218 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



this the Blue Limestone is cut off by the Mosquito fault at the sharp spur run- 

 ning out southwest from East Ball Mountain toward Dyer gulch. Actual 

 outcrops of Blue Limestone and Parting Quartzite, dipping steeply to the 

 east, are found on the saddle east of Ball Mountain. The summit of Ball 

 Mountain, from the saddle westward to the fault, is formed of White Por- 

 phyry, of which a thick body here separates the Parting Quartzite from the 

 White Limestone. 



The structure of the area between the crest of Ball Mountain and the 

 south slope of Ellen Hill below the outcrop of Blue Limestone is some- 

 what obscure; but the explanation presented on the map, viz, that of a 

 quaquaversal or anticlinal fold in part cut off by the Ball Mountain fault, 

 is one which best fits the following observed facts. 



On the north slope of Ball Mountain, immediately under its crest, can be 

 traced outcrops of White Limestone and of a portion of the Lower Quartz- 

 ite beneath it, The prominent ridge which extends out on the steep slope 

 towards the valley of South Evans consists entirely of fragments of quartzite 

 derived from the above-mentioned outcrop. .Shaft F-6, however, which 

 has penetrated this covering of debris, shows that the underlying rock is 

 White Porphyry. The John Mitchell shaft, west of this, has gone through 

 another body of Lower Quartzite and a second underlying White Porphyry 

 to granite. North of the John Mitchell, at the base of the hill, is a small 

 outcrop of granite, with a little white quartzite resting on it. Still north of 

 this are the Ocean and Seneca shafts, the former in Lower Quartzite, the 

 latter in White Limestone, dipping to the northward. It seems, therefore, 

 that the White Porphyry is here splitting the Lower Quartzite into several 

 distinct bodies, and it may naturally be inferred that somewhere in this re- 

 gion it has been intruded into the Lower Quartzite from the underlying 

 Archean. 



The Nevada tunnel discloses a body of White Limestone, resting on 

 quartzite, east of the fault, which dips at 45 to the north. This dip is some- 

 what abnormal to the quaquaversal structure deduced from observations in 

 other shafts of this region, but its proximity to the fault may account for 

 the irregularity. 



