LA VAS IN THE GREA T BASIN REGION 64 1 



being more siliceous and not approaching the later ones more 

 nearly than by 2.25 per cent, of silica. 1 At Washoe, however, 

 the pyroxenic andesites, which precede the more siliceous ande- 

 sites representing at Eureka the earlier group, become equally 

 basic with the andesites of the second period. 7n other portions 

 of the Great Basin also the andesites belonging to the first and 

 second periods are often indistinguishable. 



On studying the general succession, as partially set forth in 

 the table of correlations, we find that the break between Nos.. 

 3 and 4 in the succession is as abrupt as that between Nos. 1 and 

 2. On the other hand, between 2 and 3 there are many transi- 

 tional phases, and also between 4 and 5. 



We may represent the facts above noted, graphically, as fol- 

 lows : 



1. Rhyolite. 



2. Andesite. 



3. • Rhyolite (and basalt). 



4. Andesite. 



5. Basalt (and rhyolite). 



The break between 3 and 4 was noted by Mr. Hague at 

 Eureka. 2 He ascribed it to a change of magmas and argued 

 that the first members of the succession at Eureka, beginning 

 with hornblende andesite and ending with acid rhyolite, were 

 derived from a magma distinct from that which produced the 

 later members of the succession, beginning with pyroxene ande- 

 site and ending with basalt. In the first group Mr. Hague 

 found, between the andesite and the rhyolite, gradual transitions 

 which grew continually more siliceous ; likewise in the second 

 group he found gradual transitions between the pyroxene ande- 

 site and the basalt. This implies two distinct processes of differ- 

 entiation, the first of which proceeded from intermediate to 

 acid, while the second followed the opposite order, from interme- 

 diate to basic. Mr. Hague's interpretation of the development of 

 lavas at Eureka may be graphically represented as follows : 



'Monograph XX, U. S. Geol. Surv., p. 269. 



2 Monograph XX, U. S. Geol. Surv., pp. 254, 269, 270, 271. 



