112 GEOLOGY AND MINING INDUSTRY OF LEADVILLE, 



Russia mine on the east, immediately below a bed of Lincolu Porphyry. 

 Along the edge of the steep ravine which descends directly from the summit 

 of Lincoln an irregular dike of porphyry crops out here and there, colored 

 brilliant red and yellow on its surface, but so much decomposed that its 

 original structure can no longer be determined. As shown in the sketch, it 

 is only the Silurian (c) and Cambrian (b) strata which form continuous out- 

 crops across the cliff tace, and these are somewhat broken by transverse 

 dikes of eruptive rock. Within the Cambrian quartzite is an intrusive sheet 

 of Lincoln Porphyry, whose darker color contrasts strongly with the bleached 

 weathered surfaces of the summit rock. The base line of the Cambrian, 

 where it rests on the Archean, appears more irregular in the sketch tlian it 

 is in nature, but it is evident that the Cambrian sea bottom was not so 

 smooth here as it is shown to be in other cliff sections. 



In the ravine ne.xt east from that already mentioned is a dike of White 

 Porphyry, which can be traced, as shown in the sketch, from the gneiss of 

 the Archean across the Cambrian quartzites into the White Limestone. This 

 is dike No. 1, whose rock lias already been described under that which 

 occurs on the north face of Lincoln. Its outline is e.xtrenielv irregular, and 

 its contact surfaces with sedimentary rocks, which are distinctly visible, 

 .show none of the contact phenomena supposed to result from the heat of a 

 fused mass. In its upper portion it is rounded, and curves over one of the 

 heavier beds of White Limestone in an oval mass. On its east side, near its 

 summit, the tliinner beds of limestone are bent upwards, as if displaced at 

 the time of its intrusion, and the lower shale beds of the While Limestone 

 belt are more or less serpentinized. It also sends out offshoots a few inches 

 wide through the natural joints of the sedimentary beds. About fifteen to 

 twenty feet above the ba.se of the Lower Quartzite it crosses an interbedded 

 mass of porphvrv of a dark-green color, which is here some thirty feet in 

 thickness. Tiiis interbedded porphyry is thoroughly decomposed, the only 

 crj'stals visible being rounded quartz grains, which resemble those of the 

 Lincoln Porphyry. All its cleavage planes are covered by a dark-green 

 coating of chloritic nature, and it is crossed by thin perpendicular fissures, 

 from one to two inches in thickness, containing pyrites and having a bright- 

 yellow weathered surface. A comparative!}* fresh specimen was obtained 



