282 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the escarpment there is much gray ash with leaf impressions (1478, 1479, 

 1480). 



Similar light-colored breccia extends south and west and forms the 

 upper part of the mountain ridge through Cathedral Peak, lying between 

 Cold Creek and Jones Creek. It constitutes the main mass of Mount 

 Chittenden and the low mountains north to Raven Creek, and beyond to 

 Pelican Cone and the ridge west (1160, 1161). 



In these low mountains the character of the andesites constituting the 

 breccia varies considerably, and acid and basic andesites are often 

 intimately mingled, many fragments being hornblendic, while others are 

 wholly pyroxenic, However, it is evident in numerous places that the 

 more siliceous and hornblendic andesites predominate in the lower parts of 

 the mass, and are overlain by distinctly later accumulations of basic andesite. 

 Thus, at the falls on Raven Creek, at about the 8,200-foot contour of the 

 map, and within a short distance of the limits of the older basalt, horublende- 

 andesitic breccia is exposed in indurated beds which carry rounded bowlders 

 of the same rock. It is also found immediately south of this point on the 

 summit of the ridge, where it is capped by dark-colored basic breccia, 

 which is well bedded in places. 



On the southern side of the meadow at the head of Mist Creek and 

 near the limits of the older basalt, compact breccia of hornblende-andesite 

 is exposed in such a manner as evidently to be over the basalt, although an 

 immediate contact was not discovered. Similar breccia, not so indurated, 

 however, forms the ridge south of this locality. Many of the fragments 

 are light colored, with phenocrysts of feldspar and hornblende ; others are 

 darker, have less noticeable phenocrysts, and are more pyroxenic (1483). 

 At the knob about the middle of this ridge the hornblendic breccia is 

 capped by a remnant of dark-red basic breccia, the line of contact between 

 the two being plainly visible (1484). Similar relations exist between horn- 

 blendic breccia and basic breccia on the ridge across the head of Mist and 

 Cold creeks (1485, 1486). The various altitudes at which the overlying 

 basic breccia is found indicate a very irregular surface for the previously 

 accumulated hornblendic breccia. 



The breccia west of Raven Creek forms the mass of Porcupine Cone, 

 which is hornblende-pyroxene-andesite at its summit (1159), mostlv light 

 colored. West of the mud springs on Pelican Creek similar andesite occurs, 



