no. 1835. RELATION OF BORNITE AND CHALCOCITE— LANEY. 528 



EXPLANATION OF PLATES. 



Plate 63. 



Fig. 1. Typical vein. Exposed in railroad cut near Christie, Virginia. The irregu- 

 larities here shown are exhibited by nearly every vein that has been opened 

 in the district. 

 2. Outcropping of a large but barren quartz vein near High Hill Mine. Few 

 of the vein outcrops are as pronounced as this one. 



Plate 64. 



Fig. 1. Photograph of a specimen from the Wall mine showing relation of ore and 

 quartz. The vein was ' 'split' ' at the point where this specimen was taken 

 and the piece here figured represents the entire width of one portion of the 

 vein. 



Dark areas = ore. 



White areas = quartz. 

 2. Tracing made from a polished surface of a specimen from the Wall mine show- 

 ing the relation of the ore to the quartz. Natural size. 



Black =ore, chalcocite, and bornite. 



White = quartz. 

 The relationship here shown is typical of all the mines in the district. The 

 ore and gangue appear to be contemporaneous. 



Plate 65. 



Secondary chalcocite in bornite. X 10. Seaboard mine. This photograph shows 

 a mass of bornite penetrated in all directions by a mesh or network of chalcocite 

 which has formed in minute fractures in the bornite. This is typical of all the 

 secondary chalcocite studied, the only difference being the stage of development. 

 This one is farther advanced than any of the others shown. The white line in the 

 center of many of the chalcocite veinlets is quartz. Running diagonally across the 

 specimen is a recent fracture which cuts bornite and the older veins of chalcocite. 

 In this chalcocite is also seen to be developing. 



Plate 66. 



Fig. 1. Secondary chalcocite in bornite. X 40. Seaboard mine. Similar in all 

 respects to section shown in Plate 67, fig. 1, only the development of chalco- 

 cite is farther advanced. The same feathery line of contact between the 

 two minerals is evident. In the center of the chalcocite areas are seen films 

 of quartz which apparently mark the original fracture in which the chalco- 

 cite developed. 

 2. Secondary chalcocite in bornite. X 40. Seaboard mine. Similar to fig. 1, 

 but shows more clearly the relation of the chalcocite to the bornite. In one 

 of the fractures in the bornite there is seen a stringer of quartz, the dark 

 area running vertically through the section. On each side of the quartz, 

 between it and the bornite, there is a considerable development of chalco- 

 cite. 



Plate 67. 



Fig. 1. Secondary chalcocite in bornite. X 40. Seaboard mine. 

 Irregular lines=chalcocite. 

 Other portion of section=bornite. 

 This figure shows an incipient stage of the development of secondary chal- 

 cocite in minute fractures in bornite. The little lines of chalcocite are well 

 defined, but present an irregular or ' 'feathery' ' line of contact with the 

 bornite. 



