PYROXENE-ANDESITE. 357 



The percentage of FeO being greater than 14 per cent the orthorhombic pyroxene 

 may be classed as hypersthene. The optical character was determined in the isolated 

 crystals and corresponded to hypersthene. 



A review of the thin sections of the andesite from Richmond Mountain shows that 

 the two pyroxenes resemble one another closely in thin sectioir, but the hypersthene 

 is pleochroic to a greater or less extent, the augite not at all so. The pleochroism of 

 the hypersthene is, of course, stronger in the thicker sections, but varies among the 

 individuals in a single section and in some instances differs zonally in a single crystal, 

 being stronger in the central portion of some individuals and in the marginal portions 

 of others. It is green parallel to the c axis and light brown parallel to a and b with a>b. 

 In some cases they are nearly colorless. The augites are very light yellowish green to 

 colorless. Cleavage parallel to the prism and more rarely to the pinacoids is observed 

 in cross sections cut perpendicular to the positive bisectrix; but in many longitudinal 

 sections there is no trace of cleavage. 



The slight border of augite grains surrounding many of the pyroxenes is almost 

 exclusively confined to the porphyritic augite crystals. This is most noticeable where 

 both varieties of pyroxene have grown together in parallel crystallographic orienta- 

 tion, the hypersthene being the older secretion in most every case; the granular 

 augite border extends around the augite crystal, but ceases at the hypersthene. The 

 orthorhombic pyroxene is more readily altered than the augite, a fibration parallel to 

 the c axis sets in from the surface and along the cracks, resulting in a light green, 

 highly refracting mineral with an inclined extinction angle which reaches 15, and is 

 evidently a fibrous hornblende (actinolite). The crystals are sometimes coated with 

 brown oxide of iron (limonite), which also coats the pyroxene micro! ites and porphy- 

 ritical hornblendes. Though generally free from inclusions some individuals bear 

 numerous magnetite grains, and irregularly shaped, colorless glass inclusions with a 

 gas bubble, besides apatite needles and, rarely, imperfectly formed brown hornblende. 



The pyroxene microlites of the groundmass, varying from 0-04 or 0-05 mm in 

 length to microscopically minute proportions, are long slender prisms parallel to the 

 vertical axis, terminated by a pyramid. They are of a pale greenish color and con- 

 tain numerous magnetite grains, which are in no case associated with the feldspar 

 microlites. Their augitic nature is shown by their crystalline form, color, and high 

 index of refraction, taken in connection with their angle of extinction, which varies 

 from to more than 35, being indeed directly traceable, through occasional larger 

 individuals, to those of unquestionable augitic nature. A part, however, may be 

 hypersthene. The parallel, fibrous decomposition product is in one instance, No. 90, 

 colored red by oxide of iron, producing small prisms of a reddish yellow color, pre- 

 cisely similar to those mentioned by Prof. Zirkel as of an indeterminable nature in 

 the "trachyte" from the south bank of Palisade Canyon, Cortez Range, previously 

 referred to, which are there also traceable to augite. This microscopic angite <>t' tinul 



