RHYOLITE8 AND DACITES. 



45 



these tongues have sometimes penetrated a considerable distance, and there the 

 lava forms dikes, sometimes thinning to a very great degree or showing only in 

 occasional outcrops as "'intermittent" dikes. The lava in these dikes is glassy, 

 like the contact phase of the main mass. 



Included basalt. Inclusions of basalt were found in the Brougher dacite in 

 several places. In two places dikes sent off from the Golden Mountain neck 

 along fault planes contain augite-hornblende-basalt, like that in place on Siebert 

 Mountain. Besides augite and hornblende, anorthite and labradorite-bytownite 

 feldspar were recognized in these inclusions. At the north base of Butler Moun- 

 tain, within the conspicu- 

 ous hollow there, the da- 

 cite is packed full of in- 

 clusions of similar basalt. 



Vestiges of cinder 

 canes. At various points 

 around the base of Butler 

 Mountain, close to the in- 

 trusive neck, is a coarse 

 volcanic agglomerate of a 

 kind not seen at any 

 greater distance from the 

 mountain. It consists of 

 large angular blocks of 

 volcanic rocks, alternating 

 with finer breccia and ash. 

 On the south side of But- 

 ler Mountain this material 

 has a thickness of about 200 feet, and contains bowlders up to 5 feet in diameter. 

 These bowlders consist of lava resembling the Tonopah rhyolite-dacite. Imme- 

 diately adjacent to the intrusive Brougher dacite contact on this side of the 

 mountain there was noted a bowlder of similar Tonopah rhyolite-dacite that was 

 30 feet in diameter and lay in the Sietiert tuffs, as if it had dropped into them 

 when they were soft mud. On the north side of the mountain similar agglomer- 

 ates were observed, and here the blocks were chiefly of glassy dacite. This 

 indicates an accumulation of volcanic cinders and bombs, and their localization 

 around the base of Butler Mountain shows that this was the site of a cinder 

 and bomb cone, which was built up as a result of the resumption of volcanic- 

 activity at the close of the tuff period and perhaps following the slight basaltic 

 eruptions (manifested within the area mapped only on Siebert Mountain). On 



10 feet 



FIG. 5. Vertical section showing contact of the Golden Mountain dacite, glassy 

 along the margin, with Siebert tuff (lake beds). Location due east of Golden 

 Peak. Dotted outlines indicate a prospecting pit sunk in the tuff at the 

 contact. 



