392 GEOLOGY OF THE EUREKA DISTRICT. 



The basalt covering the large area east of Richmond Mountain is represented by 

 three thin sections from Basalt Peak, 283, 282, and 284; one from the light colored 

 vesicular variety from Strahlenberg, 285; one of compact rock from Basalt Cone, 286, 

 and another, 288, from back of the Toll House on Newark Valley road. The first 

 three are characterized by relatively large crystals, and a coarsely globulitic glass, 

 which in 284 is nearly opaque, being of a dark, rich brown in the thinnest places. 

 They all three contain olivine, which is completely decomposed in the ordinary manner 

 in 283, but is only partially altered in 282 and 284 to the red, laminated sub- 

 stance, already described. The basalt of 285 is without olivine and is in a less per- 

 fectly crystallized state. The colorless glass is almost free from globulites; the feld- 

 spar inicrolites are small, and the augite is in much larger individuals with nearly all 

 the magnetite attached. The structure of 286 is identical with that of 282, the glass 

 is coarsely globulitic and there is a small amount of serpentinized olivine present. 

 The globulitic base of 288 is crowded with feldspar microlites, with many micro- 

 scopic porphyritical augite crystals, and numerous grains of perfectly fresh olivine. 

 Of the two thin sections from the neighborhood of Pinto, 290 is in every respect like 

 that from the summit of Richmond Mountain (264), and contains no olivine, while 292 

 is identical with the basalt of Crater Cone (286), and abounds in altered olivine. 



The foregoing detail is necessary in order to emphasize the fact of the unity of 

 the somewhat scattered outflows of this rock, as it shows the slight and nonessential 

 character of the variations, the fluctuating percentage of the olivine, and that its 

 presence or absence is without influence on the microstructure of the rock. The 

 grounds, then, for separating this rock from the andesites and classifying it as a basalt 

 may be summed up as follows : (a) The great difference in microstructure between the 

 andesite of the district and this rock, which is not porphyritically developed and 

 which is very glassy, with extremely small crystals of nearly uniform size, having no 

 macroscopic phenocrysts, with rare exceptions; (&) that while the feldspar is in well 

 developed lath-shaped crystals, the augite is mostly in less regularly outlined crystals, 

 and in much greater abundance, occurring frequently in larger individuals, in addition 

 to which there is a small percentage of hypersthene; (c) the presence of oliviue, though 

 in variable quantities. It is, however, not a normal basalt, and may be considered 

 more properly an intermediate rock between basalt and pyroxene- audesite. 



Another variety of basalt is found in the southeastern corner of the district, at 

 Magpie Hill and on the southern slope of the Alhambra Ridge. Thin section 295 is 

 from the former and shows it to be a very homogeneous mixture of feldspar, augite, 

 olivine, and iron oxide, with no isotropic glass ; but there are irregularly scattered 

 patches of a light purple, cryptocrystalline substance, which may be the remains of a 

 glassy matrix. The rock is thoroughly crystalline, and as such is very different from 

 the first described variety. The body of the rock is made up of rather broad, inter- 

 penetrating, lath-shaped feldspar crystals, in which are many small augite prisms and 



