LARGE OPENINGS NEAR THE SURFACE. 1179 



mine the lower levels are as dry as the upper levels. The region is arid 

 and the mine is never wet, but the mine shows "universal evidence of a 

 secondary enrichment that must have proceeded from the surface down- 

 wards." 01 This fact Emmons regards as evidence that at the comparatively 

 recent Bonneville epoch the water level was comparatively near the surface. 



Aside from secular changes in the level of ground water, due to varia- 

 tions in rainfall extending' through geological periods, the shorter periods 

 of varying rainfall produce some effect upon the level of ground water. 

 Thus the annual and several-year period variations in rainfall cause slight 

 changes in it. 



All these changes favor alternate solution and deposition — solution 

 when the level of ground water falls, precipitation when it rises. Where 

 the ground water has been at a low level large openings to some depth 

 are likely to be produced. Where later for some reason the level of 

 ground water rises these openings are in a very favorable position to be 

 filled with ore, as a result of precipitation from ascending solutions, of the 

 reactions of descending solutions, and of the mingling of ascending and 

 descending waters. 



It might be argued that the existence of ore deposits in the large open- 

 ings of the belt of weathering is evidence that the ores were not first 

 deposited by ascending waters. However, as has been already explained, 

 in such openings there may be concentrated mineral material originally 

 distributed by ascending waters through a much greater vertical distance. 

 Thus a very rich ore deposit in large openings, formed by the reaction of 

 descending waters upon a first concentration produced by ascending waters, 

 may be bounded below by a horizon in which the ground is very close, the 

 comparatively small openings which once existed having been greatly 

 enlarged by solution during reconcentration by descending water. 



DEPTH OF EFFECT OF DESCENDING WATERS. 



There can be no doubt of the importance of downward-percolating 

 waters in the production of ore deposits in the zone in which they are 

 active. The only question which remains open is the depth to which they 

 are effective. This varies greatly in different districts and in the mines of 



"Emmons, S. F., The Delarnar and the Horn-Silver mines; two types of ore deposits in the deserts 

 of Nevada and Utah: Trans. Am. Inst. Min. Eng., vol. 31, 1902, pp. 672-673. 



