216 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



that the influence of the fault has produced a slight anticlinal fold. Pros- 

 pect Mountain, from its summit eastward to the foot of Mosquito pass, is 

 made up of coarse sandstones and micaceous shales of the Weber Grits 

 formation, which dip a little north of east. 



Little Eiien Hiii. The same beds are found to extend through the main 

 portion of Little Ellen Hill and across the upper part of South Evans gulch, 

 and outcrops where visible have a prevailing dip of 20 to the eastward. 

 The lines of structure in a series of beds of such uniform composition are 

 difficult to trace in a country where the surface is so much obscured as here. 

 It is possible, therefore, that the eastern ends of Sections A, B, C, and D, 

 which pass through this region and have been constructed somewhat theo- 

 retically, give too great a thickness for this formation; in other words, place 

 the ore horizon at too great a depth below the surface, since the structure 

 lines obtained from other portions of the region, where definite data are 

 more frequent, show no such extent of regular slope, but much more fre- 

 quent waves or folds. Such, however, have not been indicated here, as in 

 the absence of definite data they would be purely imaginary. 



Eruptive dikes. In this area are several outcrops of eruptive bodies, 

 which apparently belong rather to the dike type than to that of intrusive 

 sheets. Two of these occur on Prospect Mountain ridge, the easternmost of 

 which is a coarse-grained quartz-porphyry, with large orthoclase feldspars, 

 resembling the Lincoln Porphyry; its feldspars are partly reddish and 

 partly light green, the coloring being due to iron oxide on the one hand, 

 or to light-green mica as an alteration product on the other. The western 

 of these dikes is a fine-grained, dioritic-looking rock, similar to that found 

 in the Arkansas Valley between Indiana and Bird's Eye gulches and at the 

 heads of these gulches. On the north slope of Little Ellen Hill is an out- 

 crop of the same coarse-grained porphyry that is found in the eastern 

 dike. In the bed of Evans gulch, above the Virginius mine and extend- 

 ing up some distance on the north side of the gulch, is an eruptive mass of 

 rather irregular form, whose outlines are somewhat obscured by surface 

 accumulations. It belongs, as well as could be ascertained from the par- 

 tially decomposed specimens obtained, to the Mount Zion type of porphyry. 



