LA VAS IN THE GREA T BASIN REGION 639 



Under No. 5 we find the very basic olivine-basalts and very 

 siliceous rhyolites or tordrillites, erupted at the same period and 

 evidently connected by the closest ties. This, for example, was 

 observed to be the case in the Pleistocene volcanics of the 

 Meadow Valley Canyon, and the same is true in the basin of Lake 

 Mono, according to Russell. 1 



These closely allied ultra-basic and ultra-acid lavas are plainly 

 complementary forms, and are proofs of differentiation as con- 

 vincing as are the complementary segregations so familiar in 

 single masses of intrusive or plutonic rocks. 



Going a little further, if we write Nos. 3 and 4 of this last 

 succession together and precede it by No. 2 (which we may 

 divide into two members) we have the following grouping : 



Medium andesite. 



Acid andesite and dacite. 



3. ( Acid rhyolite (with basalt). 



4. { Medium basic andesite. 



\ Basalt. 



I Acid rhyolite. 



We have here, therefore, a series (apparently conformable to 

 Iddings' law) which begins with a rock of intermediate compo- 

 sition and progresses to extremes, as a result, probably, of dif- 

 ferentiation. 



But the first member in the order of succession as interpreted 

 by the writer, viz., the basal rhyolite, is apparently out of place 

 in this scheme. This rock has a composition essentially like the 

 later rhyolite, but appears to have no immediate connection, 

 mineralogically or chemically, with the andesites which form the 

 base of the Eureka lavas. The andesites can hardly be derived 

 from the rhyolites by any hypothetical differentiation, and even 

 in that case the order seems in direct opposition to all of the 

 hitherto propounded laws of succession. 



It was first, therefore, the conclusion of the writer that the law 

 deduced by Iddings would not hold good in the Great Basin, on 



1 Quaternary history of Mono Valley, California : Eighth Ann. Rept. U. S. Geo). 

 Surv., Part I. 



