DETAILED DESCRIPTION OF SLIDES. 121 



tion these hornblendes are of a light yellowish-brown color, and are not ac- 

 companied by black borders. Very many of the hornblendes are twinned, 

 and a few show zonal structure. The hornblende is strongly dichroitic and 

 gives angles of extinction reaching 20°. The augites are few in number 

 and minute; indeed, at first sight, there seems to be no augite at all. 



The feldspars are all triclinic, but are considerably decomposed, and it 

 is not easy to determine their angles of extinction with accuracy. Most of 

 the large crystals, however, give angles which fall within the limits of lab- 

 radorite, while the microlites seem to be oligoclase; but one crystal show- 

 ing the two ordinary striations gives angles of almost exactly 37°. This 

 then must he anorthite. It is impossible to say that the other large crystals 

 are not so, but the probabilities are that others would have been found 

 exceeding the limits of labradorite had such been the case. In one of the 

 large feldspars fluid inclusions of the kind called secondary were observed, 

 and one of these contained a slowly moving bubble. Some of the feldspars 

 also contain partially devitrified glass inclusions. 



The slide shows two or three small grains of quartz which, from the 

 arrangement of the particles of the surrounding groundmass, appear to be 

 primary. The}^ contain liquid inclusions with moving bubbles. This is 

 the only case in which primitive fluid inclusions have been detected in the 

 Washoe andesites. The opacite is, for the most part at all events, magnetite. 

 A very perfect hexagonal crystal may be a section of a dodecahedron. 

 The apatite is colorless and without peculiarities. The groundmass polarizes 

 throughout, though in places only very feebly. If it ever contained any 

 base, the glass is now nearly or quite devitrified. 



Slide 464. 1,200 feet northwest of Geiger Grade Toll House. 



Coarse-grained trachytic-lookinghornblende-andesite. Thls sHde is frOm the Same Crop- 

 ping as ;U1, and beyond the possibility of a doubt the same rock, but it 

 differs greatly in appearance, being coarse-grained, gray, and more like an 

 ordinary trachyte than a common andesite in habitus. Under the mi- 

 croscope it is manifestly the same rock, though with a modification of 

 structure, for the groundmass is granular instead of microlitic. There are a 

 few grains of quartz which carry fluid inclusions. Slide 311 c'ontains 



