BASALT. 393 



grains of magnetite, together with an abundance of larger and better developed 

 olivine crystals. There are a couple of grains of porphyril ical quartz, and calcite is 

 pretty generally disseminated through the whole mass. The feldspar is probably 

 labradorite with some little anorthite. It is not well outlined, but is in prisms, elongated 

 in the direction of the brachydiagonal, with multiple twinning after albite, a few rare 

 individuals having a second twinning at about 90. There are no characteristic 

 inclusions, some crystals being free from any, others containing glass and small 

 portions of the associated minerals, besides long, slender jointed apatite needles. The 

 augite is not abundant and occurs in irregular grains and in short prisms with pyra- 

 midal termination. It is of a light green color without pleochroism, and is intimately 

 associated with the magnetite. The olivine is in larger, quite characteristic crystals, 

 the largest l mm long. It is almost completely decomposed to the blood-red micaceous 

 mineral already described, and is in every respect similar to that found in the basalt 

 from the vicinity of Richmond Mountain. The magnetite, which is very abundant, is 

 in irregular grains and crystals, with some long, narrow forms, that suggest titan- 

 ifei'ous iron. 



The two quartz grains in 295 are very interesting. They are angular fragments 

 about 4 mm long, with distinctly corroded outlines and several conchoidal fractures; 

 they are surrounded by shells of radiating augite prisms and calcite. One of them 

 is represented in Fig. 4, PI. iv. The only inclusions are a few dihexahedral cavities 

 containing at low temperatures liquid carbon dioxide. The bubbles, which have 

 very narrow dark borders, are motionless at a moderately low temperature, but 

 become greatly agitated at about 60 or 70, and at a few degrees higher tem- 

 perature disappear, reappearing on being cooled again. The quartz is evidently 

 primary; that is, antedates the final consolidation of the rock; but the calcite, 

 which surrounds it and penetrates numerous small cavities, and also occurs in 

 irregular patches through the groundrnass of the rock, is secondary. In the augite 

 border surrounding the quartz grains occurs a strongly pleochroic mineral .in short, 

 stout, well developed crystals. It is biaxial and monoclinic, and two cross-sections 

 show the characteristic form and cleavage of epidote, with the strong absorption 

 parallel to the clino-diagonal, but the pleochroism is unusual for that mineral, being 

 reddish purple parallel to the axis c, light yellow to colorless parallel to a, and strong 

 yellow parallel to b. This is the pleochroism of pieduiontite or manganese epidote, 

 which is probably the mineral present. It is not found in any other part of the thin 

 section. 



The basalt from the south slope of the Alhambra Eidge, thin section 296, is simi- 

 lar to the last in inicrostructure, but is full of phenocrysts. The groundmass is 

 formed of the same interpenetrating crystals of feldspar, in this instance labradorite, 

 with much more augite, magnetite, and red altered olivine, and with no glass. The 

 larger phenocrysts are not sharply outlined, the feldspars, by including more and 



