THE WASHOE ROCKS. 97 



thus absolutely certain that there is somewhere in the neigh- 

 borhood of Mount Davidson real pre-Tortiary diabase, not 

 distinguishable either by definable characteristics or by 

 those more subtle properties known as habitus from Com- 

 stock diabase. The locality in Avhich these pebbles occur 

 now receives the drainage from Mt. Davidson. According 

 to my investigation of the faulting action on the Comstock, 

 this locality formerly received the drainage from the diabase 

 area at Virginia. Be this mentioned, however, only as 

 evidence that the two localities are substantially in the same 

 district. It would be impossible and is unnecessary to show 

 that these pebbles are from the particular mass which forms 

 the east wall of the lode. This occurrence throws the bur- 

 den of iDroof on to anyone who chooses to deny the pre- 

 Tertiary age of a rock which, in its characteristic exposures, 

 has a distinctly different character from representative augite 

 andesites. 



The lithological distinctions between the porphyritic dia- 

 base, and the augite andesite of Washoe are somewhat re- 

 fined, and in many cases it may be impossible from the 

 mere study of specimens to discriminate them. 



This could hardly be otherwise, for it is difficult to con- 

 ceive that a porphyritic diabase could, so differ from a por- 

 phyritic pyroxene andesite, that every hand specimen, or 

 every slide could be unhesitatingly referred to its proper 

 category. In their typical developments, however, they are 

 distinguishable without difficulty. It may be that this is 

 only because in the course of geological periods the older 

 rock has been eroded to a depth at which the glassy magma 

 had a better opportunity to crystallize and at which fluid 

 inclusions were more readily formed, while the infiltration 

 of waters for ages has produced, even in the freshest spec- 

 imens, effects familiar to the observers of the older rocks; 

 or the distinctions between the older and younger rock may 

 be due to some other cause not yet elucidated. However 

 this may be, the east wall of the Comstock, at the higher 



