136 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



eroded off the summit. No sufficient evidence was found, however, to 

 justify its indication on the map. 



On Mount Bross the Lincoln Porphyry shows a still lighter color than 

 that on Mount Lincoln, which seems due to the fact that the decomposed 

 mica, instead of remaining as chlorite, has been entirely removed Frag- 

 ments of shales and quartzitic sandstones of the Weber Grits formation are 

 mingled with the porphyry debris of the upper slopes of Bross, and out- 

 crops of these rocks are found on the ridge connecting it with Cameron, us 

 well as to the south of its summit on the ridge overlooking Buckskin guhh- 

 In the latter instance they stand at a much steeper angle than the lower 

 series of Paleozoic beds, and give evidence of some local movement. In 

 Section L, Atlas Sheet X, is shown the probable form of the Lincoln 

 porphyry body on the summit of Mount Bross, as deduced from observed 

 outcrops. It is very possible that, like that of Mount Lincoln, it stands over 

 a channel of eruption, but the evidence of this was not considered strong 

 enough to justify its being indicated on the plane of the section. 



On the north face of Mount Bross, towards Cameron amphitheater, 

 the base line of the Paleozoic formations can be traced with tolerable dis- 

 tinctness. Of dikes crossing the formation, like those on the face of Mount 

 Lincoln opposite, there are doubtless many, but only one was actually 

 traced, which is cut by the western workings of the Moose Mine. In the 

 Archean below this mine is a prominent mass of light-gray granite. The 

 workings are in the Blue Limestone, which is exposed on the east spur of the 

 mountain between the Cameron and Bross amphitheaters, forming the sur- 

 face of the spur, until cut off by its steeper slope, whose angle is greater 

 than that of the dip of the beds. This bed is completely honeycombed by 

 abandoned mine workings, but the underlying White Limestone here, as in 

 the Leadville district, seems to have yielded little or no ore. At the foot 

 of the spur, erosion has exposed the quartzite beds of the Cambrian, in 

 which is a prominent dike of porphyry running from the edge of Bross 

 amphitheater a little north of east, in the direction of the summit of Mount 

 Silverheels. It was traced as far as the secondary ridge bordering the 

 Platte Valley into the White Limestone, where it was lost in the forest. 



