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352 



RESUME AND THEORETICAL DISCUSSION 



there is no doubt in the writer's mind — and this statement is not here made 

 without many years of careful examination of important mining regions in 

 various countries — that the occurrence of metalliferous ores is rather a 

 surface phenomenon than a deep-seated one. The conditions favorable to 

 the formation of veins and vein-like or segregated masses are more likely 

 to have existed near the surface than deep down below it. Such conditions 

 would be — in part at least — diminution of temperature, relief from pres- 

 sure, and, in the case of true veins, the existence of fissures. The history 

 of mining operations shows beyond dispute that bodies of ore occurring in 

 the segregated form are, on the whole, not to be depended on for persistence. 

 And even true fissure veins must eventually give out in depth, if for no 

 other reason than the change in the character of the enclosing rock brought 

 about by intense heat. Neither would fissures be likely to continue to 

 exist, nor the materials filling them to retain a distinct form, where the 

 temperature was above the melting point. Thus, whatever theories we may 

 adopt for the formation of mineral veins, we are led to the conclusion that 

 they must, as a general rule, be better developed near the surface than at 

 great depths. The fact that in some important mining regions the very 

 upper portion of the veins has been oxidized, and then dissolved away by 

 water, is very easy of comprehension, and not in conflict with what has been 

 stated above. 



The fact must be admitted that the quartz veins and masses were the chief 

 source of the gold contained in the gravel, but at the same time it is true 

 that gold does exist in the bed-rock where no quartz is found associated with 

 it. As far as the writer's observations go, however, such occurrences are 

 pretty much limited to the immediate vicinity of veins of quartz. The Great 

 Quartz Vein, or Mother Lode of California, is in places associated with slaty 

 rocks which carry considerable gold, although this appears to be very irregu- 



1 • • 



lar in its occurrence. Such localities have never, so far as known to the 

 writer, been found persistent enough in their yield to pay for working.* As 

 far, however, as the special theory of the gravels is concerned, it is a matter 

 of but little consequence whether the gold they contain came exclusively 

 from the quartz veins, or in part also from the adjacent rock. 



* In regard to such localities as those of Quail Hill and the Harpendii.g Claim, where the slates are reported to 

 have heen highly productive in gold, but little can be said, except that they have proved, on working, to be con- 

 spicuous failures. How far the reported richness of the rock is to be accounted for by previous "salting" of the 

 places from which the specimens were taken for assay, it is not easy to say. It is very clear, however, that there 

 was more or less of fraud mixed up with the transactions at both of these localities. 



