490 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



sheet, which, as on the other hills, is generally near the base of the Blue 

 Limestone, but which does not conform strictly with the stratification 

 planes, crossing them at low angles, and extending in geological horizon 

 from the upper part of the White Limestone well up into the Blue Lime- 

 stone, across the intermediate lower White Porphyry. Besides this are 

 several smallei bodies of Gray Porphyry not found in the other hills, the 

 most important of which seems to have the form of a transverse dike. 



Of great faults like those on Carbonate and Iron Hills, there is no 

 evidence, the force of compression having only produced gentle folds and 

 some slight displacements of a few feet in extent, which are shown by sudden 

 changes of level in the ore horizon; such a one has evidently occurred along 

 the line of the north flank of the porphyry dike, which has slickensides 

 surfaces, and shows in some cases a slight difference of level in the ore hori- 

 zon on either side. 



The process of ore deposition has been evidently the same metasomatic 

 change or replacement of the limestone by ore and vein material, only it 

 has been can-ied so much farther that, instead of a body of limestone with 

 a little vein material extending irregularly from its surface downwards, 

 there is found here only a mass of vein material with occasional irregularly- 

 shaped residuary masses of unreplaced limestone or lime sand. Owing to 

 the irregular distribution of the intrusive masses of porphyry, whose con- 

 tact planes afforded channels by which the ore-bearing currents reached 

 the limestone, the evidence is naturally less striking that these currents fol- 

 lowed in general a downward course. Still it must be borne in inind that 

 the greater mass of the present bodies of vein material are the result of sec- 

 ondary alteration by surface waters, and that this alteration having been 

 much greater here, it is proportionately more difficult to trace the probable 

 form or position of the original sulphuret deposit. In spite of this it may 

 be observed that in the majority of cases the rich ore, which is presumably 

 nearer its original position than the iron oxides, is found near the upper 

 part of ore horizon. On the other hand, if the ore came directly from below, 

 according to the idea which is generally advanced with regard to the 

 source of ore deposits, the only channel which it could have followed would 

 have been the walls of the porphyr} dike. In this case we should expect to 



