138 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



zon of the juxtaposed beds on either side of the fault and in other struct- 

 ural outlines. Although its existence is so evident, yet its actual position 

 cannot be determined with absolute accuracy, the possible error of location 

 varying under different conditions from ten to one hundred or even two 

 hundred feet. The reason for this uncertainty is found in the fact that the 

 surface rocks in the immediate neighborhood of the fault are generally so 

 much altered and decomposed that their structure planes cannot be traced, 

 and that the fault plane has not been cut by any underground explora- 

 tions. The direction of the fault, as determined from points where it crosses 

 ridges or gulches, varies from N. 15 W. to N. 45 W., its average direction 

 being N. 30 W. or NW. magnetic. The great S-shaped anticlinal fold 

 which is everywhere found in close proximity to the fault lias the same 

 general direction ; nevertheless the two directions do not seem to be coinci- 

 dent for any long distance, but diverge a little from each other, so that the 

 fault cuts the fold, now in one part, now in another, but generally west of 

 the anticlinal axis. Thus from Pennsylvania Hill to Sheep Mountain it 

 corresponds closely with the axis of the syncline to the west of the great 

 anticline ; south of Sheep Mountain it gradually approaches the anticline, 

 until at the extremity of the ridge both fault and fold die out. North of 

 Pennsylvania Hill the line of the fault has a more easterly direction than 

 that of the fold, and on London Hill it cuts the fold east of the synclinal 

 axis, and a little beyond it may very nearly coincide with the anticlinal axis ; 

 but, as the sedimentary beds have been entirely eroded away from above the 

 Archean, it is no longer possible to determine the position of this axis. The 

 amount of displacement occasioned by this fault can only be determined 

 approximately, since the fact of its near coincidence with the anticlinal 

 fold introduces an unknown factor, viz, the amount of apparent displace- 

 ment that may be due to actual plication. The reason of this can best be 

 understood by reference to Sections C, D, E, F, G, and H, on Atlas Sheets 

 VIII and IX, which are drawn to scale and have been constructed with 

 great care from observed outcrops, dips, and thicknesses of formations. 

 The movement of displacement, as shown in these sections, which is prob- 

 ably a minimum, averages a little over two thousand feet. 



