334 



University of California Publications. 



[Geology 



distributed in the ore bodies thus far blocked out. But the richer 

 ground is apparently rather determined by freedom of flow of 

 descending solutions, that is, by local structural conditions in 

 the porphyry, than by any horizontal control such as the present 

 level of the ground water. In so far as the sulphides are uni- 

 formly distributed, this may possibly be ascribed to a steady 

 retreat of the ground water level, whereby the additional copper 

 ore leached from above was distributed fairly uniformly over 

 the range of that retreat, although at any given stage it was 

 precipitated chiefly at the contact with the ground water. It is 

 more probable, however, and more consistent with the distribu- 

 tion of chalcocite at Butte, Montana, that the products of the 

 down leaching from the zone of oxidation were not precipitated 

 at the ground water level, but more diffusely through the region 

 of the ground water, in the course of a slow circulation, which 

 is implied in the maintenance of a fairly constant level of that 

 water in the face of additions from above. In this view we 

 might expect below the level of the ground water an appreciable 

 but very moderate enrichment of the ore as compared with the 

 values in the still unoxidized ore above the ground water to 

 which the mine developments are at present chiefly confined. 

 This generally very moderate enrichment will doubtless be found 

 to be greatly modified by structural differences affecting free- 

 dom of circulation of the ground water, and may only be de- 

 tected when considerable bodies of the country have been blocked 

 out so as to permit of a satisfactory average value, which may 

 properly be compared with average value of the ore in the un- 

 oxidized ground above the present water plane. This zone of 

 probable moderate enrichment might from the experience at 

 Butte be expected to gradually decrease in value with depth. 



The second significant fact to which it is here desired to 

 advert, is that the ground which is being leached in the course 

 of oxidation differs in no essential particular from that which 

 is being enriched in consequence of that leaching. It has been 

 shown that the line of contact between the oxidized and the un- 

 oxidized zones, while quite irregular, is exceedingly sharply de- 

 fined and that the result of oxidation and its attendant leaching 

 is simply to lower the upper surface of what may be termed the 



