532 T. L. WATSON LITHOLOGICAL CHARACTERS OF VA. GRANITES 



Winterpock, in Chesterfield county. The evidence seems quite clear that 

 the porphyritic granite underlies in part the eastern portion of the Eich- 

 mond coal basin, but its relations to the crystalline rocks on the west can 

 not be stated, since the line of contact is concealed beneath the cover of 

 Newark rocks of the coal basin. 



Granite-gneisses 



Gneisses of granitic composition make up one of the principal rock 

 types in the "Virginia Piedmont complex. Many of these gneisses were de- 

 rived from original massive 

 granites, and they are invariably 

 of the mica type. In mineral 

 composition the granite-gneisses 

 are essentially identical with the 

 massive granites, except that 

 hornblende is associated with 

 biotite in the Eichmond and 

 Fredericksburg areas. In some 

 of the slides hornblende is 

 largely in excess of the biotite. 



The gneisses differ from the 

 granites principally in the 

 banded structure induced in the 

 former through the action of 

 pressure metamorphism. The 

 banding may be fairly regular, 

 but, as a rule, it is quite irreg- 

 ular; in either case the bands 

 are composed of alternating ones 

 of light and dark colored min- 

 erals. The individual bands 

 may vary much in thickness. 



Between the entirely massive 

 granite and the typical granite- 

 gneiss intermediate grades of 

 schistosity are easy of differ- 

 entiation over parts of the Virginia area. This undoubtedly means that 

 in some of the areas a less schistose granite may grade into a more 

 schistose one. In other areas this interpretation is not possible, for the 

 massive granites are sharply defined structurally from the schistose ones, 

 and in all cases where contacts were found it was entirely clear that the 











s ^ n 'A 



qmaTjte, 



-X.. y- 



^ 



Qr<?ntte- 



Qne/ss 









^V-^VA 









Figure 1. 



-Contact of Hornblende-biotite Gneiss with 

 Granite. 



At McGowan quarry south of Richmond. Granite 

 cuts directly across schistosity of the gneiss; the 

 pegmatites conform with the schistosity. 



