THE WASHOE ROCKS. 101 



seems sufficient evidence that the other eruption of pyrox- 

 ene andesite was far earlier and comparatively near to the 

 date of the hornblende andesite. It is worth noting that 

 most of the glassy pyroxene andesite, and perhaps all of it, 

 belongs to the eruption immediately preceding the later 

 hornblende andesite. 



DIORITE. 



Not characteristically pyroxenic. — Messrs. Hague and 

 Iddings maintain that the two walls of the Comstock 

 are the same rock and both originally in the main py- 

 roxenic. That in some cases the granular diorite of my 

 report contains fresh brown hornblende, far exceeding the 

 accompanying augite in quantity, they do not deny, but 

 they assert their belief that in the main mass of the granular 

 rock, containing green fibrous hornblende in irregular 

 patches, this mineral is uralitic. This is a case in which 

 full direct evidence is scarcely available, there being com- 

 monly no means of deciding whether the bisilicate in a 

 particular slide is a product of the degeneration of pyroxene 

 or of hornblende. During my last visit I collected a series 

 of specimens with a view to testing this question on the fine 

 exposures of the face of Mount Davidson. 



In a great portion of this rock the grains are somewhat 

 indistinct from an admixture of the minerals. In other 

 portions equally granular, the grains are sharp and appar- 

 ently free of impurities. Specimens of the latter class 

 were selected and slides from them show that they contain 

 unquestionable crystals of hornblende with characteristic 

 outlines. 



Porphyritic cliorife. — Benewed observations were also made 

 on the porphyritic patches of the mass. On the bare faulted 

 surfaces of the diorite of Mount Davidson, though consider- 

 ably more than 90 per cent, of the rock is granitoid in struc- 

 ture, there are patches of porphyritic rock surrounded by 

 granular material, and patches of granular matter sur- 



