Bojots.] LEADVILLE MINING DISTRICT. 421 



masses along channels formed during- the preceding phase — contact 

 planes between eruptive and sedimentary rock masses, and planes of 

 fracture traversing either. The deposition of ore took place mainly in 

 the Blue or Carboniferous limestone, and was a process of replacement 

 of this rock by metallic minerals in the neighborhood of these channels, 

 the original rock structure being in many cases preserved in the ore 



body. 



3. The third phase was produced by the dynamic movements which 

 resulted in folding, faulting, and displacement both of the sedimentary 



and eruptive rocks, and of their included ore bodies. 



4. The final phase has been the gradual wearing away and degrada- 

 tion of the uplifted masses and the carving out of the present mountain 

 forms, so that the ore bodies, formerly deeply buried, have been brought 

 near the surface and exposed to the action of atmospheric waters. 

 These reached them readily along the great fault lines and down the 

 gently dipping stratification planes, and changed not only their mineral- 

 ogical and chemical composition, but to a certain extent their position. 

 The great accumulations of gravels, which cover all the lower portions 

 of the hills and valleys, have probably rendered especially active the 

 decomposing action of surface waters, as they contain a sufficient admix- 

 ture of clay to render them capable of holding within their mass a great 

 deal of water, after the manner of a sponge, and thus giving it time to 

 act more thoroughly and deeply than if it were rapidly and freely 

 drained away. 



