PETROGRAPHY OP THE GRANITES 



531 



Thin-sections of the syenite reveal the following minerals : Orthoclase, 

 plagioclase, orthorhombic and monoclinic pyroxene, quartz, microcline, 

 iron ore, apatite, and zircon, with the alteration products epidote, chlorite, 

 and sericite. Hornblende is essentially absent in the thin-sections. Phalen 

 regards the nnakite as having originated from the akerite by hydromet- 

 amorphism aided by dynamic disturbances. 



The following analyses of the nnakite and the unakite-bearing rock — ■ 

 akerite — are quoted from Phalen (page 313) : 



SiO a * 



A1 2 8 



Fe 2 3 



FeO , 



MgO 



CaO 



Na a O 



K a O . 



H a O . 



P a O. 



MnO 



Cr a O, 



ZrO a 



Unakite, 

 Milams gap. 

 halen, analyst. 



Hypersthene akerite 



Milams gap. 



Phalen, analyst. 



58.32 



60.52 



15.77 



16.99 



6.56 



.60 



.89 



6.53 



.09 



1.59 



11.68 



4.58 



.32 



2.83 



4.01 



3.91 



1.73 



.88 



.48 



.74 



.13 



.25 





trace 



trace 



trace 



99.42 



Porphyritic Granite 



Unlike the crystalline region of North Carolina and Georgia, porphy- 

 ritic granites are but scantily developed in the Virginia area. The best 

 developed and most typical area in the state is that bounding the eastern 

 margin of the Eichmond coal basin near Midlothian, 13 miles west of 

 Eichmond and extending for a north-south distance of about 20 miles. 

 This marks the western limits of the even-granular granite quarried 

 around Eichmond and Petersburg. Both are biotite granites and only 

 differ from each other texturally. 



The rock in the vicinity of Midlothian is a coarse porphyritic biotite 

 granite, the porphyritically developed mineral of which is potash feld- 

 spar. The phenocrysts are of large size, 2 or more inches; idiomorphic 

 in outline and contain biotite inclusions; often twinned on the Carlsbad 

 law, and are in part orientated in a general northeast-southwest direction 

 from flowage of the rock. The porph}rritic texture, with variations, is 

 traced as far north as Gayton, in Henrico county, and as far south as 



* Including Ti0 2 . 



