EARLY BASIC BRECCIA AND FLOWS. 279 



the head of the southwestern branch of Middle Creek, and on the other 

 side of the divide at the head of Rocky Creek, beneath more recent light- 

 colored acid breccia. 



On the southern side of Sylvan Pass breccia of pyroxene-andesite is 

 highly indurated by the many dikes that traverse it, and the pyroxenes are 

 more or less uralitized. At the forks of Middle Creek, east of this, it is 

 full of large phenocrysts of pyroxene and resembles much of the basalt of 

 the Crandall Basin, and at Signal Point and near Park Point, on Yellow- 

 stone Lake, there are basalts and breccia of this type (1616-1619). Signal 

 Ridge and the mass of Grizzly Peak are composed of pyroxenic breccia 

 without prominent phenocrysts, with some olivine and little or no horn- 

 blende, while hornblende appears in the breccia at the summit of Grizzly 

 Peak (1521-1525). 



An isolated exposure of basaltic rocks belonging to this series occurs at 

 the head of the southeastern branch of Beaverdam Creek and just north of 

 Coulter Creek. At this place there are two horizontal sheets of porphyritic 

 basalt, one upon the other, and over them is a light-colored tuff of trachytic 

 rock, with many inclosed fragments of basalt and andesite. This tuff cor- 

 responds to the trachytic tuff in the neighborhood of Junction Butte. 



Almost precisely similar basaltic lavas and trachytic tuff occur at Two 

 Ocean Pass, beneath the basic andesitic breccia which forms Two Ocean 

 Plateau, and though the exact period of eruption of these basaltic lavas, as 

 compared with the basalts of the Crandall volcano, is a matter of uncertainty, 

 still on purely petrographical grounds they may be described in this con- 

 nection. 



On the northern side of Two Ocean Pass there is a ledge of basaltic 

 rocks composed of five sheets resting directly one on another. The basalt 

 of the different sheets varies somewhat and belongs to the group of sho- 

 shonites. That of the top one is a dark, dense rock, with rather small 

 phenocrysts of feldspar, augite, and decomposed olivine (1715). The top 

 surface is red, the middle of the flow dense, and the bottom somewhat 

 vesicular. The top has probably been eroded. The sheet below is similar, 

 with numerous large feldspars and altered olivines (1716). It is vesicular 

 in the upper portion, dense in the middle, and slightly vesicular at the bot- 

 tom, and is 25 feet thick. Beneath this is a layer of scoria 2 or 3 feet thick, 

 having the character of the underlying flow, which is full of large feldspars, 



