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



granular granites, (2) porphyrinic granites, and (3) schistose granites 

 or granite-gneisses. The granite-gneisses were derived from the massive 

 granites, from which they principally differ in the pronounced schistose 

 structure secondarily induced by dynamic metamorphism. 



Based on mineral composition, the Virginia granites are divisible into 

 the following leading types : ( 1 ) Biotite granite, which usually carries a 

 little muscovite in addition to the biotite, and under which a majority of 

 the granites of the state may be grouped, including the Eichmond- 

 Petersburg areas, and most of the Fredericksburg area; (2) muscovite 

 granite, of which the Hazel Eun area one mile west of Fredericksburg 

 is the only typical representative yet known; (3) hornblende-biotite 

 granite, represented by a part of the granites occurring southwest of 

 Washington, in the Falls Church area in Fairfax county; and (4) epidote 

 granite, unakite, of which there are only two known localities. 



In addition to these, pegmatite, coarse crystallizations of quartz and 

 feldspar with subordinate mica, in dike-like form, abundantly penetrate 

 the finer granites and associated crystalline rocks over much of the Pied- 

 mont region. Aplite is represented only in the granite of the Eichmond 

 area, and here principally on the western border of the Eichmond basin, 

 near Midlothian. 



Even-granular Granites 



INTRODUCTORY STATEMENT 



The granites distributed along the fall-line in the eastern part of the 

 Piedmont region in the vicinity of Eichmond, Petersburg, and Fred- 

 ericksburg are the most important economically in the state and they 

 best illustrate the types of mica-granite. Three types are here repre- 

 sented based on differences which are best brought out under the indi- 

 vidual descriptions below. The three types are: (1) The Eichmond- 

 Petersburg light gray, (2) the Eichmond-Fredericksburg dark blue gray, 

 and (3) the Fredericksburg light gray. 



To these, two additional types must be added from other parts of the 

 state. These include the Falls Church dark gray hornblende-biotite 

 type and the yellowish green and pink epidote type known as unakite. 



PETROGRAPHY OF THE GRANITES 



The Richmond-Petersburg light gray granite. — This type of granite 

 has been quarried for many years in the vicinity of Eichmond and Peters- 

 burg. It extends over parts of three counties, namely, Dinwiddie, Ches- 

 terfield, and Henrico. It is a biotite-granite, frequently containing a 



