MINING DISTEICTS. 5 



Over the whole Cordilleras are found localities of the precious metals ; 

 and it is not surprising to observe that, following its leading structural idea, 

 they appear to arrange themselves in parallel, longitudinal zones. 



This zonal parallelism was first indicated by Professor William P. Blake, 

 in a note in his "Catalogue of California Minerals." It is obviously true, as he 

 has indicated, and it is probable that the idea could be carried much further 

 than he has done. The Pacific Coast Ranges, upon the west, carry quick- 

 silver, tin, and chromic iron. The next belt is that of the Sierra Nevada and 

 Oregon Cascades, which, upon their west slope, bear two zones, a foot-hill 

 chain of coj)per mines, and a middle line of gold deposits. These gold veins 

 and the resultant placer mines extend far into Alaska, characterized by the 

 occurrence of gold in quartz, by a small amount of that metal which is 

 entangled in iron sulphurets, and by occupying splits in the upturned meta- 

 morphic strata of the Jurassic age. Lying to the east of this zone, along 

 the east base of the Sierras, and stretching southward into Mexico, is a chain 

 of silver mines, containing comparatively little base metal, and frequently 

 included in volcanic rocks. 



Through Middle Mexico, Arizona, Middle Nevada, and Central Idaho, is 

 another line of silver mines, mineralized with complicated association of the 

 base metals, and more often occurring in older rocks. Through New Mexico, 

 Utah, and Western Montana, lies another zone of argentiferous galena lodes. 

 To the east, again, the New Mexico, Colorado, Wyoming, and Montana gold 

 belt is an extremely well defined, and continuous chain of deposits. 



In the history of the entire Cordillera there are, then, two periods of 

 orographical disturbance which have been accompanied by the rending of 

 mountain chains and the ejection of igneous rocks. Such periods as these, 

 of course, afford the conditions of solfataric action and the consequent forma- 

 tio'n of metal-bearing lodes. That period which culminated in the Jurassic, 

 produced over the entire system a most profound disturbance, and is, in all 

 probability, the dating point of a large class of lodes. To the second, or Ter- ^/ 

 tiary period, may be definitely assigned those mineral veins which traverse the 

 early volcanic rocks. 



These two periods have produced two types of metalliferous lodes. 

 First ; those veins which are wholly inclosed in the granites, or in the more 



