BASIC BBECCIA OF MOUNTAIN CEEEK. 297 



sometimes with hornblende, sometimes with olivine (1638, 1639). It caps 

 the ridge to the southeast and forms its western spur (1641, 1642), and also 

 a small point on the southern spur of Coulter Peak. The upper basic 

 breccia becomes thicker and more extensive about the southern end of the 

 area of earlier breccia. At Mount Humphreys and the peak northwest it 

 is about 600 feet thick, while on Table Mountain and the Turret it is about 

 800 feet thick. It constitutes the mass of Eagle Peak. 



The breccia on the summit of Table Mountain is dark colored and is 

 composed of comparatively small fragments of basaltic andesite, with small 

 phenocrysts (1668). The layers of this mass dip gently toward the west. 

 The mass of Eagle Peak, however, presents a wholly different aspect (PI. 

 XXXV). The breccia forming this mountain is distinctly bedded, but the 

 layers are not horizontal, dipping at various angles and forming a slight 

 syncline, the dip being to the southeast, away from the light-colored 

 breccia at the head of Mountain Creek and toward the northwest from the 

 southern end of the mountain. The breccia is more slaggy and scoriaceous, 

 with less tuff than the beds on Table Mountain, and the mineralogical 

 character of the rock is more uniform. It is dark colored, mostly pyroxenic 

 with much olivine, and in places carries some hornblende. There are dense, 

 porous, and vesicular modifications of the rock (1669 to 1676). In places 

 it has been partially altered, producing layers and patches which are bright 

 red, brown, and yellow, with purple, lavender, and green. The alteration 

 seems to have followed certain layers, but was not confined to them. That 

 there is considerable divergence in the bedding of the breccia may be seen 

 from the eastern side of the mountain. The lower beds dip at steeper angles 

 than the upper ones, the general dip being to the northwest, One massive 

 bed was observed to thin out upward toward the southeast, and to thicken 

 and separate into several beds toward the northwest. 



The mountain range east of the northeastern branch of Mountain 

 Creek is composed of steeply dipping layers of similar dark breccia, in 

 places brightly colored by decomposition. The dip in general is toward 

 the northwest, The high northern portion of this range is composed of 

 nearly horizontal beds of dark breccia. North of this range the mountains 

 are made up of similar breccia in nearly horizontal layers, which occa- 

 sionally dip steeply toward the north, and thicken in the steep parts of the 

 layers. 



