96 CALIFORNIA ACADEMY OF SCIENCES. 



that there were two separate eruptions of porphyritic, 

 pyroxenic, plagioclase rocks, closely allied, indeed, but 

 presenting peculiarities in structure and occurrence which 

 made it necessary, according to the accepted criteria of the 

 time, to separate them into diabase and andesite.^ Messrs. 

 Hague and Iddings consider both of these masses literally 

 or substantially as a single Tertiary eruption. 



During the past season I have found additional reasons 

 for maintaining the existence of diabase, and also for divid- 

 ing the pyroxene andesite into two distinct outflows separa- 

 ted by a long interval of time. 



Diabase at Steamboat. — At Steamboat Springs, at the west- 

 ern foot of the Virginia range, and about six miles from 

 Virginia City, occurs an extensive series of sedimentary 

 beds. They are for the most part in a condition of great 

 alteration, much plicated, on the average nearly vertical, 

 the strike following the general direction of the Sierra. 

 Andesites and basalts have broken through them and over- 

 lie them. No trace of a fossil could be detected in these 

 rocks. They are certainly pre-Tertiary, however, for the 

 Miocene to the north and the Pliocene to the south (at Car- 

 son) are very differently characterized. This series appears 

 to be at least as old as the beds determined as Jura-Trias 

 by the geologists of the 40th parallel. These beds contain 

 pebbles of the exact physical and mineralogical character of 

 the most typical portion of the east wall of the Comstock 

 lode, which I determined as porphyritic diabase.*^ It is 



Note '". — In my memoir on the Comstock lode, it is maintained that the 

 rocks of the district, in the order of their succession, are as follows: Granite, 

 metamorphics, granular diorites, porphyritic diorites, quartz-porphyry, j)or- 

 phyritic diabase, later diabase (black dike), earlier hornblende andesite, 

 augite andesite, later hornblende andesite, basalt. It will be shown in this 

 paper that the augite andesite would be more properly entitled xDyroxene 

 andesite, and that it is divisible into two eruptions, bt^tween which, how- 

 ever, no other lava is known to have been ejected. 



Note ^. — As is almost invariably the case at Virginia, the pyroxenes are 

 represented only by pseudomorphs, but these are unmistakable. 



