1088 A TREATISE ON METAMOBPHISM. 



In the case of the copper deposits the organic matter has in all probability 

 reduced sulphites or sulphates to sulphides. The function of the organic 

 matter in the case of the g-old may have been to reduce it to metallic gold, 

 or to produce ous salts, for instance, ferrous sulphate, which reduced the 

 gold. (See pp. 1093-1095.) 



(JEXERAL STATEMENTS. 



In conclusion, it may be said that the precipitation of metallic ores 

 by the mingling of various solutions is probably the most important single 

 factor which results in the first concentration of ores. Probably next in 

 importance to this are the reactions upon the trunk streams, due to the wall 

 rocks. As lateral streams from beyond the wall rocks must pass through 

 the latter, many of these streams produce an effect due partly to materials 

 more remote than the wall rocks and partly to the wall rocks. Thus in 

 many cases the effect of solutions originating beyond the wall rocks and 

 that of solutions furnished by the wall rocks may not be discriminated. 

 Diminishing temperature and pressure, while probably subordinate in 

 their effect to the mingling of streams and reactions due to the wall rocks, 

 are in many instances undoubtedly important, and in some instances 

 dominant, factors. In general, the tendency of writers has been to 

 emphasize the effect of diminishing temperature and pressure, and to 

 minimize or even disregard altogether the effects of mingling solutions 

 or the wall rocks, or both. 



Precipitation in many cases is not produced by a single one of the 

 factors, but by two or three of them. For instance, precipitation may be 

 produced by the joint effect of a change in pressure and temperature, by 

 the reaction between gases, solutions, and solids, and by the combination 

 of change in pressure, change in temperature, and the mingling of solutions. 

 In short, all possible combinations of the various causes of precipitation 

 may and do occur in connection with ore deposits. 



COMPOUNDS DEPOSITED BY ASCENDING SOLUTIONS. 



Of the metallic ores those of iron, copper, lead, zinc, nickel, silver, 

 gold, and mercury are the more important. These metals vary greatly in 

 the forms in which they are precipitated. The deposits formed by ascend- 

 ing' water occur in the metallic form, as sulphides, tellurides, oxides, car- 

 bonates, and silicates. The metals deposited in the metallic form in impor- 



