PRECIPITATION OF THE METALS. 1085 



It is believed that the Cripple Creek deposits likewise illustrate this 

 principle. Penrose" notes that many of the rich ore shoots occur at cross 

 fissures. It is thought probable that the main cause for the formation of 

 ore shoots at such places is the reaction of solutions furnished by one set 

 of fissures upon those furnished by the other set. It is but fair to say, 

 however, that Penrose makes the explanation, the "mechanical one, in 

 deflecting the course of the ore-bearing solutions." 



What are the reactions which result in precipitation from the mingling 

 of solutions? It has been pointed out (p. 1075) that in the upper part of 

 the journey of descending waters ic salts are likely to be present, In the 

 deeper parts of the belt of cementation, where organic matter and ferrous 

 compounds are abundant, these ic salts are commonly changed to oks salts, 

 and such salts are abundant, Such solutions are likely to make their way 

 into the trunk channels. If they there meet salts of gold, silver, or copper, 

 these compounds may be thrown down. This is likely to take place if the 

 reducing solutions are abundant. That is, the reaction is under the law of 

 mass action. It has already been seen that this reaction is promoted by 

 decreasing temperature. Another reaction frequently resulting in the 

 precipitation of sulphides follows from the mingling of solutions, one of 

 which bears hydrogen sulphide and the other sulphates or other oxidized 

 salts. In trunk channels the mingling- of solutions of these kinds must be 

 very common, the oxidized solutions coming perhaps somewhat directly 

 from the surface, whereas the hydrogen-sulphide solutions have usually 

 taken a longer journey in the belt of cementation, and thus have taken on 

 a reducing character. 



While the above explanation may in many cases account for apparent 

 irregularities in the kinds and percentages of metals, other principles are 

 needed to explain the fact that metals occur in a definite order from the 

 surface downward and that many valuable metalliferous ores grow poorer 

 at a depth of 1,000 meters or less. Varying temperature and pressure are 

 important in this connection; but more influential in many instances, as 

 will be shown subsequently, is a second concentration produced by 

 descending waters. 



"Penrose, E. A. F., jr., Mining geology of Cripple Creek, Colorado: Sixteenth Ann. Rept, U. S. 

 Geol. Survey, pt. 2, 1895, pp. 164-165. 



