ACID BRECCIA OF MOUNT LAXGFOED. 285 



on this ridge 4 miles northeast of Mount Langford, when seen from the 

 southwest, appears to consist of indurated light-colored breccia, traversed 

 by numerous dikes. The same ridge a mile southwest of this point is cut by 

 ten dikes within a distance of a mile, and the breccia composing the ridge 

 at this place is hornblende-andesite, with abundant hornblende in most cases, 

 and often with very light-colored tuff. The dikes of this vicinity will be 

 described in connection with the great system of dikes at Sylvan Pass. The 

 light-colored hornblendic breccia, which has been traced from its northern 

 limit in the neighborhood of Lamar River, continues southward as far as 

 Mountain Creek. It forms the mountain ridges around the southern head 

 of Middle Creek, being well exposed in precipitous amphitheaters on either 

 side of Mount Langford. The rude bedding of the mass east of the latter 

 peak is about horizontal. The character of the breccia in places is very 

 variable, but is distinctly hornblendic (1634, 1635). Mount Langford con- 

 sists of nearly horizontal beds of light-colored breccia, and the same is 

 true of its northeastern spur and the ridge west to Mount Doane and north 

 around the head of Middle Creek. On the summit of Mount Langford the 

 breccia consists of andesites of various colors and habits, with small or larsre 

 phenocrysts, mostly hornblendic. Some fragments are pyroxene-andesite 

 (1632, 1633). On the west slope of Mount Doane, above the saddle, the 

 light-colored hornblende-andesitic breccia contains masses of hornblende- 

 mica-andesite, very light gray and pink, with abundant phenocrysts (1636, 

 1637). This breccia also forms the southern base of Mount Stevenson, the 

 bedding being about horizontal. 



The mountain ridge from Mount Stevenson as far as the Yellowstone 

 Lake is composed of dark-colored breccia, in part hornblendic, but mostly 

 of basic andesite (1623, 1624). The audesitic breccia in the ridge between 

 Rocky and Beaverclam creeks is similar to that of Mount Langford ; light 

 colored, in places dark; largely composed of hornblende-andesites of 

 various habits, together with some basic pyroxene-andesites (1625 to 1631 

 and 1447 to 1449). The same light-colored breccia forms the ridge east of 

 Beaverdam Creek, from the stream bed to its summit, except in the peak of 

 Mount Humphreys and that northwest of it, where the light-colored breccia 

 is capped by dark breccia. Bold escarpments on the northwest side of the 

 last-mentioned mountain exhibit nearly horizontal beds of light breccia 

 overlain by similarly bedded dark breccia, the bedding of the two being 



