1914] Whitman : Notes on the Copper Ores at Ely, Nevada 311 



zonite-porphyry. Along this contact the orthoclase phenocrysts in 

 the latter are uniformly oriented parallel to the contact for two or 

 three inches back from it, the phenocrysts losing their parallelism 

 gradually, until at a distance of six inches from the contact their 

 arrangement bears no relation to it. The parallelism in the arrange- 

 ment of the constituents of the rock is not limited to the orthoclase, 

 but is shared also by the hornblendes, which in many cases are half 

 an inch long. This flow structure points conclusively to the later age 

 of the Veteran monzonite-porphyry. Furthermore, the latter rock is 

 much less altered, and contains no notable amount of magnetite or 

 epidote as compared with the other. The ore-porphyry, on the other 

 hand, has suffered extreme alteration, and in many places has been 

 completely replaced by the magnetite along the contact, and at most 

 points contains conspicuous amounts of magiietite, and often of 

 epidote. The striking difference in the degree of alteration of the 

 two rocks is taken as indicating that by the time the alteration of the 

 latter had begun, that of the former was well under way ; since the 

 two rocks appear to be petrographically similar and therefore equally 

 susceptible to the same agencies of alteration. 



INCLUSIONS IN ORE-PORPHYRY 



The ore-porphyry is characterized by the presence of immense 

 included blocks of limestone, as seen in the Eureka Pit (pi. 30, fig. 2, 

 and pi. 31, fig. 2). These are of indefinite form and outline as a 

 result, largely, of the alteration they and their carrier have under- 

 gone ; but it is safe to say they range up to three hundred feet in 

 diameter. Their alteration is largely metamorphic, their constituents 

 having been changed chiefly to lime-garnet and epidote in a greenish 

 silicious matrix of uncertain composition, the chief minerals now being 

 epidote, magnetite, garnet, gypsum, calcite, quartz, and iron and 

 copper sulfids. 



ALTERATION OF ORE-PORPHYRY 



The most conspicuous change which has affected the ore-porphyry 

 is the alteration to sericite and secondary quartz, with accompanying 

 shrinkage of the whole mass and attendant fracturing and general 

 collapse. In general the alteration is greatest near the present surface, 

 and along the central portions of the areas exposed. The rock is 

 scarcely to be found near the surface in an unaltered condition, but 



