286 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



continuous and not interrupted. Occasionally one passes into the other 

 horizontally near the line of contact. The dark upper breccia consists of 

 basic andesites with more or less hornblende in phenocrysts. The greater 

 number are very dark, with minute phenocrysts of feldspar and pyroxene 

 (1638, 1639). 



Light-colored breccia forms the ridge southwest of this mountain, 

 through Coulter Peak, being overlain along the crest b}^ a horizontal layer 

 of basic breccia. The light-colored breccia passes beneath basic breccia at 

 the eastern base of the peak south of Beaverdam Creek, the lower breccia 

 being pink and hornblendic, the upper breccia dark colored and pyroxenic, 

 with a little hornblende and some olivine (1642, 1643). It is distinctly 

 bedded, and dips about 20° W. At the head of the south fork of Beaver- 

 dam Creek the light-colored breccia overlies two superimposed sheets of 

 basalt, over which is a deposit of trachytic breccia, already referred to. The 

 petrographical character of these basalts and the trachyte, as well as their 

 geological position with respect to the hornblende-andesite, relates them 

 to the older basaltic period of the Crandall volcano, though they probably 

 were emptied from some other center. 



The light-colored breccia continues around the eastern head of Trappers 

 Creek, from Mount Humphreys to Table Mountain and the Turret. At 

 the head of Mountain Creek, northwest of Eagle Peak, the light-colored 

 breccia consists of light-gray tuff and various-sized fragments of horn- 

 blende-andesite (1655 to 1660). These andesites differ in color and habit, 

 a few having large phenocrysts of hornblende. There is occasionally some 

 mica. In places the rock over considerable areas is all of the same mate- 

 rial, though apparently brecciated, solid angular masses being- cemented 

 by crumbling material of the same character. There are also large masses 

 of somewhat fissile andesite, but nothing resembling intrusive rock in place. 

 Similar andesite forms the northern spur of the peak at the head of Mountain 

 Creek, and is well exposed in the ridge west of the creek to Table Mountain. 

 It is but rudely bedded, and is capped by well-bedded, dark, basic breccia, 

 which forms the plateau of Table Mountain. It is also overlain by patches 

 of basic breccia on the peak at the head of Mountain Creek, and passes 

 beneath the mass of Eagle Peak. 



At the southern end of the ridge of Table Mountain the light-colored 

 breccia is well exposed. It is like that farther north and is highly varie- 



