6 MIXING INDUSTEY. 



or less metamorphosed strata of that series which extends, with perfect con- 

 formity, from the Azoic up to the Jurassic. The latter are generally found 

 occupying planes of stratification or jointings developed by metamorphism; and 

 although they closely conform in dip and strike to the country rock, the 

 clay selvages and striated surfaces of the quartz, together with a usually 

 unbroken continuity downward, seem to indicate an origin similar to true fis- 

 sure veins. Of this type are, prominently, the gold veins of California. 



The districts embracing what are known as the Humboldt mines are. 

 located upon tv^'-o parallel ranges, formed almost wholly of folded strata of the 

 Triassic age. The veins either occupy planes of stratification or arrange 

 themselves along prominent jointing planes, induced by the disturbances of 

 upheaval and metamorphism. . 



The districts of Reese River occur, first, in a large mass of granite, 

 accompanying a mountain fold; and, secondly, are found lying in the meta- 

 morphic rocks of the Carboniferous in positions similar to those of Hum- 

 boldt. The celebrated White Pine district, whose mineral deposits are 

 inclosed conformably between strata of Devonian limestone, is a prominent 

 example of the groups comprised wholly within the ancient rocks. 



The discovery of the geological horizon of these limestones by Professor 

 Meek is of interest, since it proves them to be among the oldest known silver- 

 bearing rocks in either of the Americas. The mode of occurrence and dis- 

 tribution of the mining districts of Colorado are somewhat unique, and will 

 be so fully treated of in the succeeding chapters by Mr. J. D. Hague, that the 

 writer prefers to make no comments upon them here. In general they be- 

 long to the ancient type. 



The second type belongs to the second or Tertiary orographical period, 

 and finds its origin in the disturbances of the volcanic ejection. While the 

 fires of the lava period were still burning, and where the deeply riven rocks 

 of the earliest volcanic outflows were repeatedly broken through by subsequent 

 eruptions, and where torrents of water poured down the hill slopes and every- 

 where penetrated the fissured rocks and came in contact with intensely heated 

 material, there were present all the elements of vein formation. That these 

 conditions actually existed is a matter of every-day geological proof The 

 lodes of this type are either wholly, or in part, inclosed in volcanic rocks. 



