Vol. 4] Smith. — Upper Region of Main Walker River. 



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a long exposed surface it turns slightly red. At a distance it 

 always appears black. Microscopically, feldspar and pyroxene- 

 make up the bulk of the rock. Opaque iron ore is relatively abun- 

 dant and in irregular form. Translucent hematite of a reddish 

 brown color is also recognizable. No olivine was with certainty 

 recognized, and nearly half the rock is ferro-magnesian mineral, 

 which is an aluminous monoclinic pyroxene. It and the feld- 

 spar are apparently contemporaneous to some extent in crystalli- 

 zation ; the pyroxene is often twinned, and has a nearly rectan- 

 gular cleavage in sections norma] to an optic axis, and has 

 oblique extinction and irregular boundary. Its color is faint 

 yellow ot' greenish, and its double refraction is strong, being blue 

 and yellow of the second order, adjacent to quartz showing gray 

 or grayish blue of the first order. Within the augite are needles 

 of apatite and also iron oxide. The feldspars are of various sizes 

 The largest are about 1 mm. in length. Then there are sections 

 nearly squared 0.5 mm., and the smallest feldspars are lath- 

 shaped. The twinning of the larger phenocrysts exhibits rather 

 broad and irregular stripes, and zonal growths are also pro- 

 nounced. Oblique intergrowths are frequent, and many of the 

 larger feldspars show resorption near the margin, and subsequent 

 growth. This growth is more acid, having a lower relief and 

 different extinction. The margin occasionally contains granular 

 inclusions agreeing quite well with the augite, which would seem 

 to indicate that it continued to crystallize up to a late date, which 

 may be as late as, or subsequent to, the period of effusion. As 

 for the phenocrysts of feldspar, a last attack is also noticeable 

 on their margin ; but not so on the small, lath-shaped ones, which 

 may have grown in part at the expense of the more acid exterior 

 of the larger ones. No glass was recognizable. Where alteration 

 has taken place the plagioclase shows no calcite. But it is thought 

 this agrees with what has usually been observed, that even should 

 the plagioclase contain more calcium, calcite does not seem to 

 form as readily as in the less basic members of the series. The 

 absence of isotropic glass and a fluidal arrangement of the lath 

 feldspars seem to indicate the ability to crystallize at a compara- 

 tively low temperature. It may be remembered in this connection 

 the apparent sheet like appearance of this lava. 



