9 50 



RESUME AND THEORETICAL DISCUSSION. 



Section VII. — Source of the Gold, and its Distribution in the Gravel 



In the first place, it may be unhesitatingly stated that the gold is almost 

 exclusively limited to the quartz and metamorphic gravel. Volcanic gravel 

 is of rare occurrence, and it still more rarely contains gold enough to be 

 worth working. It may be set down as a general rule, that the volcanic 

 rocks of the Sierra are not auriferous: and of course the detritus resulting 

 from their disintegration is barren of the precious metal and of no value. 

 This statement does not apply, however, to the older volcanic masses which 

 are included in the bed-rock series, and which have themselves been sub- 

 jected to such extensive metamorphism that they can no longer be dis- 

 tinguished from the metamorphic sedimentary rocks, except by the aid 

 of the microscope. There is reason to believe that in the process of 

 metamorphism gold and other metals have been introduced into the volcanic 

 formations, as well as into those of sedimentary origin ; and, indeed, we 

 know of no reason why this should not be so. Volcanic rocks of Tertiary 

 age on the eastern slope of the Sierra are traversed by veins of quartz and 

 other vein-stones, richly impregnated with various ores, and especially with 

 those which are both auriferous and argentiferous. These occurrences, how- 

 ever, seem limited to the oldest of the volcanic masses of the Tertiary epoch, 

 and only to such as are highly metamorphic in character, — the "propylites" 

 of Richthofen. In such cases we must conclude that the processes of vein 

 formation and impregnation with the precious metals have been part and 

 parcel of the chemical reactions which have resulted in the extensive meta- 

 morphism of the enclosing mass, or country-rock itself. 



The whole series of phenomena observed in the Sierra Nevada gives ample 

 grounds for the belief that the metamorphic rocks of the Range 



the bed- 



rock, in short 



were the original home of the gold now so generally dis- 

 tributed through the gravel. What has been already stated* in reference 

 to the occurrence of the auriferous gravels themselves, shows clearly that 

 their presence is entirely dependent on that of a body of slaty rocks, them- 

 selves enclosing great numbers of quartz veins, which in many cases are 

 sufficiently well supplied with gold to repay the expense of mining. In 

 short, the quartz-mining districts and the gravel region are essentially one 

 and the same tract of country, as will be apparent on inspection of the vari- 

 ous maps accompanying this volume. That a considerable amount of gravel 



I 



* See ante, pp. 302-314. 



