SECOND CONCENTRATION BY DESCENDING WATERS. 1139 



In some cases the deposits thus produced are rich enough to be of 

 economic importance. In these cases, which undoubtedly exist, but which 

 perhaps are less numerous than. one might at first think, a first concentra- 

 tion by deep waters has been sufficient. 



SUBCLASS 2. ORES PRECIPITATED FROM ASCENDING AND DESCENDING AQUEOUS 



SOLUTIONS. 



Thus far ores precipitated by ascending waters alone have been consid- 

 ered. However, many of the ores thus produced have been profoundly 

 modified by the action of descending waters. 



Where the point of exit of the ascending waters of the trunk channels 

 is in a valley or near the level of surface drainage the waters may reach 

 the surface. However, Avhere the openings are below slopes, ascending 

 waters ordinarily do not continue to the surface, but make their way 

 laterally from the trunk channels at and below the level of ground water. 

 (See fig. 26.) Above the level of ground water, and frequently for a 

 certain distance below it, the movement is downward in the openings. The 

 water thus moving downward includes not only that which directly passes 

 into the trunk openings at the surface, but a much larger quantity which ' 

 converges into them from the smaller openings on all sides. 



In regions in which mining is going on denudation has ordinarily 

 truncated the veins for considerable depths, in many cases to hundreds or 

 even thousands of meters. It is therefore clear that, in a majority of cases, 

 the upper portions of the fissures in which the waters are now descending 

 were in all probability much deeper below the surface, and therefore the 

 waters in many of the larger fissures were once ascending. During the 

 time the water was ascending the first concentration of sulphurets and other 

 products took place. But as a result of the downward migration of the belt 

 of weathering and the downward movement of water in that belt alteration 

 and secondary concentration of ore deposits have taken place. This second 

 concentration of ore deposits is of the greatest consequence, and I believe 

 largely explains the frequent greater richness of the upjaer 50, or 100, or 

 500 meters, and in some cases 1,000 meters, as compared with lower levels. 



As soon as denudation results in the transfer of the sulphides, 

 tellurides, and metals of the first concentration into the belt of weathering- 

 they are subjected to the action of the descending waters bearing oxygen 



