582 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



entirely wanting, no silver was found, while the trachyte and andesite, 

 which contain more of these minerals than the Black Hill rhyolite, also 

 contain more silver. 



Although the above facts are not sufficiently conclusive to afford ab- 

 solute proof that the metallic contents of the deposits were entirely derived 

 from the eruptive rocks, they certainly show the possibility and even prob- 

 ability that this source furnished a part at least of the vein materials. 



The actual percentage of metals found may seem very small; on the 

 other hand, it should be remembered that the amount of time and of water 

 allowable for the leaching process may have been almost indefinitely large. 

 The present porphyry bodies, moreover, are of enormous extent as compared 

 with the actual size of the deposits, while the amount of porphyry that has 

 been removed by erosion since the deposits were first made, though it can- 

 not be accurately estimated, must have been even larger. 



Possible contents of porphyry bodies. In order to show that even with the 

 small percentages given in the above table the possible contents of 

 the porphyry bodies are amply adequate to account for the amount of ore 

 thus far developed in the district, a hypothetical calculation will be made 

 based on these percentages and on the probable bulk of one of the por- 

 phyry bodies, taking first the amount assumed to exist now and second a 

 conservative estimate of the amount which existed at the time of original 

 ore deposition, and before any of it had been removed by erosion. For 

 this purpose the Pyritiferous Porphyry will be chosen, since a greater 

 number of tests of this rock have been made than of any other. 



The present area of outcrop of the Pyritiferous Porphyry may be 

 taken, in round numbers, as 5,000 X 10,000 feet = 50,000,000 square feet. 

 If it is assumed that it originally extended westward to the foot of Car- 

 bonate Hill, north to the line of Yankee Hill, south to that of Printer Boy 

 Hill, and but little beyond its present boundaries to the eastward, it would 

 have covered a square area of 10,000X20,000 = 200,000,000 square feet, 

 or four times the assumed area of its present outcrop. The specific gravity 

 of Pyritiferous Porphyry, obtained as an average of four specimens, is 

 2.608. Therefore one ton (2,000 pounds) of this rock would occupy 12.27 

 cubic feet; say 12 for convenience of calculation. 



