AGE RELATIONS OF THE ROCKS 375 



As indicated in the anatyses of the fresh and decayed rock of the 

 table, the change has been one of hydration — the assumption of water, 

 accompanied by the peroxidation of the iron and the partial removal of 

 the more soluble constituents, lime, magnesia, and alkalies. 



Age Relations 



Excepting the northernmost extension of the Jura-Trias to the south 

 and southeast in the vicinity of Oxford, Granville county, North Caro- 

 lina, no known elastics of definite age are found close to the area. 

 Dikes of Mesozoic diabase are reported to be rather numerous in parts 

 of Granville county, and in several instances are observed cutting the 

 rocks of this area. To the east, south, and west massive granites and 

 granitic gneisses of approximately the same mineral composition are of 

 frequent occurrence. Sufficient work has not yet been done, however, 

 to definitely determine the exact origin of the gneisses, but in many 

 cases their close mineralogical resemblance to the granites is suggestive 

 of igneous origin. Indeed, a chemical analysis quoted by Kerr * of a 

 similar granitic gneiss taken from the Raleigh quarries would strongly 

 indicate, in connection with the mineral components, an original mass- 

 ive granite sybsequently rendered schistose by pressure. 



The occurrence of similar ancient volcanic rocks in the adjoining 

 counties to the southeast of the Virgilina area, described by Williams f 

 and others J as closely resembling those of the South Mountain area, 

 are grouped as pre-Cambrian in age, and can be most likely corre- 

 lated with the rocks of the Virgilina district. 



The rocks of this district are shown to be quite similar in many re- 

 spects to the volcanics farther north in Virginia and Maryland of the 

 Catoctin belt and of South mountain, a continuation of the Catoctin 

 belt in Pennsylvania. Keith § has shown the rocks of the Catoctin belt 

 to be pre-Cambrian — Algonkian — in age. Likewise Williams || and 

 Bascom^f have shown the series of both acid and basic volcanics of 

 South mountain in Pennsylvania to be of the same age — Algonkian. 



The rocks the Virgilina district are, with few exceptions, shown to 

 be highly schistose in structure, which is a secondary structure, and 



♦ Geology of North Carolina, Geol. Survey of N. C, 1875, vol. i, p. 122. 



f Jour, of Geology, 1894, vol. ii, pp. 1-31. 



t Gold Deposits of North Carolina, Geol. Survey of N. C, Bulletin no. 3, 1896, pp. 37-43 ; ibid., Bul- 

 letin no. 10, 1897, pp. 15, 16. 



§ Geology of the Catoctin Belt, 14th Ann. Rept., U. S. Geol. Survey, 1894, p. 319. See map, plate 

 xxii, opposite p. 308. 



|| Volcanic Rocks of South Mountain in Pennsylvania and Maryland, Am. Jour. Sci., 1892, vol. 

 xliv, pp. 493, 494; Jour, of Geology, 1894, vol. ii, pp. 1-31. 



fl.The Ancient Volcanic Rocks of South Mountain, Pennsylvania, Bulletin no. 136, U. S. Geol. 

 Survey, 1896, p. HO. 



1T Jour, of Geology, 1893, vol. i, pp. 813-832. 



