THE BUNSEN PEAK MASS. 



87 



streams of rhyoiitic and basaltic lavas. The rock is light gray and fine 

 grained, with abundant small phenocrysts of feldspar and biotite. It is a 

 mica-dacite-porphyry, whose chemical composition is as follows: 



Analysis of Bunsen Peak mica-dacite-porphyry. 



Constituent. 



(102) 





70.52 



Trace. 



15. 85 



2.28 



.36 



.09 



.09 



2.59 



Trace. 



3.93 



3.43 



.17 



.29 



.35 



TiO. 



Al.,0, 





FeO 



MnO. 



MgO 



CaO 



Li,0 



Na 2 



M 



SO , 



H.0 



Total 



99.95 





This analysis was made from the rock at the summit of Bunsen Peak. 

 It is nearly the same as that of the rock from Mount Holmes (p. 65), being 

 a little lower in alumina and higher in lime and potash. 



The mineral composition and microstructure of the rock vary but 

 slightly throughout the body. Under the microscope it is seen to consist 

 of a holocrystalline groundmass of feldspar and quartz, with abundant flakes 

 of biotite and a little magnetite Small crystals of apatite and minute 

 prisms of zircon are present. There are numerous phenocrysts of irregularly 

 outlined biotite, idiomorphic feldspar, and a few corroded crystals of quartz. 

 The feldspar phenocrysts are almost all polysynthetic twins of lime-soda 

 feldspars, oligoclase-andesine, also twinned according to the Carlsbad law, 

 and having marked zonal structure. A few appear to be labradorite. Some 

 unstriated feldspars may be orthoclase. They have few inclusions, and 

 exhibit very little decomposition in some parts of the rock. The cracking 

 and secondary minerals characteristic of the feldspar phenocrysts in the 

 intrusive rocks of the Gallatin Mountains also occur in parts of this rock. 

 The biotite is brown, with strong absorption and few inclusions. It is for 

 the most part unaltered. Quartz phenocrysts are much corroded and carry 



