666 ■ GEOLOGY AND MLNING INDUSTKi' OF LEADVILLE. 



known conditions which pi'evailed where deeper-seated deposits were formed, 

 a certain amount of heat may have been added, is the one adopted by the 

 majority of students of vein phenomena to account for the removal of the 

 vein materials from place to place within that portion of the earth's crust 

 that comes under our observation. 



That it was from aqueous solutions that the Leadville vein materials 

 were deposited is a necessary corollary of the assumption that the deposi- 

 tion took place as a metasomatic interchange between them and the country 

 rocks, since the various materials of which they consist could have been 

 brought in and the dolomite and other rock substances have been removed 

 only by the agency of water. A further necessary corollary of the meta- 

 somatic interchange is that the ores were not deposited in pre-existing 

 cavities, as is generally assumed to be the case in ore deposits, particularly 

 those in limestone. The three assumptions being thus interdependent, evi- 

 dence in favor of either may be considered, in so far, a proof of the others, 

 and it will not be necessary to consider them separately. Direct evidence 

 that the original sulphides in the region were deposited in this manner is 

 necessarily difficult to obtain, where secondary alteration has gone so far as 

 it has in Leadville; but indirect and negative evidence is abundant. "When 

 the unoxidized deposits have been thoroughly opened by future explora- 

 tions, so that it will be ^jossible to study them in their unaltered and original 

 condition, an opportunity will be offered for testing the correctness of the 

 deductions here made. 



Indirect evidence. — In their present condition there can be no doubt that 

 the ore bodies are a replacement of the country rock. In the case of the 

 limestone deposits they grade off gradually into the countr}' rocks, the 

 only regular outlines of the bodies being those which are formed Ijy the 

 contact of the limestone with the adjoining porphyr}-; the other outlines 

 are irregular and ill-defined. Not only are fragments of unaltered limestone 

 found entirely inclosed within the ore bodies, but the latter sometimes 

 occupy the entire space between surrounding sheets of porphyry, which 

 the geological structure shows must have been formerly occupied by the 

 oi'iginal limestone bed. The chemical analyses of the ores and vein mate- 

 rials given above show lime and magnesia to be constant constituents, 



