, 
354 RESUME AND THEORETICAL DISCUSSION. 
rarely accompanied by the precious metals ; 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 voleanic 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 Comstock Lode, largely so.t 
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 rocks, 
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. 
+ The average value of the ores from the Comstock Lode is about 47% gold and 53% silver. 
