212 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



first to have been dropped down here by a sudden sinking of the ground. 

 Nearly parallel with the South Dyer fault is the Dyer Mountain fault, whose 

 presence is indicated by a slight discrepancy in the stratified beds at the head 

 of Dyer and North Iowa amphitheaters. Its extent as well as its movement 

 is apparently small, as it could not be traced beyond these valleys in either 

 direction, and in the Dyer amphitheater is shown only on the east wall, 

 there being apparently no break in the lines of stratification on the West 

 Dyer Mountain side. The amount of displacement caused by these two 

 faults is shown in Section M, Atlas Sheet X. 



West Sheridan. West Sheridan Mountain, which is in point of fact 

 simply a Y-shaped spur extending out westward from the main Mount Sher- 

 idan, has, as it were, three summits, two of which are capped by the remains 

 of the White Porphyry sheet, which here separates the Blue Limestone from 

 the White. The remainder of the crest of the ridge is formed by beds of 

 White Limestone, under which is a fringing outcrop of Lower Quartzite. 

 In those on the north and west slopes are several small bodies of White 

 Porphyry. An estimate of the thickness of White Limestone and Lower 

 Quartzite on the western face of the south and north spurs of West Sheri- 

 dan, respectively, gave 250 and 275 feet, the difference being accounted ior 

 by included sheets of White Porphyry. 



Dyer Mountain. Dyer Mountain, as shown in Section M, whose line runs 

 from the summit of the mountain southward through the spur represented 

 in the photograph, is composed of the following beds in descending series: 



Sacramento Porphyry. 

 Weber Grits." 

 White Porphyry (main 

 Blue Limestone. 



White Limestone. 

 Lower Quartzite. 

 White Porphyry. 

 Archeau. 



The main sheet of White Porphyry is the cap-rock of that portion of 

 spur shown in the frontispiece. The lines of stratification on the face of 

 the spur toward the observer, dipping gently to the north toward the head 

 of Dyer amphitheater, belong to the Lower Quartzite and to the underlying 

 bed of White Porphyry, which is here two hundred to three hundred feet in 

 thickness. On the south face of this spur, toward Iowa gulch, in the Archean 

 apparently near or on the line of the South Dyer fault, is an irregular out- 



