398 GEOLOGY AND MINING INDUSTRY OF LBADVILLE. 



was comparatively free from iron oxide, it seems evident that it must have 

 resulted from the reduction of galena, the lead having been removed in the 

 state of carbonate. 



In the body of the limestone, on the eighth level not far from the North 

 incline, a natural jointing plane, forming one wall of the drift, was ob- 

 served to be coated Avith fine, silky, white crystals, which chemical exami- 

 nation proved to be calamine or silicate of zinc. If the sulphureted ores, 

 which will undoubtedly be found when the mine workings shall have 

 reached the limits of the zone of oxidation, are as rich in blende as those 

 which have been found in the A. Y. mine, it seems singular that little or 

 no zinc has hitherto been found associated with the oxidized ore. This 

 occurrence would seem to show that, owing probably to greater solubility, 

 the alteration products of blende have been removed during secondary 

 deposition to a greater distance from their original location than those of 

 the other sulphurets. 



In the lower levels of the mine there has been a notable increase in 

 the proportion of unaltered galena in the ore, but as yet no pyrites or other 

 sulphurets have been found. While specimens of galena are still found 

 which average as high as 1,200 ounces of silver to the ton, the general 

 tenor of the ore is lower than near the outcrops, and the evidence afforded 

 by the records of assays, which were very systematically kept in this mine, 

 shows that there has been a gradual but comparatively steady decrease in 

 the average tenor of the ore in silver with the progress in depth. These 

 records further show, and their evidence was confirmed by numerous tests 

 made in the laboratory of the Survey, that no reliance can be placed on a 

 relation assumed by some to exist between the coarseness or fineness of 

 grain of a galena and its contents in silver. 



Explorations to the eastward beyond the Tucson shaft and in the 

 lower part of the Main incline have been carried on along the contact line 

 thus far without very remunerative results. It would seem probable that 

 ore might be found in this direction in the body of the limestone, and 

 possibly in more or less direct connection with the cross-cutting sheet of 

 Gray Porphyry, from which those above mentioned are offshoots and 

 which may be assumed to be at a considerable depth below the contact ip 

 this eastern region. 



