354 



RESUME AND THEORETICAL DISCUSSION. 



rarely accompanied by the precious metals j while in the metamorphic rocks 

 the baser metals are almost invariably argentiferous, or associated with ores 

 of silver, which sometimes also contain gold. 



The possible connection of volcanic activity with metamorphism, and with 

 the impregnation of the veins with their metalliferous contents, has already 

 been suggested in the preceding pages.* A few words may here be added 

 with special reference to the occurrence of the precious metals as being more 

 or less distinctly related to the presence of eruptive rocks. While the un- 

 metamorphosed recent lavas and other volcanic ejections are, on the whole, 

 very barren of metals, the rocks of this class which have been subjected to 

 chemical reactions are often the repositories of extensive metalliferous de- 

 posits. This statement is not only true with reference to volcanic rocks 

 older than the Tertiary, and which are often so much changed from their 



original character as to have 



in many cases, at least — escaped recogni- 



tion, but it applies especially to the Tertiary volcanic masses. These are 

 believed to be the seat of some of the most important silver mining regions. 

 In fact, the silver of the world comes chiefly from two sources : one is the 

 association of this metal with the sulphurets of the baser metals, especially 

 lead ; the other is from the proper ores of silver. The first of these class 

 of occurrences is now almost universally designated in the Cordilleras as 





" base-metal mines." These occur chiefly in metamorphic sedimentary 

 rocks ; the other class belongs essentially to the metamorphic volcanic, and 

 includes such immensely productive regions as those of Mexico, Washoe, and 

 Peru. The silver ores of these regions are always more or less auriferous, 

 and sometimes, as in the case of the mines in the Corns! ock Lode, largely so.f 

 Again, the fact is not to be ignored that the two by far most important 

 and productive gold-mining districts of the world are regions of former 

 intense volcanic activity. Australia and California exhibit the same phe- 

 nomena of rich auriferous detritus buried beneath masses of lava, as will be 

 more fully noticed further on in this chapter. Although the eruptive 



in these cases, are not the direct repositories of the precious metal, there 

 would seem to be strong reasons for believing that there is a genetic con- 

 nection between the volcanic activity and the enrichment of the adjacent 

 strata. 



The ranges of the Andes and of the North American Cordilleras certainly 



* See ante, pp. 310, 331. 



t The average value of the ores from the t'omstock Lode is about 47% gold and 53% silver. 





