Art. VII.] Watson, Virgilina Copper District. [ 1 9 



means of chemical and microscopical study to the original rock 

 type, the differences become more apparent. This difference is 

 that which distinguishes in the original rock an andesite from a 

 diabase, diorite, gabbro, et cetera ; but, as already stated, the 

 altered rock derived from these several types is closely similar. 



Sufficient study of the ancient vocanic rocks occurring to 

 the southwest of the Virgilina area, in North Carolina, is lack- 

 ing on which to base specific comparisons. That they are al- 

 tered volcanic rocks of great age, comprising both acid and 

 basic types, is established, but the exact mineral and chemical 

 composition, denoting the original types from which they are 

 derived, is yet to be investigated. Megascopic descriptions and 

 the field relations of many of the basic types indicate their 

 striking similarity to those of the Virgilina district. 



As developed from the chemical and microscopic study of 

 the rocks of the Virgilina district, the present much altered 

 rock, greenstone, clearly indicates its derivation from an orig- 

 inal andesite of an intermediate basic type as contrasted with 

 the similar Catoctin schist or greenstone, which from Keith's^ 

 description, is derived from a more basic andesite, diabase, or 

 basalt. 



According to Keith, the rocks of the Catoctin area are 

 igneous in origin and represent probably two different flows — 

 the upper, basaltic, and the lower, dioritic. In general the 

 rocks are much altered through dynamic metamorphism and 

 secular decay and now largely form greenish epidotic and chlo- 

 ritic schists, designated by Keith as the Catoctin schist. The 

 fine-grained varieties are composed of quartz, plagioclase, epi- 

 dote, magnetite, and chlorite. In the coarse-grained types the 

 original nature of the rock is well indicated. The ophitic ar- 

 ragement of the coarse feldspars is definitely marked. The ad- 

 ditional minerals in the coarse rocks are calcite, ilmenite, skele- 

 ton olivine, biotite, hematite, and, in a few instances, horn- 

 blende. The alteration products, chlorite and epidote, are 

 abundant and characteristic. An analysis of the fresh rock by 



> 14th Ann. Rcpt. U. S. G. S., 1894, p. 304 et seq. 



