MOUNT BROSS. 117 



The rock (88) of which it is composed differs from any yet described. Its 

 weathered surface is so white that at first glance it might be taken for the 

 White or Leadville Porphyry. On a fresh fracture it has a light-green 

 color and shows few macroscopical crystals. It has certain resemblances 

 to porphyrite and also to the Silverheels Porphyry, but the microscope 

 shows it to be identical with the quartz porphyry found on Loveland Hill 

 and on the north wall of Mosquito Gulch, which has been described under 

 the name of Green Porphyry. 



Bross amphitheater, like those of the other two peaks, lies nearly due 

 east of the summit, but, owing to the steeper inclination of the Paleozoic 

 beds which cap its walls, it has not been carved to so great a depth into 

 the underlying Archean schists, whose outcrops are therefore of much less 

 superficial extent. As in the others, the highest beds exposed in the cliff" 

 sections on its walls are those of the Blue Limestone. Shales, probably 

 belonging to the Weber Shale foi'mation, are disclosed in prospect holes 

 along the road which curves round its head, and very possibly a consider- 

 able portion of the area which has been given the color of porphyry on the 

 map may prove by actual excavation to be underlaid by beds of this for- 

 mation. The road which leads by the Dolly Varden and Moose mines, 

 along the north face of Mount Bross and the west face of Mount Cameron, 

 to the Present Help mine, on the south face of Mount Lincoln, is indicated 

 on the sketch in Plate IX by a light double line, the location of the respect- 

 ive mines being shown by the house outlines. The Dolly Varden mine, on 

 the spur south of the amphitheater, finds its ore in the Blue Limestone adjoin- 

 ing a dike of White Porphyry 40 feet in thickness, which crosses it at an 

 angle of 60° with the horizon. Below the Dolly Varden mine the spur 

 slopes more steeply than the beds, and at its base the Parting Quartzite of 

 the Silurian is exposed. In the basin-shaped valley called Mineral Park, 

 south of this spur, erosion must have exposed still lower beds than on the 

 spur, and it is possible that the quartzite beds said to be exposed there may 

 belong to the Cambrian. 



The ridge running south from Mount Bross, between Mineral Park and 

 Buckskin gulch, is mainly covered by easterly- dipping beds of the Blue 



