MICROSCOPIC PETROGRAPHY. 

 PLATE I. 



DRAWN FROM NATURE BY JOHN H.. CASWELL. 



Fig. 1. Rhyolite (•§•" objective, X 88, polarized light) [1*27], 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 groundmass. 

 (Seep. 500.) 



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

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

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



Fig. 3. Phonolite (J" objective, x 250) [139], Black Butte, showing the hexagons and rectangles of 

 nephelite in a large hornblende mass. Inclusions of the same nature in both sections. 

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



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

 nosite crystals, with nej>helite sectioDs and small hornblendes in the feldspathic ground- 

 mass. Fluid-like structure around the nosite crystal. (See p. 504.) 



