182 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



and consequently loses the distinctive form due to glacial erosion. The 

 succession of beds crossed in descending the ridge from the north point of 

 Empire Hill to this meadow is sufficiently indicated on the map. 



On the north side of the gulch the structure is even more complicated 

 than on the south, and the rock surface is more obscured by morainal and 

 other detrital material. Were it not for the numerous prospect holes this 

 structure could hardly have been unraveled. It is shown in much more 

 detail on the large map of Leadville and vicinity, and its description is re- 

 served for the chapter which treats of that region. 



Leaving aside then, for the moment, the region included within the 

 limits of this map, the crest of the range and that portion of its western 

 slope not included therein will next be described. 



Main crest north of Ptarmigan Peak. At Ptarmigan Peak and for some dis- 

 tance north the entire ridge is composed of Archean, in which granite and 

 coarse porphyritic gneiss are the main components; thence north to Horse- 

 shoe Mountain successive shells of Lower Quartzite, White Limestone, 

 Parting Quartzite, and Blue Limestone form the crest. Round the head of 

 Empire gulch their outcrops form a semicircular rim, sweeping round the 

 western point of Mount Sheridan, while the crest of the ridge is covered by 

 the main body of White Porphyry. Under Peerless Mountain a second 

 body of White Porphyry comes in between the Blue and White Limestones, 

 and extends as far north as the base of Dyer Mountain, where it seems to 

 pass down to successively lower horizons, until in Dyer amphitheater it is 

 found quite at the base of the lower Paleozoic series. Remnants of this 

 second body of White Porphyry form the cap-rock on the western spur of 

 Mount Sheridan, known as West Sheridan, whose mass, by the slight move- 

 ment of displacement of Sheridan fault which runs through the saddle sep- 

 arating these two peaks, has been let down relatively to the mass of Mount 

 Sheridan itself; in other words, its upthrow is to the eastward. This rather 

 singular fault passes partly across the head of Iowa Amphitheater, where 

 it is joined by a fault at right angles to it, or running nearly east and west; 

 as the result of their movement, a little segment of beds of the Lower 

 Quartzite, White Limestone, and overlying White Porphyry is left in the 



