522 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



traded until after a strong schistosity had been imposed upon the 

 andesitic rocks, and is a type of magma the intrusion of which is 

 frequently attended by more or less mineralization in the intruded 

 or adjacent rocks. It is also well able to furnish the acidic material 

 of the veins, and in its effects upon the intruded rocks, through hydro- 

 thermal metamorphism, could very well have been responsible for the 

 development of the calcite, epidote, and probably the chlorite. In 

 fact, it appears to be the only rock in the region that could have fur- 

 nished the feldspars of the veins or have been responsible for the peg- 

 matite-like character of certain portions of some of the veins. It is, 

 therefore, believed that the deformation attendant upon the intru- 

 sion of the granite produced the fractures in which the veins now are, 

 and that the filling of these, both gangue and ores, was supplied by 

 the granitic magma, and that it came in as a phenomenon attendant 

 upon or immediately following the intrusion. 



As to the conditions of the deposition, there is little or no positive 

 evidence. Since the ore deposits are confined to the more basic facies 

 of the schists, it may be surmised that the basic character of the rock 

 was a factor of prime importance in the deposition of the ores. 



SUMMARY AND CONCLUSIONS. 



The rocks of the Virgilina District are greenstone and sericitic 

 schists, which in places have been intruded by granite and gabbro. 

 The intrusive rocks show none of the schistosity of the other rocks. 

 The schists have been derived from a series of volcano-sedimentary 

 rocks of two types — andesite and quartz porphyry, with a preponder- 

 ating amount of tuffs corresponding to these rock types. Their age 

 is probably early Paleozoic. 



The veins are true fissure veins which have a more northerly trend 

 than the schistosity of the country rock, and the filling of which is 

 quartz — about 70 per cent silica — with local and varying amounts of 

 epidote and calcite. The ore-bearing veins are confined to the more 

 basic portions of the greenstone schists, and the values lie in well- 

 defined ore shoots. 



The ore minerals are bornite and chalcocite. They apparently pre- 

 fer the quartz, but are not confined to anyone of the gangue minerals. 

 Bornite is present in slight excess over chalcocite and is apparently of 

 only one period of deposition. Chalcocite is clearly of two periods: 

 One confined to the upper portions of the vein, younger than, and fill- 

 ing a network of minute fractures in, the bornite; the other contempo- 

 raneous and intergrown often crystallographically with it. There is 

 no evidence that any of the bornite is of secondary origin. It is, 

 therefore, clear that in the Virgilina District the greater part of the 

 chalcocite is a primary mineral contemporaneous with the bornite 

 and in no way derived from it, or from any other copper mineral, by 

 processes of secondary alteration. 



