126 Bulletin of Laboratories of Denison University [Voi. xn 



The rocks of the Virgilina district are, with few exceptions, 

 shown to be highly schistose in structure, which is a secondary- 

 structure, and indicates that the area has been subject to long- 

 continued dynamo-metamorphism. In view of these facts and 

 in the absence of contradictory field evidence, the rocks are 

 placed as pre-Cambrian in age. This is in accord with the work 

 of Kerr and Holmes, who agree in assigning the rocks of this 

 area to the Huronian (Algonkian),^ and with that of Keith,' 

 Williams,^ and Bascom^ for somewhat similar volcanics occur- 

 ring to the north in Virginia, Maryland, and Pennsylvania. 



It further harmonizes with the work of Williams^ and 

 Nitze^ in the adjoning counties to the southwest of the Virginia 

 district, where similar rocks are described and classified as pre- 

 Cambrian in age. Subsequent work will probably establish the 

 contemporaneous origin of the rocks for the several scattered 

 areas. 



Conclusions. 



The principal points developed in this study may be sum- 

 marized as follows : 



1. The rocks of the area here described have been greatly 

 altered through pressure and chemical metamorphim, as indi- 

 cated in the prevailing secondary schistose structure and the 

 abundant development of the secondary minerals — chlorite, 

 epidote, and hornblende — and small amounts of others. The 

 alteration has advanced sufficiently far in the schistose phases 

 to destroy in most cases the original structure and minerals of 

 the rock. 



2. From structural, petrographic, and chemical evidences 

 the rocks are shown to have been derived from an original ande- 

 site, but in their present much altered state they are, according 

 to present usage, more properly designated meta-andesites ; 

 that these are intimately associated with the corresponding 

 volcanic elastics. Furthermore, the popular name greenstone 



1 Van Hise, C. R.: Correlation Papers, Bulletin No. 86, U. S. Geol. Survey. 



2 Op. cit. ^ Op. cit. ■• Op. cit. * Op. cit. « Op. cit. 



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