16 



UNIVEESITY OF VIRGINIA PUBLICATIOXS 



Virginia. 



Mineral composition. — The granitic rocks of Virginia are predominautl}' 

 of the biotite type. The principal minerals are the same as those enumerated 

 for Marj'land on page 13, except that allanite has not been observed in the 

 Virginia rocks. Muscovite in subordinate amount usuall}- accompanier the 

 biotite in the principal areas of granitic rocks in the State, and it is the 

 principal component in the rock of the Hazel Run area west of Fredericks- 

 burg. Hornblende is an important constituent in the rocks of the Falls 

 Church area in Fairfax coimty, and near Sase in Charlotte county. Epidote 

 is a prominent constituent in the variety of granite known as unakite, 

 occurring in Page, Madison, and Carroll counties. 



Based on mineral composition the Virginia granitic rocks may be classi- 

 fied into (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; (2) muscovite granite, of which the Hazel Eun area 

 near Fredericksburg is the only tj^pical representative yet known; (3) horn- 

 blende-biotite granite, represented b}^ a part of the granites in the Falls 

 Church area, and near Saxe in Charlotte coimty; and (4) epidote granite, 

 unakite, of which there are only two known localities in the State. 



The rocks vary in structure from massive to schistose, and in texture 

 from even-granular to porphyritic. On this basis the three usual types of 

 the rocks are distinguished, even-granular massive, porphyritic, and schistose. 



Chemical composition. — Complete analyses of the Virginia rocks show 

 characteristically high silica, alumina, ajid alkalies, and correspondingly low 

 iron, lime, and mag^aesia. The difference in silica is slight, but in general 

 the iron, magnesia, and lime show a slight tendency to increase with decreas- 

 ing silica, indicating proportionately more of the dark minerals and plagio- 

 clase, and less quartz. The range in silica, lime, and alkalies is sho^^Ti in 

 the following table : 



D. Table of partial analyses of Virginia granites. 

 (Wm. M. Thornton, Jr., Analyst.) 



I. Medium-textured and medium gray, biotite granite. Westham granite quar- 

 ries, 4.5 miles west of Eielimond, Chesterfield countv. 



