208 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



idan line, is found directly over the upper sheet of White Porphyry in very 

 considerable thickness. How great this thickness may have been there is no 

 direct means of determining, since, except on Prospect Mountain (where no 

 shafts have been sunk to any great depth), no sedimentary rock remains 

 above it to give its upper limits. Its greatest observed thickness is 420 feet 

 in the Independent shaft. What was the original extent to the southward 

 of this Gray Porphyry sheet before any portion of it had been removed 

 by erosion, there is also no means of determining. At present it extends 

 but little beyond the median line of the map. Its source must probably be 

 looked for to the northward, beyond the limits of the map, since in that 

 direction it passes into Lincoln or Eagle River Porphyry, of which it seems 

 to be merely a decomposed variety. 



Pyritiferous Porphyry. Next in importance in point of superficial extent, 

 and possibly of greater importance in its bearing on the ore deposits of the 

 region, is the Pyritiferous Porphyry. The main sheet of this porphyry, 

 which covers the lower slopes of Breece Hill, seems to be a stratigraphical 

 replacer of the Gray Porphyry, which however, along the line of the Breece 

 fault, it overlaps. It may therefore be supposed to have been in point of 

 time a later intrusion. As is shown in Section G, one of the vents, and 

 possibly the sole vent, probably existed beneath California gulch Its ex- 

 tent to the north and east could not have been much greater than at present. 

 To the south and west, however, it may have covered a considerably larger 

 area immediately above the White Porphyry. Of the upper sheets in the 

 Weber Grits no opinion can be formed, so completely has all trace of this 

 formation been removed by erosion. It is perhaps fair to assume that it ex- 

 tended to the south as far as the present crest of Long and Derry Ridge, 

 and to the westward over Irpn Hill, and, possibly, as far as Carbonate Hill. 



other porphyries. Of the other bodies of porphyry, the most important in 

 their bearing upon the ore deposition of the region are those which are 

 essentially of the same rock as the main sheet of Gray Porphyry, though 

 having no apparent connection with it. The most extensive sheet of this 

 rock is found under Iron and Carbonate Hills, near the base of the Blue 

 Limestone, and cutting up across this horizon to the westward; various irreg- 

 ular, dike-like bodies found in the different mines are, doubtless, offshoots 



