Plate 17 



Photomicrograph of no. 479. Mahopac granite. Taken with crossed nicols, 

 magnification about 30 diameters. 



Taken to show a characteristic field of this t^pe. Note particularly 



a Large microcline grains 



b Numerous granular aggregates of quartz 



c Little biotite 



d Very small amount of accessories 



The rock as a whole is characterized by rather large feldspars, chiefly 

 microline with occasional very slight perthitic development. There is little 

 biotite and a very large amount of quartz which is distributed in grouped 

 aggregates interstitially, and in isolated grains in lesser amount, and also 

 in very minute amount intergrown micrographically with feldspar. 



