144 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



mine, viz, that these much metamorphosed and vertical beds are the lower 

 Paleozoic strata coming up from under a sharp syncline, compressed and 

 altered beyond recognition by the dynamic movement at the time of and 

 subsequently to the faulting. This would seem at first glance to be an 

 explanation inconsistent with that which is offered for the conditions which 

 obtain on Pennsylvania Hill, on the opposite side of the gulch ; but the fact 

 that the fault line comes in the one case east of the synclinal axis and in 

 the other nearly coincides with it, and the supposition that compression sub- 

 sequent to the faulting has not only produced sufficient heat to alter the 

 original character of the beds, but has steepened the dips of the already 

 inclined beds and actually made them thinner, sufficiently explain the appar- 

 ent incongruity. 



Pennsylvania Hill west of London fault. — The Western end of Pennsylvania Hill, 

 through which the London fault runs, is deserving of detailed description. 

 Its structure is shown in section D, with the ideal position of the beds 

 in depth. The observed facts are the.se : Ascending the wedge-shaped 

 western point of the ridge from the saddle which divides South Mosquito 

 from Sacramento amphitheater, one crosses a regular series of sedimentary 

 beds, dipping 20° to the eastward, with two interbedded sheets of porphyry 

 apparently conformable with the sedimentary beds. The riflge has almost 

 perpendicular walls both to the north and south, on which the structure 

 lines can be distinctly seen. The horizon of the beds which cap the divid- 

 ing saddle at the base of the ridge is estimated to be 150 or 200 feet higher 

 than the limestone beds which occur about the middle of the Weber Grits 

 formation. About half wav up the steep western slope, which is mainly 

 composed of coarse sandstone with some few intercalated beds of shale, is a 

 bod}' of interbedded Sacramento Porphyry, of a thickness of 15 to 20 feet. 

 Near the top of this steeper slope is a bed of black sandstone, composed of 

 white quartz sand and fine grains of carbonaceous material in the nature of 

 anthracite or graphite, which is very characteristic of this formation. The 

 ver}' summit of the steeper ridge is formed by a second body of porphyry, a 

 fine-grained gray rock with conchoidal fracture, resembling the Silverheels 

 Porphyr)', whose thickness is 25 to 30 feet. Above the steeper slope 

 of the ridge the surface is nearly flat and widens out so that the succeeding 



