378 University of California Publications in Geology [Vol. 10 



tical distance downward of 37 feet. Some of the analyses from the 

 Capote mine, Cananea, Mexico, show also a neutralization of acid at 

 comparatively shallow depth. The geologic data are comparatively 

 in harmony with this conclusion, for there is abundant evidence that 

 descending acid waters attack alkaline or alkaline-earth silicates and 

 alkaline-earth carbonates ; acid reacting with feldspars or sericite forms 

 kaolin, and these reactions are attended by the solution of alkalies or 

 alkaline-earths as sulphates." 



It is probable that the sulphate solutions upon reaching the ground 

 water-level, or the reducing zone, lost free acid and became neutral. 

 The sulphate was then reduced to the sulphide and the pyrite-ore 

 body formed as a metasomatic replacement in the rhyolite. 



Prom the character and disposition of the ore deposits at Leona 

 Heights it seems probable that the rhyolite at this place is underlain 

 by the Knoxville shale ; indeed, from the exposures of this shale near 

 by, as shown by the geologic map, it may be safely inferred that the 

 shale lies at a comparatively short distance below the present workings 

 in the ore body. The ore body described by Mace 23 rests on a carbo- 

 naceous shale, and he concludes that organic matter from the shale 

 was taken into the underground circulation and produced a local 

 reducing water table. The ore, as far as has been exploited, appears 

 to have a downward trend or pitch towards the northeast, so that it 

 may extend to the shale. The shale may have supplied reducing agents 

 in the form of carbonaceous matter and H,S to the ground water. The 

 downward percolating water contained in solution the metallic salts. 

 The surface of the underground water was probably near the top of 

 the shale formation and the water was supplied with carbonaceous 

 matter and H 2 S derived from the shale. When the two waters met 

 at the water table the ferrous sulphate was completely reduced to 

 pyrite. If this be so, the influence of the shale on the chemical con- 

 dition of the ground water may have been a potent factor in the local- 

 ization of the pyrite deposit. Much light will be thrown on this point 

 when the lower limit of the ore is determined, and the probability of 

 shale lying not far below the present mine workings should be consid- 

 ered in plans for exploitation at lower levels. 



23 Op. cit., p. 1320. 



