ACID BRECCIA OF PYRAMID PEAK. 281 



augite, and olivine, and occasionally hypersthene, in different proportions 

 in different cases. In some rocks the phenociysts are large; in others, small. 

 Angite is usually the most prominent mineral. The groundmass is glassy 

 and microlitic, or noncrystalline, with lime-soda feldspar, augite, and mag- 

 netite, and having the various modifications of microstructure characteristic 

 of basic andesitic lavas. Olivine, which is abundant, is in some cases partly 

 decomposed to green serpentine, in others to orange or red serpentine, which 

 appears to pass into the mica-like mineral having similar colors and indices 

 of refraction, but marked pleochroism, with strong absorption for rays 

 vibrating parallel to the plane of perfect cleavage. The perfect cleavage 

 appears to be parallel to some plane in the prismatic zone of olivine. 



Other bodies of these rocks have an exceptional composition, and for 

 this reason will be described in detail in Chapter IX, together with other 

 similar rocks in the region. 



XATE ACID BRECCIA. 



Overlying the massive basalts that top the early basic breccia, and in 

 marked contrast to their dark-gray or black color, are breccias and tuffs of 

 light-colored and brightly variegated andesites. This superposition is 

 clearly shown in Pyramid Peak and on the fiat-topped mountain east of it. 

 West of the ponds on the latter mountain there are scattered exposures of 

 compact and also of friable beds of light-gray tuff, composed of small frag- 

 ments of hornblende-mica-andesite and hornblende-andesite, with larg-er 

 rounded and subangular pieces of the same kinds of rock (1481, 1482). 



Just west of the saddle east of Pyramid Peak the basalt is exposed 

 with overlying beds composed of fine grains of hornblende-mica-andesite, 

 acting as a cement for fragments and rounded masses of vesicular basalt, 

 similar to the basalt of the neighborhood. Above this the pyramidal 

 mountain is formed of light-colored andesitic breccia and tuff to its summit. 

 The whole mass is exposed in bold escarpment, and consists of nearly hori- 

 zontal beds of tuff and breccia, with layers of large fragments that are 

 subangular, rounded, and also angular. There is great variety of color 

 and habit in the andesite. The greater part is hornblende-andesite, some is 

 hornblende-mica-andesite, and some pyroxene-andesite; portions of it are 

 dense and compact, other portions porous. The beds are brown and gray, 

 some being very thick without distinct bedding. At the southern end of 



