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University of California Publications in Geology [Vol. 8 



Pyrite could not, of course, form in the upper zone of oxidation, 

 but it can exist unimpaired in the lower zone of oxidation away from 

 the open fissures, and therefore might easily form there, since those 

 conditions must be conditions of stability for pyrite. And in this 

 region it is conceivable that the pyrite should have an irregular dis- 

 tribution, that is. being disseminated sporadically through the rock 

 away from open fissures, on account of the diffused and slow circu- 

 lation there. This being granted, the rest follows, since the conditions 

 of precipitation of the copper svdfids are supplied by local centers con- 

 sisting of grains of pyrite. 



Presumably those main channels of descent for the vadose waters 

 must terminate at or near the ground- water level, and in the neigh- 

 borhood of those points of entry deposit their metallic burdens in more 

 or less concentrated form ; but nowhere in these excavations do these 

 channels terminate nor do such concentrations of sulfid occur. Thus, 

 the evidence for which one would naturally look to indicate the present 

 or past level of the ground water is not to be found. 



QUARTZ BLOUT 



An interesting phenomenon of the district is the "blout" 4 or 

 jasperoid. This is a compact granular silicious rock, generally of a 

 yellowish-brown color, forming bold outcrops along the copper belt. 

 In a general way it is arranged peripherally to the ore-porphyry ; but 

 the arrangement is irregular and ambiguous. Lawson has advanced 

 the hypothesis that these deposits may be of the nature of an encasing 

 shell about the porphyry due to contact action. But the better 

 exposures due to mining operations of recent years have revealed 

 evidence which does not support this view. The blout in many eases 

 caps the porphyry but is nowhere found within it, while it is certain 

 that a considerable thickness of the intrusive has been eroded away 

 over the points where it is found. The occurrences of blout are also 

 notably conformable to the present topography. Had it originated 

 from magmatic emanations it must have incrusted the original periph- 

 ery of the intrusion in addition to being distributed through the 

 neighboring limestones and shales as it is. These hard shells of silica, 

 then, should have controlled the topography, preserving the under- 

 lying rock from weathering and erosion. But this is not the case. 

 Many of the hills have jasperoid outcrops on the summits, but they 



* Lawson, op. tit., pp. 324-330. 



