134 GEOLOGY OP THE GOMSTOCK LODE. 



patches of magnetite which are evidently exaggerated black borders. 

 Some of these patches are ]jseudomorphs after hornblende, but mica plainly- 

 predominated. Of this enough is left to determine that its color was brown. 

 Tlie slide contains a single grain of quartz, which is evidently primary. 

 The groundmass is thoroughly crystalline and microlitic. It contains much 

 magnetite, and shows fluidal structure in places. 



Slide 476. Divide between Mount Rose and Mount Emma. 



Light-gray porous variety. — A light-gray rock witli a large amount of visible 

 mica and hornblende. The feldspar is largely in simple crystals, few, if 

 any, of which, however, are orthoclastic. The microlites are developed 

 with imusual sharpness. The feldspars contain bubble-bearing glass inclu- 

 sions and jjatches of groundmass. Hornblende is much more abundant in 

 this slide than mica, and is remarkable for the fact that it shows scarcely a 

 trace of black border. Much of it occurs as minute brown spiculse dis- 

 seminated through the groundmass. The mica is present in well-developed 

 crystals, and cleavage scales show the ordinary sensibly uniaxial interfer- 

 ence figure of biotite. The iron ore is magnetite, and it and the feldspar 

 microlites of the groundmass seem to be imbedded in a colorless glass. 



BASALT. 

 Slide 457. Basalt mesa, just west of Silver City. 



Only variety. — Tliis is a dark ordinary basalt, with numerous visible fresh 

 amber-colored olivines. Under the microscope it is seen to be a crystalline 

 mixture of olivine, augite, feldspar, and magnetite. The olivine is nearly 

 colorless iu the section, but has a faint yellowish tinge. It occurs for the 

 most part in grains showing only one or two crystalline faces or none at 

 all. A few of the larger crystals have good hexagonal or octagonal out- 

 lines. Besides irregular cracks, there are occasional indications of imper- 

 fect cleavage. The olivine is wholly undichi-oitic and polarizes brilliantly. 

 As inclusions it contains crystals of magnetite and a few particles of augite. 

 It shows only occasional traces of decomposition. The augite is of the 

 common brown color, but of a rather deeper tint than usual. Some of the 



