400 GEOLOGY OF THE EUEEKA DliSTKlCT. 



PLATE V. 



1. I'LAGIOCLASE KELDSPAK IN llOKNBLENDE-MICA-iVNDESITlS. 



Thin section •12a, magnified 45 diameters. Between crossed nicols, esJiibiting zona' .striictuio and 

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

 Tvork of irregular cracks. 



2. A MICROPEGMATITIC PnENOCRYST IX RHYOLITE. 



Thin section 141, magnified 19 diameter.s. Three sanidino crystals surrr>uuding a ])lagiocl;ise, 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. ri..\GIOCLASE FELDSPAU FROM HORNBLENDE-MICA-ANDESITE. 



Thiu section 35, magnified 35 diameters. Between crossed nicols, exhibiting polysyutlietic twiuuiug, 

 zonal structure, and microscopic inclusions of glass. 



4. PL.A.GIOCLASE FELDSPAR AD,JACENT TO FIG. 3. 



In same thin section, magnified 35 diameter.s. Between crossed nicols, exhibiting polysyuthetic 

 twinning and microscopic inclusions of glass. 



