PYROXENE-ANDESITES. 301 



it in a variety of ways. They form a narrow zone of small prisms around 

 a large augite in one instance, and in another ease hornblende contains 

 small grains of augite as inclusions. It is also intergrown with pyroxene 

 in the manner already described in other occurrences. 



pyroxene-andesites. — The pyroxeiie-andesites of the late basic breccia are 

 quite the same as those of the late acid breccia just described. Most of 

 them have glassy microlitic groundmasses, the glass being brown in the 

 majority of cases. Some have noncrystalline groundmasses. The habit 

 of most of the modifications is that produced by abundant small pheno- 

 crysts and a microlitic groundmass. In some cases distinct phenocrysts 

 are wanting, but there are many microscopic prismatic feldspars in the 

 groundmass, usually arranged in curving parallel lines, nroducing fluidal 

 structure. 



In a few rocks these rectangular microscopic feldspars are in excess of 

 the microlitic groundmass. In the noncrystalline varieties the dark color 

 of the groundmass gives way to gray shades. The degree of crystallization 

 does not exceed that of holocrystalline-microlitic or finely microgranular. 

 The phenocrysts are lime-soda feldspar, hypersthene, and augite, with small 

 crystals of magnetite. Rarely hornblende or olivine is present in small 

 amounts. 



The feldspars are labradorite-bytownite, with marked twinning and 

 zonal structure, and generally rectangular outline in cross section. Glass 

 inclusions are frequent. In some varieties of this rock the feldspar phe- 

 nocrysts are anorthite. Hypersthene and augite have the same colors, 

 forms, and microscopical characters as in the pyroxene-andesites already 

 described. In some cases pale-yellowish augites are twinned on the ortho- 

 pinacoid, and, being cut so as to exhibit symmetrical extinction angles on 

 both sides of the twinning plane, furnish an inclination of the bisectrix a 

 equal to 42° to the trace of the twinning plane, indicating hedenbergite or 

 augite. Zonal structure is occasionally exhibited between crossed nicols, and 

 sometimes by variations in color. Glass inclusions are often present. The 

 pyroxenes are mostly fresh and unaltered. An alteration of hypersthene to 

 pale-green pleochroic amphibole is sometimes observed, the fibers of amphi- 

 bole lying parallel to the vertical axis of the hypersthene. A narrow opaque 

 border and a darker-colored margin are sometimes present. Occasionally a 

 serpentinous mass is included and suggests the former presence of olivine. 



