54 NEW YORK STATE MUSEUM 



In thin section it is seen to be a medium-grained granitoid rock 

 composed of orthoclase, acid plagioclase (albite and oligoclase), 

 quartz, biotite, epidote and zoisite, and in some specimens a blue soda 

 amphibole. The accessory minerals are apatite, garnet, muscovite, 

 titanite, rutile in the form of sagenite, and rarely allanite. 



The peculiar habit of the feldspar serves to distinguish this from 

 the other rocks of the region, especially in these two points: 



1 They are extremely poikilitic and contain countless small inclu- 

 sions of zoisite and mica which \-ary in size from a line dust to 

 clear, well-formed crystals. 



2 The feldspars show strong granulation of the edges of the crys- 

 tals (mortar structure). In some cases the entire crystal has been 

 broken up. Part of the quartz shows this effect, the rest of it with 

 the biotite and epidote, occurs undisturbed distributed through the 

 interstitial crushed feldspar material and along the crush lines. 



The foliated appearance of the rock is caused by the orientation 

 of the mica. 



The quartz which has not been crushed is of the vein type with 

 marked wavy extinction. The biotite is of a greenish brown color, 

 not strongly pleochroic. It is occasionally intergrown with mus- 

 covite and frequently with epidote. It occurs in two generations, 

 as inclusions in the feldspar and as a late product of crystallization. 

 The epidote or zoisite is variable in amount and character. It, like 

 the biotite, occurs in two generations — a line inclusion in the 

 feldspar, and a coarser grained associate of the interstitial biotite 

 and quartz. The garnet is in medium-sized grains, of a faintly 

 pinkish color, in rounded but poorly developed crystals. 



Some of these characters are shown well by the accompanying- 

 photomicrographs . 



No. 430-a (plate 13) is taken to show the prominence of the zoi- 

 site grains referred to in the general description. 



No. 332-0 (plate 14) shows a typical granulation effect. 



No. 16-^ shows a case where strong schistosity is developed in 

 addition to granulation (plate 16). 



No. 430-Zj (plate 15) is another variety with both garnet and 

 allanite. 



Mahopac Granite. The rock is a medium-grained, pinkish, gneiss- 

 oid granite, distinctly but not strongly foliated. It is made of color- 

 less quartz, slightly pink feldspar, and dark biotite. The biotite is in 

 rather fine bands, more abundant than in the Canada Hill granite. 



