390 GEOLOGY AIS^D MINING INDUSTRY OF LEADVILLE. 



ment of masses of Chinese tale in the ore body, at some distance from the 

 contact The inch'ne has not yet reached a second ore shoot in depth, 

 though there is every probabiHty that one will eventually be found there. 



The ore in both these mines is mainlj- a hard carbonate, very rich in 

 lead, but of comparatively low grade in silver. It is ver}' thoroughly oxi- 

 dized, and in some cases a red oxide of lead lias been found in it. It 

 occurs in bodies sometimes of considerable thickness and always at or near 

 the contact. At the contact the alteration of porphyry into the so-called 

 Chinese talc is very persistent, and when found in the ore body, as in the 

 Dome mine, shows that offshoots of the porphyry had probably penetrated 

 the limestone previous to the replacement of the latter by vein material. 



Sections E and G, Atlas Sheet XXV, which pass through the Rock 

 workings, show the fold in the limestone, which affords a good illustration 

 of the tendency of the ore currents to deposit their load immediately above 

 any sharp bend in the stratification. 



La Plata, Stone, and A. Y. — The La Plata claim is Opened by a tunnel 

 800 feet long, running south from near the bed of the gulch. Its direction 

 was intended no doubt to correspond witli the strike of the formation, but 

 in point of fact it diverges a little to the westward, so that while at the 

 mouth it is at the actual contact of the White Porphyry and Blue Limestone, 

 it departs from it more and more as it advances. At the extremit}-, however, 

 the contact bends sharply down to the south, so that a winze has been sunk 

 70 feet to reach it. It is noticeable that this bend is on a line with the east- 

 ward continuation of the California fault. 



Below the mouth of the tunnel and in the body of the limestone is 

 found the Gra}' Porphyry sheet, which to the north and south is found 

 above the Blue Limestone and separating it from the White Porphyry. The 

 contact in this mine was not found verj^ productive. A small body of ore was 

 found east of the tunnel, near its mouth, and a prospecting drift running to 

 the Gneisson shaft, and continued some distance beyond it, found the usual 

 evidence of mineralizing action, but no pay ore ; it showed, however, a 

 steepening of the dip of the formation of 35°. This, with the sudden steepen- 

 ing at the end of the tunnel, shows how difficult it is to count on any regu- 

 larity in the dip of the formation until it has been actuall}- proved. The 



