98 CALIFORNIA ACADEMY OF SCIENCES. 



levels, and particularly on the 500-foot level, is entirely 

 similar to diabases of well ascertained age, while the 

 augite andesite found over a thousand feet lower is equally 

 typical augite andesite. Of these two types, one answers 

 perfectly to the pebbles in the pre-Tertiary rocks at Steam- 

 boat, while the other does not. There are thus good litho- 

 logical arguments, as such arguments go, for the assertion 

 that the east wall of the Comstock is diabase, though these 

 do not by any means complete the chain of evidence ad- 

 ducible. 



Pyroxene Andesite at Steamhoat. — There are at. Steamboat 

 Springs extensive masses of pyroxene andesite, indistin- 

 guishable from a portion of the indubitable Washoe ande- 

 sites. This rock also cuts ordinary hornblende andesite 

 in dikes at the springs, and furthermore, passes by insen- 

 sible gradations and in good exposures into an extremely 

 micaceous "trachytic" andesite in all respects similar 

 to the later hornblende andesite of the Comstock region. 

 The hypersthene seems to be gradually suppressed and 

 replaced by mica, the augite at the same time yield- 

 ing to hornblende. So gradual is the transition, that 

 in some croppings of the intermediate rocks one may 

 search for half an hour before detecting a flake of mica, 

 and from this rock with a vanishing trace of mica to one 

 which looks as if it contained 30 or 40 per cent, of biotite, 

 every degree of admixture can be found. In large adjoin- 

 ing areas on the other hand the pyroxene andesite appears 

 to be entirely free from mica. The exposures are so good 

 and so extensive that there can be no mistake about these 

 facts. At Steamboat Springs then, only six miles from the 

 Comstock and on the same mountain range, there is cer- 

 tainly a pre-Tertiary porphyritic pyroxene rock in pebbles 

 and a very recent porphyritic pyroxene lava in large masses. 

 The latter is certainly more recent than the ordinary dense 

 hornblende andesite. 



Micaceous Pyroxene Andesites at JVasJwe. — Having studied 



