584 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



trace with any degree of certainty the processes of original ore deposition. 

 The most that could be hoped for was to indicate the possible methods by 

 which the deposition might have taken place and to weigh the probabilities 

 afforded by ascertained facts in favor of one or the other of these methods. 

 The foregoing reasons seem to favor the probability that the ores may have 

 been derived, in part at least, from one or more of the bodies of porphyry 

 which occur in the region, and the above figures show that the small per- 

 centages of the metals still existing in these rocks might furnish, an ade- 

 quate amount of material to form the known ore bodies. 



The most uncertain element in all these calculations and hypotheses is 

 the form and extent of the porphyry bodies in depth beyond the limit of 

 present explorations. The form given to these bodies in the sections is, as 

 has already been stated, only hypothetical, though founded on deductions- 

 from many actual observations and in all probability correct in its main 

 outlines. Still it is probable that there are more vents or channels from 

 below through which the bodies have reached their present position than 

 are shown there, but their number and position can only be determined 

 by actual exploration It is possible that in future years, when mine 

 workings shall have been extended over areas where the ore horizon 

 exists at considerable depths below the surface and other eruptive channels 

 have been found and critically examined, evidence may be obtained that 

 ore solutions have ascended along these channels from below. Such evi- 

 dence will not, however, necessarily preclude the derivation of part of the 

 metals from the country rocks, and at present that derivation is the only 

 one which has the support of actual though somewhat indirect proof. 



Another element of uncertainty, and one which renders it difficult to 

 decide from what particular variety of porphyry the metals of the deposits 

 were derived, is the impossibility of determining the form and character of 

 the porphyry bodies, which have been removed by erosion, as they existed 

 at the time of original ore deposition, and upon this point future explora- 

 tion will throw little or no light. 



