1138 A TREATISE ON METAMORPHISM. 



In the case of a given ore deposit, by wise combination of the criteria 

 given, it is in general possible to make a somewhat reliable judgment as to 

 whether or not the ore was produced solely by ascending- waters. 



GENERAL STATEMENTS. 



The foregoing statements explain to some extent the source, nature, and 

 manner of deposition of the compounds deposited by ascending waters. But 

 it is not the intention here to discuss their application to the known dis- 

 tricts. This I do not attempt, because I lack the necessary accurate obser- 

 vations upon which such a discussion should be based. To tell in what man- 

 ner the deposits of an individual district are formed requires very detailed 

 investigation of that district. 



While it is not the purpose here to take up the solution and deposition 

 of the compounds which occur in individual ore deposits, it is well again to 

 recall the law of mass action. Other things being equal, those compounds 

 which are abundant will be dissolved in larger degree during the downward 

 course of the waters, and will be most abundantly precipitated in the trunk 

 channels. It is well known that, with the exception of aluminum, which 

 does not form a sulphide, iron is the most plentiful of all the metallic com- 

 pounds in the crust of the earth, and therefore iron sulphide occurs in greater 

 abundance than the sulphide of any other metal. It is well known that in 

 man}^ cases the deeper a mine goes below the level of ground water the 

 greater becomes the proportion of iron sulphide and the less that of the 

 other metals. As a result of this, combined with increased cost of working, 

 it frequently does not pay to mine a deposit beyond a certain depth. The 

 law of mass action explains the abundance of the iron sulphide; it does not 

 explain the frequent relative increase of the iron sulphide and the decrease 

 of more valuable sulphides as one passes from the level of ground water into 

 deeper workings. To explain this we must take into account the effect of 

 the downward-moving waters, discussed under the succeeding heading. 



We have now seen that the zone of fracture is traversed by the perco- 

 lating waters; that metalliferous materials taken into solution by the down- 

 ward and lateral moving- waters are earned to the trunk channels of 

 underground circulation, and that in these trunk channels the metalliferous 

 materials are precipitated in various ways. Thus a first concentration by 

 ascending waters, giving sulphurets, tellurides, and metals of some of the 

 elements, is fully accounted for. 



