536 



T. L. WATSON LITHOLOGICAL CHARACTERS OF VA. GRANITES 



Petersburg, and Fredericksburg granites show comparative freedom from 

 them, and usually they are of very small dimensions. 



The Aplites and the Pegmatites 



The aplites are only occasionally met with in the Virginia granite 

 areas. They have been noted by the writer only in the Eichmond granite 

 area. A banded aplite-pegmatite of small dimensions, shown in figure 4, 

 penetrates the dark blue granite of the McGowan quarry south of Eich- 

 mond. In the vicinity of Midlothian, 13 miles west of Eichmond, the 

 porphyritic biotite granite is cut by a number of small aplite dikes. 



Where these have been observed they are 

 ^'\^\ , 7\iV r i?/^ X 'VC plainly of an intrusive nature, cutting the 

 7 l X^'\-'\'*j'^f\ \' t v i\ granite proper and not the inclosing gneisses. 

 ~^'1\ ^ ra J , !^t^^ / ^ s Pegmatites are abundantly developed in 



<*%/>> l \ \ ^- "\ J. v -•" \T^ the Eichmond-Fredericksburg areas and at 

 '■ I A > — ' • / - f — N x x ^ times are of large size. Only in one or two 



.A ' ~~*Z \^ . .-< instances do they seriously interfere with 



* << Pegmatite J( quarrying operations. They are of granitic 



^ . .^ "*- . mineralogy, without the occurrence of un- 



^s / - ' - > usual or rare minerals noted in them, and 



;^^^ ^)V/W x$\?\\ C}\ ^ they cut alike the granites and the gneisses. 

 yO s\\ \\-~ '' J \f n They consist of coarse aggregates of feld- 



/ -y N _\^ '\ ^ s y \ \ \\{. s P ar and quartz, with more or less black bio- 



• ~^a Qrantte ■£.> '^^v tite and a little muscovite. In the Fred- 

 dy / ' " v -~ \' -L ' ~" * 



^-\y\^\^ \V 'J. x N "\ ericksburg quarries of dark blue granite, 

 ' • / ' v / s N i n / , x "f \^*J where the pegmatites are particularly abun- 

 Fiau re 4.— Banded Apiite-Pegmatite dant, massive granular magnetite and large 



Intersecting the Blue Granite. and gmall per f ect re d Crystals of gamet are 



McGowan quarry, south of Rich- not i n f reqT1 ent constituents. The dodeca- 



mond. Scale, 1" = 5". ^ 



hedron and trapezohedron are the commonest 

 forms of the garnet. The feldspar exhibits a variety of colors, from 

 white opaque and pink to a decided medium green, the former two being 

 the commonest shades. Thin-sections show both orthoclase and micro- 

 cline under the microscope. An acid plagioclase is present in some of 

 these veins. The numerous quarries in the Eichmond area afford excel- 

 lent opportunity for studying the pegmatites and some interesting data 

 have been obtained bearing on their comparative age relations. 



It has been shown above that there are three granites, including the 

 granite-gneiss in this area, representing as many periods of intrusion. 

 The order of these intrusions, beginning with the oldest, has been shown 



