AKDESITES OF ABSAROKA RANGE. • 291 



The groundmass in tlie different varieties of this rock varies from 

 microcryptocrystalline and microlitic to microcrystalline with micropoikilitic 

 structure, which is very fine grained, the structures and mineral composi- 

 tions being typically andesitic. 



Hornbiende-andesites. — The habit of these rocks is generally similar to that of 

 the andesites just described — a holocrystalline groundmass with abundant 

 minute phenocrysts of feldspar and hornblende. In one case feldspar pheno- 

 crysts are wanting. The hornblendes have the same varieties of color and 

 other characteristics as those in the andesites last described. In most of the 

 cases studied they are red. Black borders occur in some varieties, but with 

 no special connection with any particular microstructure of the ground- 

 mass. Biotite is present in several rocks in small amounts. Small crystals 

 of pyroxene are rare. The feldspar phenocrysts are similar to those 

 described in the hornblende-mica-andesites. The groundmass structures 

 are also similar — holocrystalline and microlitic, with andesitic habit. 



Hornblende-pyroxene-andesite. The 101 thill Sections of this killd of aildesite 



which were studied exhibit a range of mineral composition from rocks with 

 much ferromagnesian phenocrysts to others with few, and from varieties 

 with much hornblende and little pyroxene, which are closely connected with 

 hornblende-andesite, to those with more pyroxene than hornblende, which 

 might be classed as hornblende-bearing pyroxene-andesites. The ground- 

 mass also differs greatly in various rocks, and the amount, size, and shape of 

 the feldspar phenocrysts are equally variable. In general, however, the 

 habit is like that of the rocks just described — a groundmass crowded with 

 small phenocrysts. The actual character of the groundmass can be dis- 

 covered only with the microscope. It is glassy and microlitic in many cases 

 and holocrystalline in others. 



The feldspar phenocrysts, which are present in great abundance, are 

 mostly rectangular and elongated in thin sections. They are all plagio- 

 clase, and, like those before described, probably labradorite. But there 

 appears to be some variability in the kinds, and some small crystals exhibit 

 extinction angles suggesting anorthite (33°). Polysynthetic twinning is 

 always present to a 'greater or less extent, and zonal structure is highly 

 developed in some crystals, over 100 alternating zones of light and shade 

 having been counted in one feldspar, which was 0.46 mm. from center to 

 margin, the average width of a zone being 0.0002 of an inch. In some cases 



