518 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



Ores. — Though apparently preferring the quartz, the ore is so 

 intimately associated with all the gangue material as to make it almost 

 certain that all were deposited contemporaneously. (See pi. 64.) The 

 copper-bearing minerals are bornite and chalcocite, with the oxidized 

 products derived from them. Chalcopyrite is present in such small and 

 varying amounts that unless careful search is made it will not be found 

 at all, and it is apparently no more abundant in one portion of a mine 

 than in another. Certainly there is no increase with depth in the 

 amount of this mineral. In fact the mine which shows it most abun- 

 dantly is only about 150 feet deep, and here it was as abundant in the 

 first sulphides encountered as in those in the bottom of the shaft. In 

 two of the deepest mines — the Holloway, 450 feet in depth, and the 

 Durgy, about 400 feet — it is so rare that one can hardly find it. 



Chalcocite occurs in two very distinct relations with the bornite; 

 secondary to and filling fractures in the bornite, and intergrown, 

 sometimes clearly crystallographically, with it. Bornite is the most 

 important mineral in all the mines in the district except the Holloway, 

 in which it is subordinate to chalcocite. It appears, too, from even 

 a casual observation of the ores that there has been considerable 

 shattering since their original deposition. This is especially promi- 

 nent in the ore from the Seaboard mine, which furnishes the purest 

 bornite in the district. In the fractures in the bornite from this 

 mine, be they ever so minute, are developed veinlets of chalcocite, 

 which penetrate the bornite in all directions, and vary in size from 

 the finest line, often not visible to the unaided eye, but perfectly 

 clear under the microscope (pi. 67, fig. 1), up to areas a quarter of 

 an inch in diameter (see pis. 65 and 66) . In the center of many of 

 these chalcocite-filled fractures are films of quartz which evidently 

 mark the original fracture in which the chalcocite began to develop, 

 thus showing that at the beginning of or prior to the development of 

 the chalcocite, there were solutions carrying considerable quartz. In 

 the interior of some of the largest quartz veinlets thus formed there 

 occur particles of chalcocite so related to each other as to indicate a 

 growth of the quartz since the beginning of the deposition of the 

 secondary chalcocite (see pi. 66). Also in a few instances the vein 

 of chalcocite when deeply etched presents a kind of spongy skeleton 

 of quartz appearing as if quartz and chalcocite were deposited sim- 

 ultaneously. The boundary between these veinlets of chalcocite 

 and the bornite is exceedingly irregular, usually presenting a 

 somewhat feathery outline, though always perfectly distinct and 

 clear-cut. There is absolutely no gradation of one into the other. 

 There is certainly a growth of the chalcocite, but how it takes 

 place is not made clear by the microscopic study of the veinlets. 

 It appears, however, that it takes place at the periphery of the 



