314 University of California Publications in Geology [Vol. 8 



as has been pointed out, the channels of greatest alteration are 

 anastomozing with a general vertical trend. Still another significant 

 thing is the fact that many small, nearly flat, gouge-filled faults dip 

 toward the central parts of the ore-porphyry masses. This all points 

 to the main axes of these masses as the loci of greatest alteration and 

 enrichment. It was above these axes that the greatest thickness of 

 ore-porphyry was eroded away, with accompanying liberation of 

 sulphuric acid, metallic sulphates, and other active agents. Also, as 

 soluble substances were abstracted from the rocks, the first being taken 

 from the central parts where action was the earliest, the consequent 

 shrinkage must have converged toward that region, resulting in small, 

 flat-lying faults ; and the coincidence of anastomozing steep channels 

 of extreme alteration, and of downward circulation is readily under- 

 stood as that of effect and cause. 



If the chalcoeite can be taken as an indication of the mode of 

 origin of the most extreme alteration, since it is characteristically 

 associated with it, then the weight of that evidence would be in favor 

 of alteration by descending meteoric waters.. The evidence of the 

 chalcoeite consists essentially in the fact that it originates through the 

 enrichment of the yellow sulfids in copper. This is seen in the general 

 occurrence of yellow sulfids in the interior of small masses of chal- 

 coeite, and the coating of grains of yellow sulfids by replacement of 

 bornite and chalcoeite. And this phenomenon is distributed and 

 diminishes radially with reference to the main axes of the ore- 

 porphyry areas and the surface, the enrichment diminishing outward 

 and downward. 



There are extreme irregularities in the distribution of the chal- 

 coeite which appear to contradict this rule, but they may usually be 

 accounted for by barriers of impervious gouge, minor flat slips or less 

 permeable rock, which have diverted the enriching solutions along 

 their upper or lower sides according to local conditions of circulation. 



The general character, also, of the alteration, would seem to point 

 to meteoric waters, since all the minerals present, with the possible 

 exception of molybdenite, are such as are recognized to be of shallow 

 origin ; and the silicification, instead of being a general impregnation, 

 is localized along the minor steep channels of circulation. But the 

 molybdenite occurs universally disseminated through the ore in small 

 grains like the chalcoeite, and, like it, is also found in small fractures 

 in the ore-porphyry and inclusions of altered limestones as if it had 

 migrated in the same way as the chalcoeite. 



