214 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Just below the Dyer mine a bed of limestone of a light steel-blue color 

 is singularly changed into a light-pink, clayey material, so different in ap- 

 pearance from the unaltered rock that a partial analysis of the two was 

 made in order to determine the chemical change that had produced this 

 appearance. The following figures were obtained: 



Unaltered limestone. Altered limestone. 



Lime 20.31 19.21 



Magnesia 10.35 0.58 



Alumina and iron 0.23 5.23 



Insoluble 31.27 34.56 



From which it is seen that the alteration consists mainly in the removal 

 of a portion of the soluble bases and a consequent relative increase in the 

 proportion of silica. It also shows that a very essential change in the 

 physical character of a rock may be made by the action of percolating 

 waters, with very little actual chemical change. 



The break in the beds north of the Dyer mine, caused by the move- 

 ment of South Dyer fault, is very evident in the Blue Limestone, but cannot 

 be traced much below that horizon. On the west wall of the Dyer amphi- 

 theater the beds slope up the face of West Dyer Mountain in an unbroken 

 line, showing no trace of the fault; the main sheet of White Porphyry 

 which forms the saddle between Dyer and Evans amphitheaters thins out very 

 rapidly to the northwest, and on the face of West Dyer Mountain shows an 

 outcrop of only about ten feet, the summit of the peak being formed by a 

 few remaining beds of Weber Grits. 



Evans amphitheater. The basin at the head of South Evans gulch, as 

 well as the main Evans amphitheater, shows mainly outcrops of Archean 

 rocks, those of the Weber Grits, which adjoin them west of the fault line, 

 being generally covered by debris. The wall of Mount Evans facing the 

 amphitheater presents similar conditions to the wall at the head of Iowa 

 gulch, namely, an eruptive mass underlaid by horizontal stratified beds, 

 and the same strong contrast in their weathered forms, the difference being 

 that in this case it is the Sacramento instead of the White Porphyry that 

 forms the intrusive mass. In a shallow ravine on this wall just south of 

 the Mosquito pass there is a slight break in the continuity of sedimentary 

 outcrops, caused by a small cross-fault with a slight upthrow on the north. 



