MICROSCOPIC PETROGRAPHY. 

 PLATE I. 



DRAWN FROM NATURE BY JOHN H. CASWELL. 



Tiu. 1. Ehyolite (f" objective, X 88, polarized light) [1<J7], near Deer Mountains, containing twin 

 crystal of quartz, the two parts not being distinguishable in ordinary light. Biotite in 

 prismatic and basal sections, and small, rectangular sanidin crystals in the groundniass. 

 (Seep. 500.) 



Fig. 2. Rhyolite (f" objective, X 88, polarized light) [137], top of Custer Peak. Biotite crystal with 

 border of magnetite grains and lines of lamination. Brilliantly polarizing quartz, abundant 

 • magnetite and indistinct sanidin crystals in the crystalline groundmass. (See p. 503.) 



Fig. 3. Phouolite (i" objective, X 250) [139], Black Butte, showing the hexagons and rectangles of 

 nephelite in a large hornblende mass. luclusions of the same nature in both sectiorts. 

 Groundmass consists of sauidin. (See p. 503.) 



Fig. 4. Phonolite (f" objective, X 88, polarized light) [139], Black Butte, containing large, dusty 

 nosite crystals, with nephelite sections and small hornblendes in the feldspathic ground- 

 mass. Fluid-like structure around the uositc Crystal. (See j}. 504.) 



