644 /. E. SPURR 



f Pyroxene-hornblende andesite. 



/ \ 

 Pyroxene andesite. Hornblende-pyroxene andesite. 



| Hornblende-pyroxene-biotite Augite-biotite-hornblende latite. 



4.^j aleutite. / \ 



Hypersthene-hornblende Biotite-augite-hornblende quartz- 



aleutite. / latite. \ 



Pyroxene aleutite. Biotite-hornblende dacite. 



. / \ 



Pyroxene olivine-aleutite. Hornblende-biotite latite. 



/- \ 



r Hornblende basalt. Biotite-hornblende rhyolite. 



/ \ 



t>.\ Augite basalt. Biotite rhyolite. 



r / x 



L Olivine-basalt. Ultra acid rhyolite or 



TORDRILL1TE 



It therefore appears that we have the representatives of two 

 complete cycles of differentiation, and probably the end of a 

 still earlier cycle. During the first completely recorded cycle 

 (beginning with the earlier andesites) the more siliceous rocks 

 were continually extravasated in preference to the more basic 

 complements, so that the acid-growing series is far more promi- 

 nent in the record than the basic-growing one. On the contrary,, 

 during the second or last complete cycle the more basic rocks 

 were continually thrown out in preference to the more siliceous 

 ones, making the basic-growing series more prominent. The 

 laws which govern these apparently eccentric preferences of 

 eruption are yet unknown. 



I have looked in vain in the field for the earlier representa- 

 tives of the first cycle of differentiation, to which No. I belongs. 

 If they were erupted previous to the appearance of the rhyolite 

 (No. 1) they have not yet been determined. 1 



1 What may possibly be one of the members of the earlier differentiation cycle, 

 older than the rhyolite, occurs in the Walker River Range. Here the granite and alas- 

 kite, which are considered by the writer as probably the deeper-seated equivalents of 

 the basal rhyolite of the Great Basin section, are evidently younger than and at times 

 intrusive into a great body of biotite-hornblende quartz-monzonite. A similar horn- 

 blende-biotite quartz-monzonite was found east cf the Walker River Range, in Gabb's 



