independent type, and it is therefore difhcult to locate accurately. 

 The country southeast of Peekskill Hollow creek, bounded on the 

 south by Peekskill creek, the Peekskill granite, Osceola lake, and 

 Lake Mahopac, has this granite as its chief intrusive. The best 

 exposures are along the road running from Kent clififs to Mahopac 

 mines, especially on the fault scarp north of Mahopac mines, and 

 also in the hill north of Peekskill in contact with the Poughquag 

 quartzite. 



The Peekskill Hollow boundary is a sharp one, as here there is 

 an unconformity and the Cambrian quartzite is laid down on the 

 old eroded granite surface. The contact with the Reservoir granite 

 is not at all clear. The best evidence as to the character of the con- 

 tact is shown on the Kent clififs-Mahopac mines road at a point just 

 off the edge of the quadrangle. Here a pink granita dike with 

 indefinite boundaries lies in contact with the gray, granite, and little 

 wavering stringers of pink penetrate the gray blending Avith it. The 

 rocks are evidently of approximately the same age. The res6m- 



