472 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The porphyry dike is, as before, well defined on the hanging- wall side, 

 having a clay selvage and some appearance of slickensides ; its angle of 

 dip is no longer as steep, averaging from 45 to 50, and its thickness is 

 also very variable, at one point being only 18 feet, at others thirty to forty 

 feet, and in one case a drift was run in it 70 feet, and a raise was then made 

 up to the Wash. It must be borne in mind, however, that the portion of 

 the dike exposed by the few mine drifts which cut it is very small, relative 

 to the whole mass, and that the variation in dip may, in many cases, only 

 represent irregularities in the form of the body, and not variations in the 

 dip of the mass as a whole. 



The stringers of porphyry seen in the Little Pittsburgh ground have 

 here enlarged into extensive sheets, which split up the ore horizon into three 

 portions. The upper portion represents the greater part of the Blue Lime- 

 stone body and furnishes the main supply of ore, the second and third ore 

 bodies being simply irregularly-shaped portions, which were separated at 

 the time of the injection of the porphyry, and have since been changed to 

 vein material by the action of the ore currents. As these lower bodies have 

 yielded but little pay ore, they have not been as thoroughly explored as the 

 upper one, and their outlines, as given on Sections A and H, are more or 

 less hypothetical. 



The ore of the Amie mine is, as a rule, much richer than those already 

 described. Even the iron vein material often averages ten to twelve ounces 

 per ton in silver, in large masses, and, being comparatively free from silica, 

 has been profitably employed as a flux by the smelters, in place of the 

 Breece Iron ore which they had hitherto been using, and which was rela- 

 tively much more expensive. The rich ore, mostly dark sand carbonates, 

 generally occurs at the top of the ore horizon, immediately under the over- 

 lying porphyry, a clayey, iron oxide, with more or less manganiferous or 

 black iron, forming the base of the horizon. Chert is much less widely 

 developed than in the previously-described mines. A considerable amount 

 of so-called "Chinese talc" is found throughout the rich ore bodies, doubt- 

 less the product of alteration of stringers of porphyry in the original 

 limestone. South of the dike no considerable quantity of rich ore had 

 been found at the time of examination, as the map shows ; explorations had, 



