THE GENESIS OF ORES 537 



waters. It is even possible in some cases to determine by the char- 

 acter of the minerals the origin and nature of the causative solutions. 



As to the relative importance of the work of ascending and de- 

 scending waters there is also divergence of opinion. There are few 

 who still doubt the agency of descending waters in the formation of 

 the oxidized ores, such as carbonates, silicates and oxides of copper, 

 lead and zinc and silver chloride, or in the superficial or shallow altera- 

 tion of the sulphides, arsenides or antimonides. The iron ores of the 

 Lake Superior region, for example, are generally believed to owe their 

 concentration to descending solutions, in this respect differing from 

 many of the Scandinavian iron ores, according to recent descriptions. 



It is not, however, the oxidized or " dry " ores alone that are now 

 believed to owe their formation in large part to the action of descending 

 waters; but the base ores consisting of chemical combinations of the 

 metals with sulphur, arsenic, antimony, tellurium and some rarer 

 elements. It is only within the past decade that it has been considered 

 possible that the sulphide minerals are produced by reaction between 

 sulphate or carbonate solutions and undecomposed sulphides or other 

 minerals found in veins. Laboratory experiments have, however, 

 shown that the operation is not only possible, but easily accomplished 

 and duplicated under normal conditions as to temperature and pres- 

 sure. 2 This is a fact of great importance and wide significance, for it 

 aids in the explanation of many formerly puzzling phenomena of 

 mines and mining geology. 



It has long been noticed by the students of ore deposits that by 

 far the greater number of mines become exhausted at comparatively 

 shallow depths; that veins, instead of continuing downward uniform 

 in size and composition, like dikes of diabase and porphyry, become 

 smaller and of lower value with depth, and often disappear altogether. 

 It is noticed also that the shape of many ore deposits and the distribu- 

 tion and paragenesis of the minerals which they contain can often be 

 better explained on the theory of descending than of ascending miner- 

 alizers. Moreover, it is apparent that there are changes constantly in 

 progress in those portions of sulphide ore bodies lying nearest the 

 surface of the ground. These changes consist in the oxidation of the 

 sulphides and their solution as sulphates. These sulphate solutions 

 percolate downward into the veins or rocks below along the most open 

 channels; and thus, by degrees, the upper zone of the vein is robbed 

 of most or all of its sulphide minerals, and only a gossan or iron cap 

 remains. 



The process of oxidizing and leaching out of the sulphides in the 

 superficial zone of ore deposits tends, first of all, to disguise the nature 



2 H. V. Wincliell, "The Synthesis of Chaleoeite," Bull. Geol. Soc. Am., 

 Vol. XIV., pp. 269-276, 1903. 



