272 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Analyses of rocks occurring in early acid breccia, AbsaroJca Range. 



Constituent. 



l 



2 



3 





61. 56 j 

 .87 | 

 14. 73 ' 

 4.47 

 1.23 ■ 



.34 

 3.57 

 4.87 



64.61 



None. 



18.62 



2.78 



.95 



Trace. 



.85 



4.20 



67.95 



.45 



14.98 



2.33 



.95 



.09 



1.42 



3.98 



.23 



4.39 



2.86 



TiO, 



ALO, 



FeO, 



FeO 



MnO 



MgO 



CaO 



BaO 



Na ,0 



5.10 

 2.24 



4 37 



2.36 



.01 



.30 



K-0 



Li ,0 



p.,0-, 



.04 



.07 

 .11 

 .37 

 .61 



so 



Loss at 105° 













H,0 



1.42 



.93 







100.44 



99. 88 



100. 79 



The first is a massive rock in the early acid breccia near the head of 

 Tower Creek. The second is from the breccia of Crescent Hill, and is 

 above the average in percentage of silica, judging from the mineral com- 

 position of the breccia as a whole. The third is from early acid breccia of 

 Sepulchre Mountain, and is considerably above the average for silica. It 

 is probable that the average jDercentage of silica for the whole of these 

 early acid breccias. is 62 or 63. 



These rocks are usually light gray, with greenish, reddish, and purplish 

 tones, sometimes having darker colors. The appearance of the breccia is 

 generally variegated. It ma}' be loosely aggregated, but is oftener compact 

 and indurated. The habit of the andesite fragments varies from those with 

 abundant small phenocrysts of feldspar, biotite, hornblende, and pyroxene 

 to those with fewer and larger phenocrysts of the same minerals, which 

 seldom exceed 5 mm. 



The groundmass is glassy in some cases, usually colorless glass filled 

 with rectangular and prismatic crystals of feldspar and fewer microscopic 

 crystals of the other constituent minerals, including pyroxene in many cases. 

 Or the groundmass may be microlitic with little or no glass, or holocrystal- 



