EARLY ACID BRECCIA, ABSAROKA RANGE. 271 



tuff and altered perlite having distinct perlitic structure (1027, 1028, 1031, 

 1032). Trachyte also forms the base of Junction Butte, and rests upon 

 the gneiss directly north. It also forms the banks of the river at the 

 mouth of Slough Creek. The acid andesitic breccia extends several miles 

 farther up the Lamar River and is overlain by a lava sheet of porphyritic 

 basalt (1129). The breccia is dense and dark colored and might be 

 mistaken for basic breccia, but contains much biotite and even minute 

 phenocrysts of quartz. The acid breccia is cut by a 3-foot dike of pyroxene- 

 andesite, which is dark colored and has small phenocrysts of pyroxene and 

 feldspar (1038). The relative age of these rocks is thus plainly shown in 

 this locality. 



Mica-bearing andesitic breccia occurs at the northern base of Speci- 

 men Ridge, about a mile west of Crystal Creek. It is green, compact, and 

 carries dark-colored fragments. It may be a mixture with more basic 

 andesites. The only other exposures of the earliest acid breccia in this 

 vicinity are in Cache Creek and near Cook City. These have been 

 described in connection with the Crandall volcano. The acid breccias in 

 all of these localities are the same. But in some cases there is more or less 

 of an admixture of basic material. Of course there are localities where 

 the basic breccia rests directly upon the nonvolcanic rocks. Either the 

 first acid breccia was not so extensive as the basic breccia or it was com- 

 pletely removed by erosion in some places. 



The microscropical study of specimens collected from bodies of the 

 earliest acid breccia shows it to vary in mineral composition, the varieties 

 falline 1 under three classes: Hornblende-mica-andesite, hornblende-andesite, 

 and hornblende-pyroxene-andesite. The fragments constituting any large 

 mass often differ considerably among themselves in habit, color, and min- 

 eral composition. Sometimes their characters are nearly constant f< >r large 

 bodies of breccia. Massive bodies occur either as lava streams or as intru- 

 sive masses. In most places mica-bearing varieties abound. They are 

 seldom absent. But the relative proportions of the different varieties is not 

 constant enough to permit a close estimate to be made of the average com- 

 position of the whole. The following analyses represent the chemical 

 composition of the three varieties, one of which has been already given iu 

 connection with the description of the Electric volcano. 



