PETROGRAPHY OP THE GRANITES 529 



same from the two areas, but in those quarries opened in the vicinity of 

 Kichmond, there is perceptible variation in the degree of color. 



The Fredericksburg light gray granite. — This type occurs about one 

 mile west of Fredericksburg, on Hazel run, and consists of anhedra 

 averaging about 2 millimeters. It is essentially a muscovite granite, 

 containing quite a sprinkle of closely associated biotite with the muscovite. 

 It somewhat resembles the Stone Mountain light gray biotite-bearing 

 muscovite granite of Georgia. An occasional crystal of red garnet is 

 observed in the rock. 



The effects of dynamic metamorphism are manifested in the rock in a 

 thinly foliated structure which is only discernible on close examination. 

 Thin-sections show quartz, orthoclase, microcline, plagioclase, muscovite, 

 biotite, apatite, rutile, and zircon. The principal secondary minerals are 

 chlorite and muscovite. Muscovite is partly primary and partly secondary. 

 Microcline and plagioclase are in large amount. Intergrowths of the 

 feldspars, and of the feldspars with quartz, are quite frequent. Micro- 

 poikilitic structure in the larger feldspars is strongly indicated. Partial 

 orientation of the mica along parallel directions ; an occasional bent and 

 broken muscovite shred; fractured quartzes with wavy extinction, and 

 the perfect granulations of the quartz feldspar individuals and the in- 

 terstices filled in with the fine mozaic of the two minerals are pressure 

 effects plainly marked in the thin-sections. 



The Falls Church types. — The granites in the vicinity of Falls Church, 

 Fairfax county, are of two varieties. One is a medium to finely crystal- 

 line rock and contains dominant biotite with some muscovite as the third 

 essential component. The ratio of muscovite to biotite is variable, and 

 while muscovite may be considerable in a few instances, the granites of 

 this area contain dominant biotite. The other, a light and dark speckled 

 rock, is a fraction more coarsely crystalline and is a hornblende-biotite 

 granite. It is the only representative of a hornblende granite* yet found 

 in the state and it is somewhat closely allied with quartz-diorite. It is 

 closely associated in the field with diorite masses on the one hand and with 

 foliated mica-granites on the other. The associated diorites are partly 

 altered to metadiorites. 



Petrographically it more properly belongs with the grano-diorites, dif- 

 fering from the diorites proper in increased quartz and potash feldspar. 

 Thin-sections show quartz, plagioclase, orthoclase, green and brown horn- 

 blende, biotite, apatite, rutile, and the secondary minerals epidote, sericite, 



♦Hornblende syenite occurs at several localities in the state, but no recognition is taken of 

 this rock in this paper. The boundaries of the syenite areas are unknown at present, but it is 

 possible, that future study will reveal increased quartz in sufficient quantity to differentiate a 

 part of the rock of these areas as a hornblende granite. 



