GRANITE-GNEISSES 



533 



massive rock was younger and had been intruded into the schistose rock. 

 The evidence for this is discussed elsewhere in this paper. 



The Kichmond-Fredericksburg Gneiss 



This is the oldest of the acid rocks in the Eichmond-Fredericksburg 

 areas and it is invaded by both the light gray and dark blue massive 

 granite. Figure 1 shows the relations between the dark blue granite and 

 the gneiss at the McGowan quarry, south of Eichmond. Similar rela- 

 tions obtain in the quarries 3 miles north of Fredericksburg. The period 

 of deformation inducing the 



schistose structure in the gneiss 

 antedates that of the intrusion 

 of the massive granite, as evi- 

 denced by the massive granites 

 cutting across the schistosity of 

 the gneiss and by inclusions of 

 the gneiss in the granites which 

 preserve perfectly the gneissic 

 structure. 



The rock is a hornblende- 

 biotite gneiss of medium text- 

 ure and irregularly banded. 

 The principal minerals are 

 quartz, orthoclase, plagioclase, 

 hornblende, biotite, sphene, and 

 apatite. Chlorite occurs as an 

 alteration from biotite. The 

 principal features to be noted 

 in the composition of the gneiss 

 are : ( 1 ) The presence of essen- 

 tial hornblende, which greatly 

 exceeds biotite in amount in 

 some of the sections; (2) the absence of microcline — a mineral always 

 present in the massive granites; and (3) the presence of much plagio- 

 clase, which is the dominant feldspar in a few sections and nearly or quite 

 equals orthoclase in amount in others. 



Structural Eelations of the Granites in the Eichmond- 

 Fredericksburg Areas 



TYPES 



Three types of rocks of granitic composition are represented in the 

 Eichmond area ; two are massive granites ; the third is a schistose granite 



Figure 2.- 



• Contacts between the Gray and Blue 

 Granites. 



Locality Netherwood quarry. Inclusions of the 

 older gray granite are shown in the blue granite. 

 Scale, 1" = 9". 



