COPPER. 151 



Mexico, this district has much in common with those already 

 mentioned. A great dike of felsite cuts limestones, and along the 

 contact, as well as in the felsite itself, copper ores are found. 



1. Contact deposits in limestone. These afforded the usual ox- 

 idized ores, but were not found to extend to any great depth, and 

 while for a time productive, they were soon exhausted. 



2. Deposits in felsite. These consisted of pellets and sheets of 

 native copper in the dike itself, which were oxidized to cuprite 

 near the surface. (Cf. Lake Superior amygdaloids, Example 13.) 

 They were worked by the Mexicans in the early part of the pres- 

 ent century. 1 



2.04.24. Example 20e. Black Range District. The mines of 

 this region differ from those described above, and in some respects 

 resemble the pyrite beds of the Alleghanies. (Example 19.) The 

 ore occurs in one or more great fissure veins, along the contact of 

 vertical slates with a great dike of porphyrite. The veins run com- 

 pletely into the porphyrite and into the slate, and afford oxidized 

 ores above, changing into chalcopyrite below. They contain con- 

 siderable silver and gold, as well as arsenic and antimony. The 

 principal mines are the Hampton and Eureka. They are situated 

 in the valley of the Verde River, twenty miles east of Prescott. 2 



2.04.25. Example 20/. Copper Basin. Beds of closely text- 

 ured conglomerate and sandstone, resting on granite and gneiss, 

 and having a cement of copper carbonates. Copper Basin lies 

 about twenty miles southwest of Prescott, and is formed by a de- 

 pression in greatly decomposed granite, which is traversed by 

 numerous small veinlets of copper ores. The granite is pierced by 

 porphyry dikes, and covered by the sedimentary conglomerates 

 and sandstones into which its copper is thought by Blake to have 

 partially leached and precipitated as a cement. Reference, by 

 way of comparison, may be made to the Lake Superior conglomer- 

 ates, in which, in part, the native copper serves as a cement. 3 



1 A. F. Wendt, "Copper Ores of the Southwest," M. E., XV. 27. 

 Wislizenus, "On the Santa Rita Mines: Memoir of a Tour in Northern 

 Mexico, 1846-47," p. 47 ; Amer. Jour. Sci., ii., VI. 385, 1848. 



2 J. F. Blandy, "The Mining Region around Prescott, Ariz.," M. E., 

 XL 286. G. K. Gilbert, "On the General Geology of the Black Mountain 

 District," Wheeler's Survey, III., p. 35. A. R. Marvine, " Brief Details of 

 the Verde Valley," Wheeler's Survey, III., p. 209. A. F. Wendt, " Copper 

 Ores of the Southwest," M. E., XV. 63. Rec. 



3 W. P. Blake, V The Copper Deposits of Copper Basin, Arizona, and 

 Their Origin," M. E., XVII. 479. 



