306 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



three dikes with a southerly trend. One of these dikes consists of horn- 

 blende-andesite with many phenocrysts of hornblende and feldspar. The 

 rock is columnar, with horizontal prisms. 



In the gulch west of Avalanche Peak the breccia of hornblende- 

 andesite is indurated, and is cut by bodies of hornblende-andesite that have 

 been much fractured and cracked. Similar indurated breccia forms the 

 ridge west, and is traversed by ten or more dikes of hornblende-andesite 

 with variable trends, mostly north and south or northeasterly. On the 

 west end of this ridge are other dikes, whose trends were not observed. 



Other dikes of hornblende-andesite cut the ridge from Avalanche Peak 

 to Mount Chittenden. They have a general north-south trend, as indicated 

 on the map, several trending northeast and one east. Dikes may be seen 

 cutting the southern face of Silver Tip Peak. At the head of Crow Creek 

 several of the dikes traverse the sheet of massive hornblende-pyroxene- 

 andesite, cutting it in different directions. 



These dikes extend into the head of Jones Creek, the most northerly 

 intersecting the eastern edge of the summit of Mount Chittenden. Here 

 this dike is 50 or 60 feet wide, is horizontally columnar, and trends a little 

 east of north. The rock is hornblende-andesite (1498 to 1500), dense, fine 

 grained, with abundant small phenocrysts of hornblende, sometimes in 

 stellate groups, and less prominent feldspar. No dikes were observed in 

 the mountains northeast of Mount Chittenden, or in those in the vicinity 

 of Pyramid Peak, or in Castor and Pollux peaks. 



In the region of Sylvan Pass, the mountain mass of Hoyt Peak 

 consists of indurated breccia of hornblende-andesite with some pyroxene- 

 andesite. It is traversed in various directions by dikes, some trending 

 northeast and east: and others southeast. Those cutting the northern 

 crest of the mountain are mostly hornblende-andesite. They are from 

 4 to 10 feet wide, and in one instance 20 feet. The rock of the 20-foot 

 dike (1535) is dense, light gray, with a hackly fracture, and is crowded 

 with small phenocrysts of minute feldspars and larger hornblendes, with a 

 very little mica. One of the andesite dikes is very fine grained, with few 

 phenocrysts. The dikes on the south slope are largely covered by talus. 

 Their general trend is toward the southeast. Among them, near the bottom 

 of the slope, are several dikes of quartz-mica-dacite, with phenocrysts of 

 these minerals Farther down the slope, just east of the divide, there is 



