102 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



outcrop of coarse white sandstone a little east of the shaft, has a dip of 30 

 to the westward, with a strike of about N. 25 W. On the slope of the 

 pass towards the Platte Valley the Dead-Broke tunnel discloses what is 

 probably the same bed of limestone, with sandstones dipping in the same 

 direction. A body of light-colored mica- porphy rite is also cut in the end 

 of the tunnel. 



The existence of a synclinal fold, as proved by these western dips, is in 

 complete accord with the evidence, obtained farther south along the flanks 

 of the ridge, of a secondary roll or minor fold in the strata parallel to the 

 great fold of the center of the range, and explains the great thickness of 

 exposures of Weber Grits beds. That the fold may have been accompanied 

 by faulting is possible, but, as already stated, no direct evidence of a fault 

 was found. Perhaps, had time permitted, a careful exploration of the ravine 



. at the head of the Blue River and on the west face of Hoosier ridge might have 

 afforded more definite proof. As it is, the geological outlines given on the 

 map are generalized from observations made on the spur connecting it with 

 Hoosier pass. The results of these observations are graphically shown in 

 section A A, Atlas Sheet VIII, for the eastern end of which, beyond the 

 Platte Valley, they furnished the data. The largest body of porphyry 

 there shown, which forms the shoulder of the spur above Hoosier pass to 

 the east, consists of typical Lincoln Porphyry (54). It contains the usual 

 large pinkish crystals of feldspar, which in this rock, however, seem excep- 

 tionally susceptible to alteration and, instead of being fresh and rather 

 glassy in appearance, are opaque and often quite kaolinized. The micro- 

 scope shows rather more plagioclase than in the type rock, which may be 

 accidental. The quartz occurs in small, double-pointed, hexagonal pyramids 

 showing also the development of the prism ; and on the crest of the spur, 

 where, owing to the gentle slope and accumulation of soil, decomposition 

 seems to have gone on most freely, the rock surface is covered with a coarse 

 sand made up almost entirely of such quartz crystals, often with well 

 defined angles and facets. 



The steep north slope of the spur, facing the basin-shaped head of 

 an eastern tributary of the Platte, shows a cliff wall of this rock with 



characteristic cross-jointings and vertical cleavage, almost amounting to a 



