642 J. E. SPURR 



('Hornblende Andesite. 



Feldspathic Magma 



Series growing gradually more siliceous. 



V 



[Acid Rhyolite. 



f Pyroxene Andesite. 



Pryoxenic Magma A 



Series growing gradually more basic. 



[Basalt. 



The general succession in the Great Basin region, as set forth 

 in the table which we have made opposite this page, corre- 

 sponds to Mr. Hague's conception (leaving out of the question 

 the rhyolite No. 1, which is not exposed in the Eureka district). 

 The relations of the lavas of the whole region, therefore, omit- 

 ting the minor exceptions, might be represented as follows: 



1. Rhyolite. 



Break. 



2. Andesite. 



Gradual transitions. 



V 



3. Rhyolite. 



Break. 



4. Andesite. 



Gradual transitions. 



V 



5. Basalt. 



Nevertheless, the exceptions are frequent enough to demand 

 recognition. Observations at several points outside of the 

 Eureka district prove that basalt No. 5 has a closely associated 

 rhyolite which is plainly complementary. Similarly, rhyolite 

 No. 3 has in the Sierras an associated olivine-basalt, which is 

 also probably complementary. Upon looking carefully, we find 

 that there are complementary phases for the stages intermediate 

 between the andesite No. 4 and the basalt No 5 ; and we become 



