122 GEOLOGY AND MINING INDUSTRY C F LEADVILLE. 



mineral is the result of a change within the rocks themselves, and, proba- 

 bly in great part, of the alteration of pyroxene and amphibole in the lime- 

 stones. 



Eastern foot-hills. — The higher part of the Lincoln massive thus far 

 described may be considered structurally to form })art of the crest of an 

 original great anticlinal fold, inasmuch as the average inclination of the 

 beds is com})aratively small. The wooded ridges between the foot of the 

 steeper slope and the Platte Valley, which form a low slioulder to the Lin- 

 coln massive, and where steeper dips prevail, would tbrin the actual eastern 

 member of the fold. On the ridge between Quartz\ ille and ^lontgoniery, 

 for instance, the beds dip as steeply as 45°. South of this a wider region 

 is included between the outcrops of Blue Limestone and the Platte Valley, 

 and, were the steep dips continued without interruption, an immense thick- 

 ness of beds would be represented. There are reversed dips found, however, 

 notably in the ravine below Quartzvilleand in Buckskin gorge above Alma, 

 wliich give evidence of the existence of a secondary flexure parallel to the 

 main fold, a sort of minor rip])le following at the heels of the great breaker 

 or wave which caused the main uplift of the range, such as is almost inva- 

 riably found along lines of great ])lication. Another noticeable feature in 

 the structure is a decided change of strike, which commences opposite the 

 east spur of Mount Bross, or between the Bross and Cameron amphithea- 

 ters. Noi'th of this line the average strike of the beds is north or a little 

 east of north; south of it the strike bends more and more to the east of 

 north; and on the southeast slopes of Bross the strata have a d\\) with the 

 slope to the southeast. 



The outer wooded ridge above mentioned is composed of coarse sand- 

 stones of the Weber Grits formation and of various intrusive bodies of por- 

 phyry. Porphyry bodies similarly situated were observed on four distinct 

 section lines followed across this ridge, but the assumption that they form 

 part of a continuous body, as indicated on the map, is here, as in the case 

 of those on the east side of the Platte Valley, not founded on the tracing of 

 a continuous line of outcrops, as in the canon sections. They generally 

 belong to the Lincoln Porphyry class. That found in the ravine above 

 Dudley in considerable thickness has the round pink quartz grains, but wants 



