POSSIBLE CONTENTS OF PORPHYRY BODIES. 



583 



This porphyry is assumed to contain 4 per cent, of pyrite (FeS 2 rr4.00). 

 From Table III, Appendix B, its contents of protoxide of lead, as an aver- 

 age of the eight specimens tested, is 0.1'02025 per cent, in the soluble por- 

 tion ; or, assuming, from the proportion found in the insoluble portion of the 

 three specimens in which it was tested, that this represents only two-fifths 

 of the entire lead contents, the average contents of the whole rock would 

 be PbO = 0.0050625 per cent. From Table IV the average of ten speci- 

 mens assayed for silver is found to be 0.2773 ounce per ton, or, rejecting 

 the richest of these ten specimens as above the normal, the average of the 

 remaining nine specimens is 0.0265 ounce silver per ton of Pyritiferous 

 Porphyry. 



The probable thickness of the porphyry sheets it is rather difficult to 

 estimate. The sections as drawn give a maximum thickness of about fifteen 

 hundred feet, and an unknown thickness has been eroded away. It may 

 not be unreasonable to assume 1,000 feet as the average thickness of the 

 original body. From the above-assumed data would be obtained, as the 

 contents of the present and original areas of Pyritiferous Porphyry, re- 

 spectively, and on the basis of the two different values for lead and silver 

 given above, the following: 



To obtain an actual average of the metallic contents of this or any 

 other body of porphyry would have required a systematic sampling of the 

 rock and the taking of specimens at given and equal distances, not only on 

 its surface but through its mass in depth, for the tests already made show 

 that the metals are not evenly distributed, but vary in an apparently arbi- 

 trary manner. Such a sampling is manifestly not practicable, nor would 

 the expenditure of labor and time required by it be advisable if it were, 

 since in the present state of explorations in this region it is impossible to 



