DEPTH OF EFFECT OF DESCENDING WATERS. 1181 



therefore showing that the oxidizing, carbonating waters above the level of 

 ground water have been extremely effective. 



Up to this point there will be no disagreement on the part of anyone. 

 The question now arises as to the depths below the level of ground water to 

 which descending waters produce their effects. This is a question to be 

 answered, not by deduction, but by observation. Even Posepny, who 

 emphasizes the effect of ascending waters, agrees that oxidized products are 

 the evidence of the work of vadose circulation, or the circulation of lateral 

 and downward-moving waters. Furthermore, Posepny agrees that the 

 iron ores of the Lake Superior region," which are oxidized products, have 

 been produced by downward-moving waters. A number of the mines 

 have been worked to a depth of 500 meters or more below the level of 

 ground water; and in these cases it is perfectly clear that to this depth the 

 downward-percolating waters have produced an oxidizing effect. This is 

 true in a region in which the level of ground water is relatively near the 

 land surface, and which is not mountainous. In various other regions 

 oxidized products are found also to a very considerable depth below the 

 level of the ground water. 



In the San Jiian district of Colorado, in the Revenue tunnel of the 

 Virginius mine, the effect of oxidizing waters has been observed to the 

 depth of nearly 1,000 meters, as shown by iron-oxide stains through the 

 sulphides. How much deeper oxidation may be effective is not known, 

 and below this depth there may be a belt in which the sulphides may 

 have been enriched by descending solutions from above without the actual 

 production of oxidized products. In the Cripple Creek district of Colorado 

 oxidized products are not found to a great depth, but if the suggestion is 

 correct that the high-grade tellurides are due to the effect of descending- 

 waters, such waters must have been effective to a depth of 400 meters. 

 As yet the lowest limits of the enriched belt has not been reached, and 

 how much deeper it extends is unknown. Both these districts are humid 

 and of very accentuated topography, and thus furnish exceptionally favor- 

 able conditions for descending waters to produce deep effects. 



The illustrations given furnish positive evidence that the belt in which 

 descending waters are effective in producing rich secondary concentrates 

 under favorable conditions extends to a very considerable depth. 



"Posepny, F., Genesis of ore deposits t_ discussion): Trans. Am. Inst. Min. Eng., vol. 24, 1895, 

 pp. 966-967. 



