578 



GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



Among the rocks tested during this investigation it was found in a single 

 specimen only of sedimentary rock, a sandstone impregnated with pyrite, 

 probably derived from the adjoining Pyritiferous Porphyry. As the quanti- 

 tative results are given in full in Table III, Appendix B, they need only 

 be presented in the following abbreviated form here: 



Lead in eruptire rock*. 



Of the above specimens all except granite and porphyrite belong to a 

 higher geological horizon than the Blue Limestone. The results given in 

 Table III are those which were obtained from that portion of the rock solu- 

 ble in strong acids. In only three cases was the insoluble part fused and 

 subjected to further treatment. In these cases more lead was obtained from 

 the insoluble part than from the soluble. It may therefore be assumed that 

 had the same treatment been pursued in each case the results would have 

 been even more conclusive as to the prevalence of lead in appreciable quan- 

 tities in the igneous rocks of the region. The greater portion obtained in 

 the second treatment existed undoubtedly in the form of silicate, since most 

 of the sulphide contained in the rock, whether original or secondary, would 

 have yielded to the acid treatment. But it does not necessarily follow that 

 all the lead obtained by the first treatment was associated with the pyrite, 

 for the analysis of pyrite from the Pyritiferous Porphyry treated separately 

 gives only 0.0019 per cent. PbO, while the average of eight specimens of the 

 rock gives 0.0020 in the soluble portion alone. 



As a further illustration, Table III gives the result of an examination 

 for zinc of two rocks from the Ten-Mile district, where the ores are highly 



