PETROGRAPHICAL PROVINCE OF ESSEX COUNTY 293 



creamy white, weathering to brown, very fine-grained and with a 

 dull luster, and a tendency to schistosity, which largely accounts 

 for the earlier view that this was a sandstone. A few glistening 

 white phenocrysts of anorthoclase are visible. 



In thin section the phenocrysts show the characters described 

 by Rosenbusch and Sears. The groundmass is trachytic with 

 pronounced flow-structure, and is composed largely of small 

 alkali-feldspar laths, these being generally clear. The interstitial 

 matter is clear and colorless with low refractive index, partly 

 isotropic and partly feebly doubly refracting. Some of it seems 

 to be glass and some kaolinized feldspar. There is considerable 

 "dust" and many small black and brown specks, the remains of 

 former ferro-magnesian minerals, which, however, never were 

 present in a large amount. Very few traces of these remain, 

 only rare, small biotite flakes being seen. A little quartz is pres- 

 ent, but is rare. 



Two analyses of this keratophyr are given, one by myself 

 and the other by Dr. Chatard, of the United States Geological 

 Survey, for Mr. Sears. They resemble each other very well, 

 though mine shows a little more silica. It will be noticed that 

 they are not markedly different from the rhyolite, though in this 

 lime is higher. 



Si0 2 / - 



Ti0 2 



A1 2 3 - 



Fe 2 3 



FeO ... . 



MnO .... 



MgO --- - 



CaO 



Na 2 - 



K 2 .... 



H 2 0(uo n ) - 



H s O(ignit.) 



P 2 5 - 



100.62 100.42 



