DISTRIBUTION AND COMPOSITION 525 



In addition to these there are several minor areas, chief among which are 

 those of Fairfax, Prince Edward, Fluvanna, and Goochland counties. 



The massive granites will be described under the following types : The 

 Richmond-Fredericksburg, light gray; the Richmond-Fredericksburg, 

 dark blue ; the Fredericksburg, light gray ; the Falls Church and Annan- 

 dale types, and the Unakite type. 



Mineral Composition of the Granites 



Conforming with well known granites elsewhere, the Virginia rocks 

 are mixtures of feldspar and quartz, with biotite as the third essential 

 component, in the most important areas. Muscovite in subordinate 

 amount usually accompanies the biotite in all the granitic areas of the 

 state, and it becomes a principal component, replacing biotite in the 

 granite of the Hazel Run area west of Fredericksburg. Hornblende is 

 an important constituent in only a part of the granites of the Falls 

 Church area southwest of Washington, and it is almost unknown in the 

 granites of the other areas. It is essentially absent from the most 

 economically important granites of the state. 



In the unique variety of granite known as unakite, occurring near 

 Luray, in Page and Madison counties, in the Blue ridge, and near Trout- 

 dale, in Grayson county, epidote is a principal constituent and the f erro- 

 magnesian silicates nearly or entirely fail. The epidote is wholly a 

 secondary constituent. 



The dominant species of feldspar in the Virginia granites is usually 

 orthoclase, though in many instances microcline or plagioclase may equal 

 or even exceed orthoclase in amount. One of the most striking features 

 in the mineralogical constitution of these rocks is their usual richness in 

 both microcline and plagioclase, especially the latter. Both of these 

 feldspars, however, are subject to much variation, and in a few thin- 

 sections examined their poverty was a noticeable feature. It seems prob- 

 able that a part of the microcline in the Virginia rocks has developed in 

 part from orthoclase by pressure metamorphism. 



Besides the above minerals, there occur apatite, zircon, sphene, magne- 

 tite, and occasionally some other minerals. 



Kinds of Granite 



Conforming with the granites of the southeastern Atlantic states in 

 general, the granites of Virginia vary in structure from massive to 

 schistose, and in texture from even-granular to porphyritic rocks. On 

 this basis three types of the rocks are distinguished: (1) Massive even- 



