572 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



source. It would seem proper, therefore, in a practical treatise like the 

 present, to leave out of consideration altogether the ultimate and purely 

 speculative source and to confine the investigation to the more immediate 

 source, about which it is possible to obtain some actual and demonstrable 

 evidence. 



As circulating waters must take up as well as throw down their metal- 

 lic contents, it is evident that under varying conditions the same material 

 may have been deposited more than once and in more than one form since 

 it reached that part of the rocky crust of the earth which is open to actual 

 observation. There may be, therefore, intermediate sources between the 

 ultimate and the immediate, but which, like the ultimate, are removed from 

 actual demonstration. 



It is common practice to say of any ore deposit, not distinctly sedi- 

 mentary, that it has come from below, and to rest content with this state- 

 ment, which, even if not susceptible of direct proof, has the merit that in 

 one sense it cannot be disproved. This practice evidently had its origin 

 in the fact that early writers upon ore deposits used as the type deposit, 

 upon which to found their theories, the nearly vertical fissure vein. This 

 they assumed to be the filling of a pre-existing open crack, extending indefi- 

 nitely toward the center of the earth, by heated solutions arising from 

 great depths; as these solutions approached the surface of the earth and 

 were consequently relieved of the great pressure to which they were sub- 

 ject in the depths, by reason of that relief they gradually deposited their 

 contents on the walls of the fissure until it was completely filled. While 

 under the theoretical conditions assumed this hypothesis might afford an 

 adequate explanation of the manner of deposition in such a vein, it is by 

 no means proved that such conditions exist in nature, and therefore the 

 explanation, so readily given in most cases, is generally inadequate and not 

 founded upon a sufficient study of the geological conditions. 



In the case of the Leadville deposits the inadequacy and even falsity 

 of this explanation, except as applied to the ultimate source from which the 

 metals may have been derived, is readily apparent. 



In the first place, the geological study of the district has shown that 

 thev must have been formed beneath a thickness of at least ten thousand 



