580 GEOLOGY AND MINING INDUSTEY OF LEADVILLE. 



but the rock of dikes was found to most nearly approach this condition. 

 The three specimens which were found to contain silver came from dikes, 

 while all the others came from the main sheet overlying the Blue Lime- 

 stone. Considering this fact alone, it might be assumed that the metallic 

 contents had already been leached out of this sheet. On the other hand, 

 the rock in its unaltered, normal condition apparently contained a very 

 small proportion of basic silicate ; and of pyrite, if it existed as an original 

 constituent, but little trace is left. 



On the other hand, the Pyritiferous Porphyry, which stands at the 

 opposite end of the scale as regards its contents in gold and silver, although 

 generally decomposed at the surface, is less so than the White Porphyry, 

 and its interior is less deeply exposed, either by erosion or by underground 

 workings. It also contains a larger proportion of basic silicates. Its most 

 striking feature is the enormous amount of pyrite that it contains, amount- 

 ing, on an average, to about 4 per cent, of its mass. Part, at least, of 

 this pyrite is original, as it is found included within the crystals of quartz. 

 Both pyrite and galena are occasionally found, however, coating the joint- 

 ing planes of the rock, in which case they are undoubtedly secondary. 



From a consideration of the quantitative results given for this rock in 

 Table IV, it is evident that, while the traces of gold in the rock might have 

 been contained in the pyrite, all the silver could not thus be accounted for. 

 The average assay of the ten specimens of Pyritiferous Porphyry is 0.2773 

 ounces Ag per ton ; but one of these might be considered abnormal, since 

 it alone contains more than the sum of the other nine. Rejecting this, the 

 average of the other nine specimens is 0.0265 ounce Ag. The pyrite, sep- 

 arated and assayed alone, gave 0.890 ounce Ag, or 0.00134 per cent.; but 

 in a rock containing 4 per cent, of such pyrite, which was the estimate 

 obtained by a careful mechanical separation of this material from a Pyritif- 

 erous Porphyry of average composition, there would only be 0.0156 ounce 

 Ag per ton, or less than three-fifths of the above average of nine specimens. 



A mixture of galena and pyrite, also separated from the rock and 

 assayed by itself, gave 2.4 ounces Ag to the ton, or 0.00823 per cent. From 

 Table III it is found that the eight specimens of Pyritiferous Porphyry 

 tested have an average of 0.002025 per cent PbO in the soluble portion, or 



