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



tals. The color varies from nearly colorless to reddish brown, with notice- 

 able pleochroism in the deeper colored ones. 



The other accessories present no special interest from their usual 

 occurrence in granites. 



Pressure metamorphic effects are clearly denned in all the thin-sections 

 of this type of granite. They are more strongly marked in the granite 

 from the Petersburg part of the area than that from the Eichmond por- 

 tion. As seen under the microscope, the pressure effects are shown in an 

 optical disturbance of the quartzes and feldspars, more marked in the 

 former, in the form of a wavy or undulous extinction ; in a fracturing of 

 the quartz and feldspar, which is quite strongly developed in .some of the 

 plagioclase individuals of thin-sections from the Petersburg area; and, 

 lastly, in peripheral shattering or granulation of the larger quartz and 

 feldspar individuals in the granite from the Petersburg area; also, in 

 several instances biotite shreds were noticed broken across, and in still 

 other cases the folia were markedly curved or bent. 



At the Netherwood quarry, several miles west of Eichmond, this type 

 of granite is typically shown, and in some of the largest quarried blocks 

 dressed up during the sumer of 1905 a distinct schistosity was discern- 

 ible. As a rule, however, the rock from this quarry and from the Eich- 

 mond area in general belonging to this type appears massive. In the 

 Petersburg part of the area the dynamic effects are the most pronounced 

 in thin-sections of this type of granite and a tendency toward a rough 

 parallel arrangement of the minerals or schistosity is shown on close ex- 

 amination of the rock in nearly every opening. 



The Richmond-Fredericksburg dark blue granite. — This type does not 

 differ essentially in mineralogy from the light gray type described above, 

 although the two bear no resemblance to each other in hand specimens. 

 The dark blue is much more finely crystalline, the anhedra averaging less 

 than 0.5 millimeter. The biotite is very uniformly distributed through 

 the rock in minute irregular shreds which impart the pronounced dark 

 blue color to the granite. Like the light gray t}^pe, this is a biotite granite 

 containing a very little muscovite associated with the biotite. Plagio- 

 clase and microcline, in quantity and occurrence, characterize equally the 

 dark blue type as the light gray. 



Microscopically the two types are unlike in the degree of pressure 

 effects indicated. The dark blue granite is entirely massive, and the only 

 effects of pressure metamorphism discernible in the thin-sections is that 

 of undulous extinction of, and occasional fractures in, the quartz. In the 

 Fredericksburg portion of the area the rock is a shade darker in color 

 than most of the same type in the Eichmond area. The texture is the 



