238 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



the southwest foot of Cfiuterbury Hill, iust below the forks of Little Evans 

 gulch. To this line converges also the northern end of the Iron fault, whose 

 throw becomes null at the crest of the fold. The simplest expression of the 

 structure of the region between Fryer Hill and Weston fault north of 

 Stray Horse gulch is that of a synclinal basin in Little Stray Horse Park, 

 the eroded crest of an anticline at Yankee Hill, and a syncline farther 

 eastward, whose rim is partially cut off by the Weston fault. The intrusive 

 masses of porphyry here associated with the regular sedimentary series are 

 a lower sheet of White Porphyry between the White Limestone and Part- 

 ing Quartzite, an upper sheet of White Porphyry above the Blue Limestone, 

 and the main sheet of Gray Porphyry above it. This comparatively simple 

 structure, resulting from folding alone, which obtains along the line of Big 

 Evans gulch, on the north slope of Yankee Hill, is complicated on the south, 

 first, by the displacement of the Iron fault, which cuts diagonal!}' into the 

 crest of the fold after crossing Stray Horse gulch west of the Argentine 

 tunnel and passing between the Double-Decker and Highland Mary shafts, 

 the east and west shafts of the Hard Cash mine, and through the eastern end 

 of the Chieftain tunnel ; secondly, by the movement of the Adelaide cross- 

 fault, which extends from the Iron fault opposite the mouth of the Argentine 

 tunnel, just west of the Laura Lynn shaft, to the saddle between Adelaide 

 Park and the head of Nugget gulch; and, thirdly, by the intrusion of sev- 

 eral irregular masses of Gray Porphyry. 



Syncline east of Yankee Hill. — The greater part of the surface between 

 Yankee Hill on tlie west, the mouth of Lincoln gulch on the east, and the 

 steep slopes of Prospect Mountain on the north is covered by the main 

 sheet of Gray Porphyr\-, which directly overlies the upper sheet of White 

 Porphyry. The White Porpliyry only comes to the surface along the flanks 

 of the Yankee Hill anticline and in the valley of Upper Stray Horse gulch 

 and Adelaide Park. The contours of the map in the latter region would 

 seem at first glance to negative the idea that the exposure of porphyry was 

 simply due to a deeper erosion, since they show in the White Porphyr}' 

 area not only a valley but also the summit of a ridge. It must be borne 

 in mind, however, that these contours represent the actiuil surface of the 

 ground and not the rock surface, whereas the geological outlines refer oiilv 



