44 GEOLOGY OF THE COMSTOCK LODE. 



Character in detail. — Tlic principal constitxieiits are plagioclase, hornblende, 

 and mica, often with the addition of quartz; wliile the subsidiary minerals 

 are titanic iron, apatite, spliene, zircon, and in one case tourmaline. The 

 hornblende is for the most part fibrous and bluish green. But this does not 

 appear to have been its original color. The centers of considerable masses 

 of hornblende appear under the microscope wholly colorless, and the asso- 

 ciation of the two varieties described in detail under slide 295 is such as to 

 lead almost inevitably to tlie conclusion that the green color is secondar}-. 

 In many specimens the hornblende is present in great quantities, and micro- 

 lites so crowd the feldspars that their striations are almost imperceptible and 

 their species indeterminable. To a considerable extent the hornblende is 

 decomposed into chlorite and epidote, the latter mineral appearing in unu- 

 sually fine crystals. Mica is present in smaller (piantities than hornblende, 

 and gives the interference figure of biotite. Some specimens contain augite. 



In the less hornblendic varieties the feldspars are well developed, many 

 of them with sharp, rectilinear outlines, like those in the more porphyritic 

 diabases. The lamella' are exceedingly attenuated; they show angles of ex- 

 tinction appropriate to oligoclase, and pericline twinning is occasionally 

 visible. A few" orthoclase crj'stals also occur. Quartz grains are numerous 

 in the less hornblendic specimens, and do not appear to be of secondary 

 origin. They contain numerous minute fluid inclusions, and with irreg- 

 ular grains of feldspar form a kind of coarse groundmass. The titanic iron 

 is accompanied by leucoxene, which in some cases appears to pass over into 

 titanite, though a transition cannot be demonstrated. The apatite presents 

 nothing peculiar, and the zircon is in no way remarkable except in the fre- 

 quency of its occurrence. Tourmaline was found only in one slide. Of 

 course it suggests metamorphic origin, though the same mineral is known 

 to occur occasionally in rocks of eruptive origin, and, as has already been 

 mentioned, was noticed in one of the almost unquestionably eruptive 

 diorites of this very district. 



Comparatively little decomposition has been noticed in this rock, a fact 

 whicli no doubt stands in intimate relation to its unusual hardness and 

 toughness, but in some limited areas it is highly chloritic, and certain speci- 

 mens woidd \nii^» for i)ro{)ylite. 



