172 GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



shoe and Twelve-Mile Creeks the shallow syncline in the Weber Grits, 

 already mentioned, can be traced as far as the Twelve-Mile gulch. It 

 is only in the deeper cuts near the crest of the ridge that the details of 

 structure are distinctly visible. Twelve- Mile Creek heads in four sepa- 

 rate basins or amphitheaters, to the distinctness and grandeur of whose 

 forms the scale of the map can do but scant justice. The exposures of 

 Archean rocks in these amphitheaters present a great variety of gneiss and 

 granite, the most noticeable of which have already been described in Chap- 

 ter III. The deeper of these amphitheaters is that to the north, whose north- 

 ern wall is capped by beds of the lower Paleozoic series, the Lower Quartzite 

 forming, as shown on the map, the crest of the range at its head. The sheet 

 of White Porphyry above the Blue Limestone has a broad outcrop, prom- 

 inent by its white color, extending across from Horseshoe Ridge and sweep- 

 ing down the wall across the mouth of the amphitheater. On the ridge 

 between this north amphitheater and the one adjoining it, a shell of Lower 

 Quartzite still remains at its eastern end. South of this, Paleozoic out- 

 crops are confined to the meadows at the lower extremity of the amphi- 

 theater, where a number of springs come from them. On the south wall of 

 the southern amphitheater the lower Paleozoic beds again sweep up for a 

 considerable distance on the spur, the white quartzite of the Cambrian and 

 the interbedded White Porphyry being prominent by their color. The 

 eastern end of this ridge is formed by the continuation of the main White 

 Porphyry body; while along its wall can be traced an. offshoot from this 

 body, cutting across the Blue Limestone and occupying the horizon between 

 the Blue and the White Limestone. The white quartzite extends nearly up 

 to the prominent shoulder of this spur, and is found again on the very summit 

 of Weston's Peak, at the head of the spur. Here it lies nearly horizontal, 

 bending over slightly on its western edge. This mass of quartzite is evi- 

 dently, as shown in Section H, a remnant of the crest of the anticlinal fold, 

 whose axis relatively to the present slope of the ground is descending to the 

 southward. The outcrops of the sedimentary beds on the east of the axis, 

 therefore, gradually rise along the eastern slopes of the ridge; their outlines 

 as shown on the map present- a series of regular curves, due to the erosion 

 of the ravines which score the surface, which are distinguishable in the 



