V 



640 /. E. SPURR 



account of the basal rhyolite. From further study, however, the 

 evidence of differentiation up to this point appears to be so good 

 that he is inclined to accept it, and to consider the basal rhyolite 

 as belonging to a different order of events. 



This basal rhyolite is, in chemical and mineralogical compo- 

 sition, much like the latest rhyolite, No. 5, in the writer's order 

 of succession. Like it, it often becomes extremely siliceous. 

 From this circumstance, and from the apparent break in compo- 

 sition between the rhyolite No. 1 and the andesite No. 2, the 

 writer conceived the idea that the two rhyolites are recurrent 

 lavas — that is, that they represent a similar development in dis- 

 tinct but similar processes of differentiation. The development 

 of lavas might then be interpreted as follows : 



1. Acid rhyolite. 



Revolution and beginning of new epoch. 



2. Medium to acid andesite and dacite. 



3. Acid rhyolite (with basalt). 



4. Medium basic andesite. 

 Basic basalt. 



I Acid rhyolite. 



In case this is the current grouping, it is probable that No. 1 

 represents the end product of a differentiation, and is similar to 

 the rhyolite under No. 5 ; and that 2 to 5 inclusive represent an 

 independent differentiation process. 



The difficulty with the above arrangement is, first, the olivine- 

 basalt, which we are obliged to couple with the rhyolite under 

 No. 3. The existence and relations of this basalt in the Sierra 

 Nevada seems to be well established. The second difficulty 

 arises from the fact that the rhyolite No. 3 is of exactly the same 

 acid type as the rhyolites under 1 and 5. From this fact the 

 idea originates that rhyolite No. 3 may be also a recurrent lava,, 

 and that we have in the whole volcanic succession portions of 

 three instead of two cycles of differentiation. 



On testing this hypothesis we are struck with the fact that 

 andesites 2 and 4 have identical phases, although the groups as 

 a whole differ as stated in the above list. At Eureka the earlier 

 and later andesites were held to be separate, the earlier ones 



