368 T. L. WATSON — VIRGILINA COPRER DISTRICT 



Since the rocks are usually no longer massive, but instead are highly 

 schistose in structure, the weathered surfaces for structural reasons would 

 be expected to more closely simulate those of sedimentary masses. 



The extension of the belt as traced" from the rock outcrops for many 

 miles in an approximately north-south direction, with comparatively a 

 very narrow cross-section, is certainly suggestive. Their weight, color, 

 texture, and not unfrequent massive structure are properties more char- 

 acteristic of igneous than of sedimentary rocks. 



Massive granites and granitic gneisses, and in places dikes of diabase, 

 limit the area on the east and west sides. In several instances the dia- 

 base is found cutting the rocks of the greenstone area. Some evidence, 

 both field and microscopic, is at hand for regarding some of the rocks at 

 several places in the belt as altered andesite tuffs or elastics composed 

 of fragments of the igneous rock.* The study has not been sufficiently 

 extended, however, if, indeed, it were possible, to differentiate the clas- 

 tic volcanics (tuffs) from the direct igneous masses of the area. 



MICROSCOPICA L E VIDENCE 



The evidence of the igneous origin of these rocks is not entirely that 

 of field relationSj but is derived largely from microscopic structure, min- 

 eral and chemical composition. In many instances the thin-sections 

 show both stout and acicular forms of striated feldspar partially or 

 wholly preserved, embedded in a fine-grained groundmass composed 

 principally of green chlorite and hornblende, epidote, altered feldspar 

 and iron oxide. This arrangement is not confined to the massive and 

 least altered forms of the rocks, but is indicated to some degree in the 

 partial skeleton outlines of some of the original minerals in several slides 

 of the perfectly schistose rocks. In many cases chemical and structural 

 metamorphism have progressed so far that all trace of the original 

 structure, as well as that of every original mineral, has been destroyed. 



The occurrence of lath-shaped polysynthetically twinned crystals of 

 plagioclase which appears to have formed an essential constituent of the 

 rocks is characteristic of rocks of igneous origin. Furthermore, the micro- 

 ophitic and poikilitic structures of the feldspars of some of the rocks in 

 thin-section under the microscope are common only to igneous masses. 

 The structures bear certain striking resemblances to similar rocks of 

 igneous origin described by Williams f and Clements X from the Lake 

 Superior region. Professor Williams § reproduces a photomicrograph of 



♦ Weed, op. cit. 



f Williams, G. H., Bulletin no. 62, U. S. Geol. Survey, 1890. 



J Clements, J. Morgan, Jour, of Geology, 1895, vol. iii, pp. 801-822; Monograph, no. xxxvi, U. S. 

 Geol. Survey, pp. 98-103. 



§ Williams, G. H., op. cit., p. 226, plate x, figure 2. 



