DIKES NEAR SYLVAN PASS. 305 



In the valley of Sylvan Pass and on the slopes of the mountains on 

 both sides the breccias have been highly indurated by the intrusion of dikes 

 close to one another, and of larger bodies of magma. This is particularly 

 the case on the southern side of the valley. This metamorphism is limited 

 in extent north and south, but continues a greater distance east and west. 

 The induration of the breccia and the presence of massive rocks produce 

 mountain slopes very different in character from those of the breccias of the 

 region. The indurated and massive rocks weather in small angular frag- 

 ments that form long, bare talus slopes, giving to the southern flanks of 

 Avalanche Peak and of the mountain southeast a smooth, light-colored 

 appearance, which is quite distinctive. 



The southern slope of Avalanche Peak has the character just noted. 

 It consists of indurated breccia of hornblende-andesite, traversed by intru- 

 sive bodies of hornblende-mica-andesite and of hornblende-andesite. Some 

 of these bodies have been crushed and fractured so as to resemble breccia 

 in places. They furnish evidence of dynamic movements. One body of 

 hornblende-mica-andesite (1532), whose outline is obscured by talus, is 

 altered considerably. The phenocrysts of mica are large, as are also some 

 of hornblende and feldspar It is liner grained and fissile near the south- 

 ern contact wall, and is distinctly mottled in planes parallel to this contact, 

 and carries inclosed fragments of other rocks. The southern end of the 

 summit of the mountain is formed of a broad dike of hornblende- 

 andesite (1533), compact and gray, with abundant small phenocrysts of 

 hornblende and feldspar. The trend of this dike is east and west, and that 

 of three or four others cutting the summit of the peak is northwesterly. 

 These are hornblende-andesite. Another dike on this summit, trending 

 south and north, is hornblende-mica-andesite (1534). It is greenish gray 

 and dense, with a multitude of small phenocrysts — white and prominent 

 feldspars, altered hornblendes, and fresh biotite. 



The northern end of the summit consists of massive rock with horn- 

 blende and pyroxene phenocrysts. The northern mass of the peak is com- 

 posed of chaotic breccia of hornblende-andesite not specially indurated. 

 Six dikes cut the northwestern spur, five having a general east-west trend, 

 and one, lower down, a north-south trend. Three dikes cut the north- 

 eastern spur. They have a northeast trend. On the eastern spin- there are 



mon xxxir, pt if 20 



