594 J' E- SPURR 



the southern end of the Virginia range by a comparatively nar- 

 row valley. The lavas of one range are evidently continuous 

 with those of the other/ In the Washoe district have been 

 found eruptions of hornblende-mica-andesite and quartz-ande- 

 site or dacite, corresponding exactly to that at Mason Butte. 

 At Washoe, also, the andesite becomes, under favorable condi- 

 tions, coarsely crystalline, and Messrs. Hague and Iddings ^ 

 have noted that upon this complete crystallization quartz sepa- 

 rates out, producing a mica-quartz-diorite. 



RHYOLITIC ROCKS. 



TRANSITIONS IN TEXTURE OF THE BASAL RHYOLITES OF THE 

 PINENUT RANGE. 



Field description. — The central core of the Pinenut range is 

 made up of granitic rocks. These were examined in two locali- 

 ties : one southeast from Dayton, and one west from Wellington. 



At the first mentioned locality, granitic rocks are exposed 

 along an easterly facing scarp which is at the north end of Smith 

 Valley. The rocks are all granular, so far as observed, but 

 vary much in texture from fine to coarse ; they are often por- 

 phyritic. They show a distinct banding, resembling at a dis- 

 tance rude stratification ; this bandings is due to a zonal arrange- 

 ment of the rocks of different textures. 



In the district west from Wellington, rhyolitie and granitic 

 rocks are exposed in the spur just east from the Mountain 

 House, and are of exceptional interest. Here, beneath the 

 andesitic rocks which cover the mountain slopes, is found a 

 highly indurated volcanic conglomerate and sandstone, appa- 

 rently waterlaid, and consisting entirely of rhyolitie material. 

 A short distance farther on, the rocks from which these detri- 

 tals are derived were found in place. These original rocks show 

 great variations, passing from a fine-grained, almost aphanitic, 



' See " The Succession and Relation of Tertiary Igneous Rocks in tlie Great 

 Basin Region," Jour. Geol., Vol. VIII, p. 621. 



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



