124 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Table XL — Mineral variation in the upper, breccias of Sepulchre Mountain — Cont'd. 



Mineral 

 Group. 



Specimen 

 number. 



Phenoorysts other than feldspar. 



Pyroxene. 



Hornblende. 



Biotite. 



Quartz. 



B 4 .... 

 B 6 .... 



405 

 406 

 407 

 409 

 410 

 411 

 412 

 413 

 414 

 415 

 „ 416 

 417 

 418 

 419 

 420 



Much 



Much 



Much 



Much 



Much 



Much 



Much 



Much 



Much 



Much 



Some 



Some 



Some 



Some 



Some 



Much 







Much 













Much 







Much 







Much 







Much 













Much 







Much 







Much 







Much 







Much 







Much 







Much 



Little 









The varieties without hornblende, B x — that is, pyroxene-andesites — 

 have a groundmass of globulitic brown glass, in shades from dark to light, 

 filled with microlites of feldspar, pyroxene, and magnetite. The size and 

 abundance of the microlites vary. The feldspar microlites are plagioclase 

 with low extinction angles. Two of the varieties examined are basaltic in 

 appearance, Avith a few decomposed olivines among the phenocrysts. 



The pyroxenes are both hypersthene and augite, having much the same 

 general appearance in the same rock. Their crystal outline varies consid- 

 erably in one rock section. Some individuals are bounded by distinct 

 crystal planes, while others are rounded. Some have irregularly jagged 

 outlines, with tongues of glassy groundmass projecting into the crystal. 

 These forms appear to be the result of irregularities of growth rather than 

 of solution by the magma before its solidification. Inclusions of glass are 

 common; also those of "magnetite, and fewer of apatite. 



The hypersthene is pleochroic; green parallel to c, yellow parallel to 

 a, light red parallel to ft. It is generally light colored in thin section. But 

 one darker-colored crystal with strong pleochroism incloses thin brown 

 plates, arranged in lines at right angles to the vertical axis of the crystal. 

 These inclusions are like those in hypersthene in man}" coarsely crystalline 



