320 GEOLOGY OP THE YELLOWSTONE NATIONAL PAEK. 



hornblende has a narrow border of magnetite grains. This bodj^ of andesite 

 is columnar and has probably cut up through the breccias, its plane of 

 contact on one side trending southwest and dipping steeply to the southeast. 

 It is exposed in an easterly escarpment, where the massive andesite can be 

 seen cutting irregularly across the layers of breccia. It is also well exposed 

 in a southerly escarpment. In the eastern escarpment there is an inclined 

 sheet or dike of similar rock, which cuts up through the breccia at an angle 

 of about 20° and thins out. It appears to be an offshoot from the large 

 body. Directly east of this peak, on the ridge north of Coulter Peak, there 

 is another body of massive andesite of the same kind Its plane of contact 

 with the underlying upper basic breccia is steep and trends toward the 

 northeast. It is also columnar along its western exposure. Near it at the 

 south end is a small intruded seam or dike of similar rock, which is wholly 

 glassy (1607, 1608). It is yellowish brown near the margin and dark 

 colored at the middle. It cracks into rounded lumps and has a perlitic 

 structure on a large scale. It is a beautiful yellow glass in thin section, 

 with well-defined microlites of prismatic and of less numerous tabular feld- 

 spars, delicate prisms of pyroxene, and grains of magnetite. The very 

 small phenocrysts are brownish-green hornblende without dark border, 

 hypersthene, augite, and labradorite ; in all respects like the glassy portions 

 of the rocks just described. 



Massive andesite of the same character occurs as intrusive bodies in 

 the valley of Mountain Creek. The largest is at the mouth of the valley 

 in the base of the mountain on the north side. Its boundary is obscured 

 by soil and deljris. It cracks into plates and is compact and light gray, 

 with a few thin crystals of hornblende (1609). The rock is a fine-grained 

 aggregation- of feldspar crystals, quite like the more highly crystallized 

 portion of the rock of Coulter Peak, except that there are fewer prisms and 

 more irregularly shaped grains. ■ The component minerals are the same, 

 and the few small phenocrysts of green hornblende and brown biotite 

 inclose much magnetite. 



Several small intrusive bodies cut the breccia east of the mouth of the 

 north fork of the creek. One is a narrow vertical dike. It is light gray, 

 with a few thin crystals of hornblende, and is slightly vesicular. It 

 becomes dark colored and glassy near the contact, and- is brecciated in 

 places (1612). Parts of it are compact, reddish brown and mottled, but 



