516 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.40. 



The quartz porphyry is, for the most part, especially on the western 

 side of the area, a typical rock of its kind, but much mashed and 

 highly schistose. The phenocrysts are largely of feldspar, with a 

 variable and usually an inferior amount of quartz. The basal and 

 the upper, and at times other portions of this rock, are to a greater or 

 less extent tuffaceous. This is especially true of the eastern area, 

 where by far the greater portion is probably a very fine tuff. No 

 workable ore deposits have been found in this rock. 



The age of these rocks is not known; they have generally been 

 regarded as pre-Cambrian. They appear to be somewhat similar to 

 tuffaceous rocks intimately associated with the slate deposits lying 

 northeast of the Virgilina district. These slates have recently been 

 described by Watson J and Powell and shown to be early Paleozoic. 

 It is believed that further study may determine the volcano-sedi- 

 mentary rocks of this district to be of the same age. 



Granite. — This is the youngest intrusive rock of the region except 

 the diabase dikes, and is also the most important. Three prominent 

 areas of it are included within the district, one in the southwest corner 

 near Mill Creek post office, North Carolina, and another in the east- 

 central portion at and surrounding Buffalo Lithia Springs, and the 

 third and largest one, northwest of Redoak post office, Virginia. 

 This area of granite extends almost across the region of volcano- 

 sedimentary rocks and cuts out the ore-bearing horizon for a distance 

 of 4 or 5 miles. It is apparently massive, and therefore shows 

 nothing of the prominent schistosity of the other rocks. In all the 

 occurrences it is a rather coarsely granular, highly quartzose rock, 

 and in places, .especially at Buffalo Lithia Springs, it is deoidedly 

 porphyritic. Like all the other granites of the southeastern United 

 States, it contains a large amount of plagioclase in proportion to the 

 orthoclase, and shows well its quartz-monzonitic character. This 

 rock is of especial interest since all the field evidenc3 obtainable 

 points toward the conclusion that it is the source of the ores, and that 

 they and the veins are closely connected genetically with its intrusion. 

 In this relation it is further considered in the paragraphs relating to 

 the origin of the ores. 



Structure. — From the cross section above given, what the writer 

 considers a probable structure of the district is readily seen, that of 

 a closely compressed syncline, the axis of which has a strike of from 

 10 to 30° east of north, and which is inclined so that it has approxi- 

 mately the same dip as the schistosity — from 70 to 80° toward the 

 southeast. It is believed that at the beginning of the formation of 

 the series there was a great outpouring of acid lava — the quartz por- 

 phyry upon the mica and hornblende gneiss as a basement. This 



i Thos. L. Watson and S. L. Powell. Fossil evidence of the age of the Virginia Piedmont slates. Amer. 

 Journ. Sci., ser. 4, vol. 31, 1911, pp. 33-44. 



