COPPER ORES IN THIS COUNTRY 261 



The nnrolia])ilit3', as a rule, of the gay colored and richh^ 

 assaying surface oxidized ores, compared with the character 

 of the ore tlicy i)a8s into at depth, is very striking and im- 

 portant, and the prospector, miner, and purchaser cannot 

 be too guarded in this respect. 



So httle reliance is to be placed on these surface ores 

 and their continuity downward to any great depth, that, 

 often, not imtil the zone of unoxidized pyrites is gained, can 

 the future value of the mine be predicted, and the test of 

 the values of this unoxidized zone is often sadly disappoint- 

 ing and at great variance with the expectations derived from 

 tests on the rich surface ores. 



The principal American regions are the Atlantic coast, 

 Lake Superior, the Rockies, and Sierra Nevada. In most 

 of these, granitic and eruptive rocks predominate. In the 

 southwestern region, Paleozoic limestones of the Carboniferous 

 series are the prevailing carriers of copper ores, but only 

 where these are closely associated with eruptive rocks, which 

 may themselves contain copper, and doubtless were the means 

 of impregnating the limestone with it by metasomatic sub- 

 stitution, like the lead silver ores of Leadville, Colorado, 

 which are also at the contact of porphyries and limestones. 



The beds of volcanic ash and amygdaloid (i.e., cavities 

 formed by escape of steam in a surface lava flow and filled 

 with copper instead of, as commonly, by calcite, or zeolitic 

 minerals) are very profitable. So are the beds of conglom- 

 erate, which have produced many millions of pounds of copper. 



In the Rocky mountain and Sierra regions we appear 

 to have a variety of copper bearing rocks of different kinds 

 and different ages. We have granites and pre-cambrian 

 crj'stalline schists and gneisses, characteristic of the mines 

 of Butte, Montana, and the United Verde of Prescott, Arizona. 



Paleozoic hmestones and sandstones associated with erup- 

 tive rocks as at Bisbee, Clifton, and Globe, Arizona, and 

 Tintic in Utah, and what we may call the southwest region. 

 Mesozoic sandstones and shales metamorphosed by contact 

 with great eruptive bodies of diabase and other volcanic 

 rock, as the pyritous beds of California. 



In portions of Texas, Colorado, and New Mexico, the 



