494 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



of their deposits or of their inclosing rocks, their geological structure has 

 been more imperfectly understood by those in charge, and the work of explo- 

 ration been carried on with less system and sometimes in an utterly aimless 

 manner. Although it has been impracticable for these reasons to determine 

 with the same accuracy and detail the relations of the ore be dies in these 

 outside mines as has been done for those of Iron, Fryer, and Carbonate 

 Hills, an explanation of their general geological structure will be of value 

 as a guide for future exploitation, and a consideration of the relative amount 

 of replacement action in different portions of the Leadville region, as shown 

 by the developments thus far made in them, will afford a basis for determin- 

 ing the probable extent and direction of the original ore currents, and in 

 consequence what part of the ore horizon, which the geological outlines 

 have already located, is most likely to contain valuable ore bodies. 



An examination of the relative distribution of vein material shown 

 by the outcrops, as delineated in cross-lining on the Leadville map, shows 

 two lines or zones along which the evidence of replacement action is most 

 apparent, one running east from Fryer Hill to Little Ellen Hill, the sec- 

 ond taking more of a southeasterly course from the southern end of Carbon- 

 ate Hill to Long and Derry Hill. In the area between these two zones the 

 surface is formed by porphyry bodies which overlie the ore horizon, so that 

 no outcrops of vein material show on the map, except a few thin lines along 

 the edges of fault planes. Under these porphyry bodies in Carbonate and 

 Iron Hills a very large proportion of the area is already proved to be occu- 

 pied by valuable ore bodies, and it may, therefore, be reasonably expected 

 that similar bodies exist under the porphyry sheets between these zones, 

 farther east, although, owing to the greater depth of the ore horizon, they 

 have not yet been reached by mine workings. There is evidence of still 

 another zone of replacement extending north from Fryer Hill under Prospect 

 Mountain, but, as the ore horizon has been reached in few points and the 

 vein material has, at these few points, proved comparatively poor, the chances 

 of finding any considerable development of rich ore in that direction have 

 necessarily a smaller basis of probability than to the eastward, though the 

 general geological conditions favor it. 



