ACID BEECCIA OP MOUNT OHITTEHDEK. 283 



that on the top of the ridge being hornblende-mica-andesite (1160, 1161). 

 Resting upon the great sheet of basalt on top of Mirror Plateau are areas of 

 andesitic breccia with light-gray tuff. In the vicinity of Mirror Lake the 

 light-gray tuff is composed of bits of hornblende-mica-andesite (1158), bio- 

 tite not being very plentiful. Near the falls on Raven Creek the tuff is gray 

 and fine grained (1157), and consists of particles of hornblende-pyroxene- 

 andesite with much hornblende. The light-colored breccia of Mount Chit- 

 tenden is bedded in its northern spur, and carries hornblende and some 

 biotite. The andesitic fragments are subangular; many are glassy and 

 light colored; some are pyroxenic and vesicular (1487). Similar breccia 

 forms the western spur (1492). At the northern part of the summit of the 

 peak the breccia contains many large masses of hornblende-andesite with 

 abundant hornblendes (1488 to 1497). The acid breccia in the eastern 

 base of the mountain summit and around the head of Jones Creek is without 

 bedding and occurs in irregularly accumulated bodies. It is overlain by 

 dark-colored, well-bedded basic breccia, consisting of small angular frag- 

 ments. This is distinct along the crest of the ridge east toward Cathedral 

 Peak and on the southern slope of the summit of Mount Chittenden and 

 along the crest of the ridge to the south and southeast. A broad dike of 

 hornblende-andesite-porphyry cuts the southern summit of Mount Chitten- 

 den, and will be described with other dike rocks. 



The character of the upper basic breccia is basaltic on the summit of 

 Mount Chittenden (1504 to 1507), without prominent phenocrysts. Farther 

 south, beyond the pass, it is dark pyroxene-andesite (1514). The under- 

 lying imbedded breccia at the top of the ridge south of Mount Chittenden 

 is mostly pyroxene-andesite (1513, 1515) with little or no hornblende. It 

 extends down the long spur westward to Lake Butte, where it overlies mas- 

 sive hornblende-mica-dacite (1509, 1510). This rock is light colored, gray 

 and red, with abundant small phenocrysts of quartz, feldspar, hornblende, 

 and biotite. It is extremely porous and appears to have been a surficial 

 lava. 



Hornblende-andesitic breccia forms the ridge around the head of Crow 

 Creek to Avalanche Peak and its western spur, which lies north of Clear 

 Creek. Just north of Avalanche Peak the breccia is dark colored and 

 hornblendic, without bedding, and carries large blocks of hornblende- 

 andesite from 3 to 8 feet long (1511). Northward it passes into lighter- 



