256 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



In this area the formations have a general dip to the northeast, while 

 along an east and west line they partake of the anticlinal and synclinal 

 structure, which is already under discussion. On such a line, as shown 

 in sections C and D, it is seen that the formations developed on Fryer Hill 

 constitute the western rim of the Little Stray Horse basin, being at the 

 same time compressed into a shallow anticlinal and synclinal fold. The axis 

 of the anticline runs through the crest of the hill in the ground of the 

 Dunkin mine, on a line with the continuation of the Carbonate fault. West 

 of this is a broad, shallow synclinal fold, which takes in the ground of the 

 Little Chief, Little Pittsburgh, and Chrysolite mines, giving to the outcrop 

 of the Blue Limestone, as shown on the map, the form of an S. In the 

 western portion of the Chr3^solite mine ground, successively lower sheets 

 of the lower White Porphyry, White Limestone, and Lower Quartzite 

 come to the surface along the crest of an anticlinal fold, on whose western 

 side, so far as the meager data obtained show, these beds dip steeply under 

 the Wash and Lake beds which form the mesa-like surface of North 

 Leadville. The difficulty of reading the geological structure of this area, 

 which in the above brief statement seems simple enough, is eidianced by 

 a variety of causes. In the first place, here, as in Little Stray Horse 

 Park, there are no outcrops of rock in place, the rock surface being buried 

 beneath about 50 to 100 feet of Wash. The data have therefore to be 

 entirely obtained from shafts, and cannot be intelligently considered until 

 they have been thoroughly mapped. Secondly, the replacement action has 

 proceeded so far that practically no limestone is left, its whole mass having 

 been replaced by vein material. Thirdly, this mass has been split up 

 locally into two or more distinct sheets by the intrusion of White Porphyry. 

 Fourthly, the lower sheet of White Porphyr}^ is cutting across the forma- 

 tion; and, southwest of a line drawn diagonally through tiie corners of the 

 Fryer Hill map, a wedge-shaped portion of the Blue Limestone is left below 

 this sheet. Fifthly, there are later intrusions of Gray Porphyry extremely 

 difficult to trace, as in their decomposed state they are scarcely distinguish- 

 able from the White Porphyry. An interrupted dike of this rock runs 

 through the middle of the area in an east and west direction; and an intru- 

 sive sheet cuts diagonally across the White Limestone up into the lower 



