1192 A TREATISE ON METAMOEPHISM. 



with chalcopyrite; still lower the chalcopyrite is mixed with pyrite; and in 

 the deepest levels the pyrite is dominant. Apparently this vertical arrange- 

 ment of the sulphides can not possibly be explained by precipitation by 

 ascending waters. From such waters we would expect exactly the reverse 

 distribution. 



While the reaction between the oxidized products and the sulphides 

 has been strongly emphasized, because it is believed to be the most funda- 

 mental of the causes producing a rich upper belt, it is understood that 

 other factors may also help in this process. As already pointed out, 

 reduction and precipitation of the metals of descending solutions may take 

 place through the agency of organic matter or other reducing materials 

 contained in the rocks, or by meeting ascending solutions carrying precipi- 

 tating agents. Near the surface more than an average amount of original 

 precipitates from ascending solutions is a possibility in some cases. (See 

 p. 1077.) 



Concluding, it appears to me that the existence of a rich upper belt 

 in many deposits, and the frequent diminution of the ores in richness in 

 passing from the surface to some distance below the level of ground water, 

 can not be the work of ascending or descending waters alone, but is due to 

 ascending and descending waters combined. Ascending waters produce a 

 first concentration. A second concentration by descending waters produces 

 the rich deposits. These rich products are found in the few meters or few 

 hundred meters of the outer crust of the earth. When it is remembered 

 that the greater part of the ores which have yet to be abstracted from the 

 earth comes from the first 500 or 700 meters, and when it is further con- 

 sidered that the effect of descending waters may be felt to these depths, it 

 becomes evident that the process of second concentration by descending 

 waters is a very important one indeed, so far as the economic value of ore 

 deposits is concerned. Indeed, as a result of it there is concentrated in 

 the extreme outer shell of the crust of the earth a large portion of the 

 products which during the first concentration may in many cases have been 

 distributed over 1,500 or 3,000 meters or more, but which have now been 

 largely removed by erosion. We therefore conclude that for a large class 

 of ore deposits a second concentration by descending waters can not be 

 said to be one whit less important in the genesis of ores than a first concen- 

 tration by ascending waters. 



