MOUNT SILVERHEELS. 105 



map, so much farther west. A rough calcvilation of the difference in thick- 

 ness of given east and west sections, taking the one on a line passing 

 through Fairplay, the other through the summit of Silverheels, would give 

 an increase in thickness in the latter case of 3,000 feet, which may be 

 assumed as the aggregate mass of the intruded porphyry bodies at the latter 

 point, since on the line through Fairplay they have very largely disappeared 

 by thinning out. 



On a line eastward from Platte Valley to the summit of Silverheels the 

 succession of rocks is as follows: Beaver Ridge, immediately adjoining the 

 Platte Valley, whose steep slopes are covered with a thick forest growth 

 which impedes observation, consists of the coarse grits of the Weber for- 

 mation, with two princi])al and probably some minor bodies of Lincoln 

 Porphyry. The valley of Beaver Creek, a straight depression in the line 

 of strike, is apparently cut out of the softer shaly members at the top of 

 this formation. From its bottom up the steep face of Silverheels are many 

 porphyry bodies, whose debris often so obscures the outcrops that no con- 

 tinuous section can be obtained. In this extent five sheets of porphyry and 

 one bed of gray limestone were observed ; these alternate with shales and 

 micaceous sandstones, which pass at the summit of the peak into conglom- 

 erates. A considerable number of these conglomerates outcrop on the 

 ridge running eastward from the summit, alternating with purple and 

 green shales and with sheets of porphyry, of which no less than eight were 

 counted. The conglomerates contain an unusual number of rounded and 

 sub-angular fragments of the more resisting Ai'chean rocks, together with 

 the rounded pebbles of pinkish milky quartz which are common in all tlie 

 sandstones of a coarser nature. Beyond them the brick-red sandstones of 

 the Trias become the prevailing rock, their dip steepening on the east slope 

 to 25° and 35°. Along the west face of Silverheels the porphyry beds, 

 which resist better the action of abrasion, can be traced in curving contours 

 along the slopes, capping the more prominent shoulders of the spurs and 

 disappearing from sight in the forests which clothe the lower spurs to the 

 south. 



The type of the Silverheels porphyry (89), which is found at the sum- 

 mit, is a fine-grained rock of slightly greenish-gray color, having a con- 



