Art. VII.] Watson, Virgilina Copper District. 1 1 5 



in some of the thin sections under the microscope than in the 

 hand specimens. In such cases the porphyritic mineral con- 

 sists principally of a well striated plagioclase. 



The prevailing fineness of grain of these rocks, which is 

 equally characteristic of the freshest specimens as for the most 

 altered material, and the associated tuffs or clastic volcanics 

 suggests solidification at the surface. 



The weathered outcrops afford, as a rule, only slight indi- 

 cation of an igneous mass, although at one point a few miles to 

 the south of Virgilina, in Carolina, the spheroidal type of 

 weathering was observed. 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 sugges- 

 tive. Their weight, color, texture, and not unfrequent massive 

 structure are properties more characteristic 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 sev- 

 eral instances the diabase 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 frag- 

 ments of the igneous rock.' The study has not been sufficiently 

 extended, however, if, indeed, it were possible, to differentiate 

 the clastic volcanics (tuffs) from the direct igneous masses of 

 the area. 



Microscopical Evidence. 



The evidence of the igneous origin of these rocks is not 

 entirely that of field relations, but is derived largely from mi- 

 croscopic structure, mineral and chemical composition. In 



' Weed, op. cit. 



