MOUNT CAMERON. 1 ] 5 



dikes are about fifteen feet in thickness each, while the combined beds have 

 a thickness of from fifty to sixty feet. 



This intrusive .sheet of Lincoln Porphyry at the Cambrian horizon, 

 which seems continuous along the north wall of the Cameron amphitheater, 

 was traced out to the end of the southeast spur of Lincoln ; and what is 

 apparently the same bed was also observed lower down the slopes, in the 

 more steeply-dipping members of the same formation. Outcrops of simi- 

 larly situated bodies, as shown on the map, are also found on the south wall 

 of the Cameron amphitheater and on either wall of the Bross amphitheater. 

 Time did not permit of tracing any connection between these different out- 

 crops ; and it seems doubtful whether any exists, inasmuch as for some un- 

 known reason there seems to have been much less tendency to spread out 

 in extensive .sheets at this horizon than at that above the Blue Limestone. 

 Although this latter porphyry bed is only found to a limited extent above 

 the Blue Limestone on Mount Lincoln, there is no doubt that it once covered 

 that bed, forming a sheet comparable in extent to that of the White Por- 

 phyry in the Leadville district. 



Cameron and Bross. — The siinuuit slopcs of Mouuts Camerou and Bross. 

 except those on the cliff faces which are too steep to permit the lodgment 

 of debris, are mainly covered by fragments of Lincoln Porphyry. Eruptive 

 rocks under the action of atmospheric degradation split up into fragments 

 whose shape and relatively small weight, as compared with their superficial 

 area, render them more susceptible to being moved by melting snow, so 

 that on mountain sides they generally cover a surface disproportionately 

 large as compared with their actual outcrops. This is eminently the case 

 on Mount Bross. where angular fragments of porphyry often cover the 

 surface to a depth of ten feet or more and the character of the underlying 

 rock can often only be determined by actual excavation. 



The porphyry of the summit of Mount Cameron is remarkable for the 

 unusual development of large orthoclase crystals, often more than two 

 inches in length, which weather out from its surface. Associated with the 

 much-weathered fragments of porphyry are various brown quartzitic sand- 

 stones, which may represent a bed of the Weber Grits formation not yet 



