RHYOLITES AND DACITES. 



ODDIE RHYOLITE. 



Location. A white siliceous rhyolite makes up Mount Oddie (PI. IX, B) and 

 Rushton Hill, and extends irregularly in .spurs and lobes away from their bases. 

 A similar rhyolite occurs on the summit in an irregular area at the northwest 

 base, but not on the slopes of Ararat Mountain, and in small patches around the 

 north base of Brougher Mountain. 



Contact phenomena of Oddie- Rushton neck. By the same method of reasoning 

 applied to the Brougher dacite necks, the conclusion is reached that Mount Oddie 

 and Rushton Hill are also the necks of ancient volcanoes. On Mount Oddie and 

 Rushton Hill the rhyolite is intrusive. At man}- points along the contact there 

 is a vertical flow structure in the rhyolite and a platy structure parallel to it. 

 The rhyolite of Rushton Hill, 

 at its contact with the later 

 andesite near the Rescue 

 shaft, dips at an angle of 45 : 

 to 60 C away from the hill. 

 The Rescue shaft passed into 

 this rhyolite and has con- 

 tinued in it several hundred 

 feet up to the time of latest 

 information. 



Near the contact the 

 rhyolite is frequently gl;i>s\- 

 and resembles the Tonopah 

 rhyolite-dacite; it has also 

 been silicitied in many places 

 subsequent to its eruption. 



The rhyolite of Oddie 

 and Rushton hills sends out 

 irregular lobes into the surrounding rocks, which by reason of their superior 

 hardness, as compared with the intruded later andesite, form ridges. These also 

 are characterized by vertical flow lines and platy structure. As a whole the 

 intrusion is elongated in an east-west direction, parallel to the previously noted 

 elongation of the dacite necks. 



Contact phi'wmiena of Ararat neck. The slopes of Ararat Mountain are 

 formed by the Tonopah glassy rhyolite-dacite, which has already been described. 

 The top, however, is of rhyolite like that of Mount Oddie, and the contact 

 between the two is sharp. The white rhyolite at the top of the mountain has 

 a roughly circular outline. At its margin it is brecciated, sometimes profoundly, 

 16843 No. 4205 4 



Scale 

 to 



20 -feet 



FIG. 8. Vertical sketch section taken at a point on the east side of Butler 

 Mountain, 100 feet below the contact of Butler dacite and tuff, showing 

 dike of light-brown, semi-consolidated sand, of volcanic origin, containing 

 angular fragments of dacite glass, intrusive into Siebert tuffs (lake beds). 



