PYKOXENE-ANDESITE. 357 



The percentage of FeO being gi-eater than 14 per cent the orthorhombic pyroxene 

 may be chissed as hypersthenc. Tlie optical character was (letermincd in the isolated 

 crystals and corresponded to hypersthene. 



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

 the two pyroxenes resemble one anotiier closely in thin section, but the hypersthene 

 is pleochroic to a great^-r or less extent, the angite 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 ditfcrs zunally 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 veiy light yellowish green to 

 colorless. Cleavage parallel to the prism and more rarely to the i)inacoidsis observed 

 in cross sectionscut perpendicular to the positive bisectrix; but in many longitudinal 

 sections there is no trace of cleavage. 



The slight border of angite grains surnmnding nuvny of the ])yroxenes is almost 

 exclusively confined to the i)orp]iyritic augite crystals. This is most noticeable where 

 both \arieties 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 ('• axis sets in from the surface and along the cracks, resulting in a liglit 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 microlites and porphy- 

 ritical hornblendes. Though generally fiee from inclusions some individuals bear 

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

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



The pyroxene microlites of the gnmndmass, varying from O-O-I or O-Oo""" in 

 length to microscopically minute proportions, are long slender lu-isms i)aranel to the 

 vertical axis, terminated by a ))^Tamid. They are of a ])ale greenish cohn- 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 0° 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 ))roduct is in one instance, No. 90, 

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

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

 the "trachyte" from the soutli l)ank of Palisade Canyon, Cortez liangc, previously 

 referred to, which arc there also tracc^able to augitic This niicroscojiic augite of linal 



