400 GEOLOGY OF THE EUltEKA DISTRICT. 



PLATE V. 



1. PLAGIOCLASE FELDSPAR IN HORN'BLENDE-MICA-ANDESITE. 



Thin section 42a, magnified 45 diameters. Between crossed nicols, exhibiting xona' structure and 

 several rounded contours due to partial corrosion at different stages of its growth. Also a net 

 work of irregular cracks. 



2. A MICROPEGMATITIC PHEN'OCRYST IN RHYOLITE. 



Thin section 141, magnified 19 diameters. Three sanidine crystals surrounding a plagiochiso, most of 

 which has fallen out, leaving a hole. The sanidine is filled with irregularly shaped shreds of 

 quartz arranged as in pegmatite. Between crossed nicols. The quartz is dark, the feldspar 

 light. 



3. PLAGIOCLASE FELDSPAR FROM HORNBLENDE-MICA-ANDESITK. 



Thin section 35, magnified 35 diameters. Between crossed nicols, exhibiting polysyuthetic twinning, 

 zonal structure, and microscopic inclusions of glass. 



4. PLAGIOCLASE FELDSPAR ADJACENT TO FIG. 8. 







In same thu section, magnified 35 diameters. Between crossed nicols, exhibiting polysyuthetic 

 twinning and microscopic inclusions of glass. 



