ON STRUCTURAL FEATURES OF MINERAL VEINS. 39 



{see Example 16), the Leadville silver mines (Example 30), the 

 southwest Virginia zinc deposits at Bonsacks (Example 27), the 

 copper and silver veins at Butte, Mont. (Example lY), and others 

 in Llano County, Texas (Example I7c), may be cited. At Duck- 

 town a considerable thickness of chalcocite, melaconite, and carbon- 

 ates accumulated just at the water line and abruptly changed to 

 low-grade, unworkable pyrite and chalcopyrite below it. At Bon- 

 sacks, near Roanoke, Ya , very rich, easily treated earthy limon- 

 ite and smithsonite (30-40 % zinc) passed into a refractory, low- 

 grade (15-20 % zinc), intimate mixture of blende and pyrite. 

 Excavations in dry districts may not reach the water line for great 



y (j. 7._ illustration of the oxidized zone, or gossan, the zone of enrich- 

 ment, and the unchanged sulphides, at Ducktown, Tenn. After 

 A. F. Wendt, School of Mines Quarterly, Vol. VII., 1886. 



depths. Thus at Eureka, Nev., in the rainless region of the Great 

 Basin, the oxidized ores continue to 900 feet. 



It is worthy of remark in this connection that possibly some 

 deposits of oxidized ores may have been formed originally as such. 

 Wendt has argued this for the copper mines of the Bisbee dis- 

 trict, Arizona. (See Example 20&.) If oxidized ores are now 

 found below the water line, it may indicate a depression of the 

 rocks from a previous higher position. R. C. Hills has brought out 

 a very interesting instance of the concentration of gold and silver 

 in the lower part of the oxidized zone, or at least at a considerable 

 depth below the outcrop. The upper portion of the vein, in this 

 case with a quartz gangue, was impoverished. The gold is thought 



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