46 GEOLOGY OF THE COMSTOCK LODE. 



supposed to be obliterated, in others the crystals are so clear that striations, 

 if present, could not but be apparent. Many of the unstriated feldspars show 

 cleavages, and extinguish light at angles Avhich seem to prove their ortho- 

 clastic character. No unstriated feldspars were found to give angles of 

 extinction, reckoned from the cleavage planes, which would refer them to 

 either of the triclinic species. The triclinic crystals show for the most part 

 very narrow striations, and give angles of extinction which correspond to 

 oligoclase. Tlie microlitic feldspars of the groundmass do not appear to be 

 triclinic. On introducing a portion of the rock in a condition of fine powder 

 into the well-known solution of mercuric iodide in potassic iodide, of a 

 specific gravity of less than 2.65, a large proportion rose to the surfiice. 

 This mounted in balsam appeared to consist mainly of feldspar. The por- 

 tion which sank contained some feldspar and the other components of the 

 rock. The feldspars contain inclusions of glass and also of fluid, but a jjor- 

 tion of the latter I regard as of secondary origin. 



Quartz. — Tlic quartzcs are bounded in part by straight lines and in j)art 

 by curves. In some cases the imperfectly developed crystals appear to 

 have been broken, and the fragments are now separated by narrow bands 

 of groundmass; in other cases they contain deep sinuous bays of the same 

 material. The quartz shows both fluid and glass inclusions, but their distri- 

 bution is somewhat uneven. In some slides they are present in nearly equal 

 numbers, while in others one or the other preponderates, or even occurs 

 exclusively; but this is exceptional. The inclusions are not thickly set, but 

 a glass inclusion and a fluid inclusion, with a moving bubble, can often be 

 seen in the same field with a Hartnack No. 7 objective. Of the hornblendes, 

 which were all black-bordered, none now remain in a fresh condition. 

 They have been replaced by the usual products of decomposition — chlorite, 

 epidote, quartz, and calcite. The mica, too, is in great part decomposed, 

 but occasional scales remain, and these give the interference figure of bio- 

 tite. The groundmass in every case shows fluidal and pseudospherolitic 

 structure. In some cases a base is also present; in others it is either wanting 

 or devitrified. When glass is present, it shows a preference for elongated 

 sinuous forms, and often the central line is marked by aggregations of iron 

 ore from which, as axes, black trichites sometimes spread into the surrounding 



