628 /. E. SPURR 



transitions from typical fine-grained extrusives to porphyritic 

 and to coarse granular intrusive rocks are highly interesting in 

 themselves and by comparison with the similar phenomena 

 which Messrs. Hague and Iddings 1 have found in the -andesite 

 and basalts of the Washoe district. The present writer has also 

 found similar transitions, to be described elsewhere. 



SUCCESSION OF LAVAS AT OTHER POINTS IN THE GREAT BASINS 



During the past season's field work the writer has observed 

 the succession of lavas at many different points in the Great 

 Basin. From the nature of the work the time for study was in 

 each case very restricted, so that the records given below are 

 sometimes very likely incomplete. 



Pine Nut Range . — The Pine Nut Range is interesting on account 

 of lying immediately south of the Virginia Range and the 

 Washoe district, and because the lavas of this district are easily 

 recognizable in it. The range was crossed at two points, one 

 east from Dayton and one east from Genoa. The succession of 

 lavas appears to be as follows : 



i. Rhyolite (intimately connected and probably contemporaneous with 

 granite of similar constitution). 

 Rhyolite sands and conglomerates (formed during long period of 

 erosion). 



2. Hornblende-pyroxene-biotite andesite (in portions sufficiently removed 



from the surface the typical glassy or microcrystalline groundmass 

 of the lava becomes coarser, leading to the formation of diorite 

 porphyry and monzonite porphyry). 



3. Hornblende-mica andesite. 



Period of denudation. 



4. Rhyolite (Shoshone Lake period?). 



5. Hornblende-pyroxene andesite, tuffs, and breccias (Shoshone Lake 



period). 



6. Rhyolite. 



7. Basalt (Pleistocene). 



Of these extrusives the first appears to have been greatest in 

 amount and the latter ones in general progressively less and 

 less, in the order of their arrival. 



1 Bull. 17, U. S. Geol. Surv. 



