170 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



The surface of the South Park, east of Sheep Ridge, is uniformly 

 covered to a considerable depth by Quaternary gravels, the only outcrop 

 of underlying beds within the limits of the map being north of the bend 

 of Four-Mile Creek, where an anticline in the Weber Grits can be seen, a 

 continuation of the secondary roll already noticed to the north. East of 

 the limits of the map the approximate location of the Triassic beds is indi- 

 cated by the red color of the soil, and the more resisting beds of this and 

 the higher formations form low north and south ridges, which rib the surface 

 of the park. With these are associated sheets of eruptive rock, probably 

 analogous to the intrusive 'sheets already described. On one of these ridges 

 was found the rhyolite tufa which is described in Appendix A. 



Black Hill. Out of the South Park plain, at the extreme southeast corner 

 of the map, rises to a height ef about GOO feet an isolated, forest-covered 

 hill, nearly circular in shape, known as Black Hill, only the northern edge 

 of which comes within the limits of the map. It is entirely composed of 

 rhyolite (140). The occurrence is interesting on account of the rarity of 

 Tertiary eruptive rocks in the region under consideration. It is noticeable 

 that it is on a direct line with the continuation of the London fault, and 

 that the prolongation of the same fault to the northwest would nearly pass 

 through the other occurrences of rhyolite in Chalk Mountain, on the north- 

 ern edge of the map. The whole mass of the hill is composed of rhyolite, 

 as far as can be distinguished. The outflow has apparently taken place 

 through the upturned sedimentary beds and spread out over their edges, 

 without, however, exercising any very marked influence on their structure 

 lines, as is the case with the secondary intrusive masses. The outcrops of 

 these sedimentary beds are somewhat obscure, being mostly covered by sur- 

 face accumulations, but, from their lithological character and from the succes- 

 sion observed along the valley of the Little Platte, south of the limits of the 

 map, they are assumed to belong to the horizon of the Upper Coal Measure 

 formation. On the northern base of the hill is a considerable accumulation 

 of impure gypsum in mud shales. Directly south of the hill, along the basin 

 of the Little Platte, quite a succession of thin-bedded clay shales, with some 

 limestone beds, is found, standing nearly vertical and striking due north 

 and south. In a prospect hole these shales are seen to be remarkably con- 



