618 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.45. 



characterized by a close association with chalcopyrite, of specialized 

 development. The enargite forms an apparent fracture-filling in' 

 massive pyrite and incloses microscopic ramifications of chalcopyrite. 

 These inclosures do not pass into the adjacent pyiite, nor do they 

 possess any arrangement suggestive of subsequent penetration of the 

 enargite by chalcopyritc-bearing solutions. On the contrary, they 

 present rather strong evidence of simultaneous development along 

 with the enargite, as a kind of a residual crystallization as a result of 

 the molecular adjustment forming enargite. As the enargite has not 

 been aft'ected by the other mineralizing processes it may be inferred 

 that its formation represents a late stage of the depositional epoch. 



The microscopic study points unmistakably to the formation of the 

 ores through replacement of the minerals of the schists; and the seri- 

 citic and chloritic components have been the first to be attacked and 

 substituted. Gradual transitions from unmineralized rock to solid ore 

 are often seen. In many places a schistose pattern delineated by 

 residual quartzes has been inherited by massive pieces of ore. (See 

 fig. 1, PL 50.) In most sections unreplaced shreds and fragments of the 

 ordinal schists may be detected. One section disclosed a hexagonal 

 quartz crystal with embayments filled with pyrite and enargite, show- 

 ing in striking manner the corrosive effects of sulphide solutions even 

 upon that mineral. (See fig. 1, PI. 51.) Accompanying dominant 

 replacement a certain amount of interlaminal deposition is also evi- 

 denced, but few examples are free from some replacement as well; 

 and this process is merely a preliminary to the dominant one. 



The study leads also to the conception that the ore deposition took 

 place during a distinct mineralizing epoch marked by solutions pro- 

 gressively changing in composition and depositing a series of sulphide 

 minerals in sequential and transitional stages. The order of deposi- 

 tion, as evolved, runs from cupriferous pyrite through chalcopyrite, 

 bornite, and chalcocite, to the tetrahedrite-enargite group (accom- 

 panied by chalcopyrite of a second generation). From this it must 

 not be inferred that the formation of any one ore mineral was con- 

 fined to any one period, or that the sequence was absolute; on the 

 contrary there is ample evidence of transitions and overlappings, and 

 many complications undoubtedly intervened to make the process even 

 more involved. What is strongly manifest, however, is that the de- 

 position of any one of the sulphide minerals, in so far as it is a prime 

 essential in the ore as a whole, was confined to some given period in 

 the evolution. 



A further generality, so persistently applicable as to seem not with- 

 out significance, is one involving a relation between three broad fea- 

 tures of the principal ore minerals, namely, the proportion between 

 their respective iron and copper contents, the order of deposition evi- 

 denced by them, and the extent of their individual participation in 



