324 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



north on Buffalo Plateau and southeast of Sunset Peak. These rocks are 

 also identical microscopically with this glassy modification of the trachytic 

 lavas north of Crescent Hill. 



Other parts of the rock are brecciated flows, with lumps that are 

 devitrified perlitic glass, with microgranular and microcryptocrystalline por- 

 tions. These retain the small phenocrysts and microlites intact. Some are 

 composed of minute fragments of glass with very irregular shapes, welded 

 together as in many rh)-olites. They are devitrified and microcryptocrys- 

 talline. Some of the groundmasses are colored yellow, orange, or red by 

 clouded particles of hydrous oxide of iron. Less often the color is green, 

 from chloritic infiltration. 



The groundmass is wholly devitrified in many cases where the feldspar 

 phenocrysts are still fresh. In a number of instances the rock is wholly 

 altered, both kinds of feldspar having been reduced to kaolin, and the other 

 phenocrysts being decomposed and the groundmass reduced to a micro- 

 crvstalline to microcryptocrystalline aggregation. One modification of the 

 rock is somewhat andesitic, all of the feldspar phenocrysts being- lime-soda 

 feldspar (709). 



The chemical composition of the rock is shown by analysis 1. It 

 is of a variety (679) which has fresh phenocrysts of sanidine in abundance 

 and fresh labradorite and a little biotite. The groundmass is devitrified, and 

 there is some chlorite. The loss upon ignition is high and indicates partial 

 alteration. If anhydrous the silica would be 66 per cent, and the rock 

 would be about on the dividing line between quartz-soda-trachyte or highly 

 siliceous soda-trachyte and rhyolite. The presence of labradorite in a rock 

 with so little lime and so much soda is surprising, and it is evident that the 

 orthoclases must be rich in soda, The chemical composition of the very similar 

 rock near Sunset Peak on Bear Gulch, mapped in the Livingston folio 

 (No. 1) of the Geologic Atlas, is shown in the accompanying analysis 2. 

 It is higher in silica and in alkalies, the potash being exceptionally high for 

 rocks of this region. This rock is chemically trachytic rhyolite. Mineral- 

 ogically all of these rocks are characterized by the absence of quartz as 

 phenocrysts. The groundmass is highly siliceous, and quartz is present in 

 the microcrystalline modifications. 



