284 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



colored breccia. The northern face of Avalanche Peak is composed of 

 imbedded breccia, while the breccia of the northwestern spur is bedded. 

 At the northern end of the summit of the peak it is variegated and full of 

 hornblende phenocrysts (1516). Hornblende-andesitic breccia forms the 

 main mass of the peak southeast of Avalanche Peak, and extends along the 

 crest of the mountain ridge eastward between Crow and Middle creeks. 

 Here it rests upon the basalt sheets already mentioned. South of the peak 

 on this ridge, If miles west of the one hundred and tenth meridian, it is 

 well exposed in bare bluffs and spurs. It is light-colored, variegated, and 

 well-bedcled breccia, mostly hornblendic, some fragments having abundant 

 pyroxene phenocrysts (1528, 1529). 



It is overlain by massive laminated andesite, which is rudely columnar, 

 and forms the high peak just mentioned. This mass was undoubtedly a 

 surficial flow over an uneven surface of breccia which sloped to the north 

 and also to the south at this point. The mass is at present about 400 feet 

 thick, and at its base is dense, gray, and crystalline, with no prominent 

 phenocrysts (1614). Farther east on the crest of the ridge, and 400 feet 

 lower, the same massive andesite forms a capping to the breccia. It is 

 laminated and finely columnar, having large vertical columns in the middle, 

 and smaller ones beneath, with irregular parting at the top (1615). Near 

 it the breccia consists of similar pyroxene-andesite. It can be seen from 

 this ridge that light-colored breccia forms the crest of the ridge north of 

 Crow Creek and overlies the basic breccia and basalt sheets constituting: the 

 base of the ridge. The light-colored breccia is in turn capped by massive 

 columnar lava on Silver Tip Peak and on the next peak east. Another 

 remnant of massive andesite rests on the breccia at the head of Crow Creek. 

 It is about 200 feet thick, is jointed horizontally, and consists of hornblende- 

 andesite with inconspicuous phenocrysts (1512). 



From the summit of the ridge north of Middle Creek it also apjjears 

 that the mountain mass south of this creek is similarly constructed, its 

 lower portion of basic breccia, with basalt sheets, forming high, flat-topped 

 spurs about 1,000 feet above the valley. The upper portion consists of a 

 high ridge of light-colored, well-bedded breccia, with a number of pinnacles 

 on its northern slopes, the pinnacles being beautifully columnar, dark-colored 

 rock. The southwestern end of the upper part of this ridge has been visited, 

 and consists of light-colored breccia of hornblende-andesite. The high peak 



