SECOND CONCENTRATION BY DESCENDING WATERS. 1143 



produced. Thus Stokes lias shown that zinc carbonate and sodium 

 carbonate tog-ether react upon iron sulphide as follows: 



8FeS 2 +14ZiiC0 3 +N& 2 C0 3 =14ZnS-J-4Fe 2 03+Na 2 S 2 3 +15C0 2 



The particular reactions in an individual case depend upon the relative 

 proportions and solubilities of the compounds present, upon the existing 

 conditions of pressure and temperature, the amount of oxygen, etc. This 

 will more clearly appear upon subsequent pages. 



The deep belt in which the sulphides of iron are preponderant is that 

 of the first concentration already considered on pi-evious pages. 



The concentrations by ascending and descending waters have been 

 spoken of as if they were successive. In some instances this may be the 

 case; but frequently it is much more probable that ascending and descending 

 waters are at work upon the same fissure at the same time, and that their 

 products are, to a certain extent, simultaneously deposited. For instance, 

 under the conditions represented by fig. 26 there is taking place in the 

 lower part of the fissure a first concenti-ation by ascending waters and, in 

 the upjier part, a second concentration by descending waters. Between 

 the two there is a belt in which both ascending and descending- waters are 

 at work. In an individual case the rich upper part of an ore deposit which 

 is worked may now be in the place where ascending waters alone were first 

 acting, where later, as a consequence of denudation, both ascending and 

 descending waters were at work, and still later, where descending waters 

 alone were at work. The more accurate statement for this class of ore 

 deposits, therefore, is that deep or ascending waters are likely to be potent 

 in an early stage of the process, that both ascending and descending waters 

 may work together at an intermediate stage, and that descending waters 

 are likely to be important in the closing stage. 



The above general statement may perhaps be better understood if 

 supplemented by a consideration of specific associations of the metals. The 

 associations which are chosen for illustrative purposes are as follows: 

 Associations (1) of lead, zinc, and iron; (2) of copper and iron; (3) of silver 

 and gold with the base metals. 



