MOUNT BKOSS. 119 



Fuot. 



Blue Limestone, covering surface of spur ? 



a-, . < Parting Quartzite (exposed in prospect holes) t 



\ White Limestone, partly covered by debris, estimated . . 200 



Shales and sandy limestones 35 



Gray quartzite, impregnated with metallic mineral 20 



Massive white quart zite G 



Cambrian < Greenish quartzite, with calcareous layers 8 



White saccharoidal quartzite 10 



Greenish-white, compact, thin-bedded limestone 3 



i, White saccharoidal quartzite 55 



137 



Archean ! 



The limestone bed in this section is of interest as being the only one 

 examined from this region which was not a dolomite. It contained 25.48 

 per cent, carbonate of lime, 4.03 carbonate of magnesia, with traces of chlo- 

 rine, the residue being mainly silica. It has already been noted that the 

 Cambrian beds in their upper part are often more or less calcareous, but 

 generally resemble a sandstone on the surface, whereas this bed has the 

 compact, even texture and clean fracture of a limestone. The strata at this 

 point dip 15 to the east, with a strike a little east of north. 



Red amphitheater. Nearly under the summit of Mount Bross and high 

 up on the east wall of Buckskin gulch is the Red amphitheater, a semi- 

 circular recess in the cliff-wall nearly a thousand feet above the bed of the 

 valley. The scale of the rnap does not permit an adequate expression of 

 the form of this remarkable basin, which is rendered still more prominent 

 by the brilliant red and yellow coloring of its walls. This color comes 

 from a thin coating of ocherous clay, which covers the rock fragments of 

 debris piles, and which contains, besides oxide of iron, traces of arsenic, 

 antimony, and sulphur. The rock fragments thus coated are so much 

 decomposed that it is seldom possible to determine their original character, 

 and it would have taken much more time than was available to thoroughly 

 decipher the geological history of this remarkable locality, which has evi- 

 dently been the scene of long-continued metamorphic action, probably a 

 sequence of the eruption of the igneous rocks now forming dikes and intru- 



