ACID BRECCIA OF MOUNTAIN CREEK. 287 



gated (1652 to 1654). It carries large masses of andesite, from 1 to 8 feet 

 in diameter. The bedding is irregular — in some places horizontal, in others 

 dipping 20° W. It is topped by well-bedded basic breccia, which forms 

 the summit of the Turret and covers what was once a very uneven surface 

 of older breccia. The light-colored breccia extends down to the valley of 

 Trappers Creek on the west and down to the bottom of Mountain Creek on 

 the east, but the southern end of the ridge for 1,000 feet above the creek 

 is pyroxene-andesitic breccia, whose relation to the hornblendic breccia was 

 not discovered. 



The most southern exposure of what has been definitely recognized as 

 the light-colored hornblende-andesitic breccia is in the vicinity of the forks 

 of Mountain Creek. On the eastern bank of the north fork it appears to 

 be somewhat indurated in places. It has abundant hornblendes, and some 

 fragments are hornblende-biotite-andesite (1661 to 1666). At one place it 

 consists of horizontal layers of tuff with intercalated layers of large water- 

 worn and rounded fragments. It is cut by small bodies of intrusive andes- 

 ite. It forms the lower end of the spur between the' main forks of the 

 creek, extending up it to a point 500 feet above the stream, where there is 

 an exposure of massive hornblende-andesite, porous and light red, with 

 some large, stout hornblende phenocrysts and segregations of red horn- 

 blende and biotite (1664, 1667). In this vicinity the hornblendic breccia is 

 overlain by dark-colored basic breccia, the superposition being shown in a 

 number of places. 



South of this point no distinct accumulation of light-colored hornblende- 

 andesitic breccia has been observed. It is possible, however, that the light- 

 colored layer of breccia at the western base of the most northern prong of 

 The Trident may represent this series of breccias. It is not separated into 

 thin layers or beds, but weathers into a massive cliff with vertical prisms. 

 The exposed layer is over 1 00 feet thick, and consists of light-colored tuffs 

 and breccia, with andesitic fragments of various kinds. They are mostly 

 pyroxene-andesite, but many carry a little hornblende and others a little 

 mica, while some are decidedly hornblendic, as is also the light-gray tuff 

 (1683, 1684). This layer is overlain by distinctly bedded, dark-colored 

 breccia, which continues up to the summit of The Trident. 



The tuff-breccia and occasional lava flows that have just been described 

 consist of andesites, for the most part light colored and hornblendic. By far 



