DEPOSITS OF THE DEEP CIRCULATION. 1137 



Lindgren cites the gold veins of Nevada City and Grass Valley, Cali- 

 fornia, as another case of constant tenor. According to him, a "It can be 

 confidently stated that there is no gradual diminution of the tenor of the 

 ore in the pay shoots below the zone of. surface decomposition," although 

 within the same shoot there are many and great variations in richness. 

 This statement is applicable to deposits which reach a vertical depth of 500 

 or 600 meters. If Lindgren is correct in thinking that the gold-quartz 

 veins of the Sierra Nevada do not diminish in depth below an extremely 

 superficial upper zone, this would be a case in which sulphuret ores were 

 sufficiently concentrated by ascending waters alone to afford workable 

 ore deposits. 



In conclusion, it may be said that even tenors, in a vertical direction 

 for any considerable depth, is very strong evidence of a concentration by 

 ascending waters; but the reverse can not be said, that variability in a 

 vertical direction is evidence that the deposits were not deposited by 

 ascending waters. 



Fourth. The absence of oxidized products in deposits, especially of 

 limonite, favors the view that the ores were deposited by the deep circula- 

 tion and were unaffected by secondary concentration; but this criterion is 

 severely limited by the fact that enrichment of the sulphides takes place 

 below the level of ground water in consequence of secondary descending 

 solutions. An excess of base sulphides prevents the formation of the more 

 palpable evidence of oxidizing waters. 



Another difficulty in reference to using oxides as a criterion is that, in 

 consequence of denudation, the ores deposited by the deep circulations rise 

 into the belt of oxidation and are modified by the oxidizing waters to a 

 variable extent. Very early in the process such oxidized products as hema- 

 tite and limonite form. The ores are still essentially those of the deep 

 circulation. From this stage to that in which the secondary action of 

 oxidizing waters is of such consequence as to produce abundant hydrated 

 hematite and limonite there are all gradations; but in many cases the 

 secondary effect of descending, oxidizing waters can be allowed for and a 

 judgment reached as to the nature of the deposit before it was modified by 

 descending waters. 



ffi Lindgren, Waldeinar, The gold-quartz veins of Nevada City and Grass Valle}', California: Seven- 

 teenth Ann. Kept. TJ. S. Geol. Survey, pt. 2, 1896, p. 163. 



MON XLVII — 04 72 



