310 



VATSOX AXD CLIXE IGXEOVS DIKES IX VIRGINIA 



Microscopic character. — The microscopic petrography of these rocks is 

 comparatively simple. In thin-sections the principal minerals composing 

 the ground-mass are a fine aggregate of feldspar (sodic plagioclase and 

 orthoclase). quartz, biotite, and magnetite. Xo glass has been identified. 

 Set in this microcrystalline ground-mass are phenocrysts of plagioclase 

 (albite), orthoclase. biotite, and augite, which usually exhibit crystal 

 boundaries. Occasionally some of the phenocrysts of each mineral con- 

 tain inclusions of the ground-mass. Secondary minerals derived from 

 the original ones by weathering are few in number, comprising chiefly 

 kaolin, chlorite, museovite, and in sections showing greatest alteration 

 calcite and limonite. 



Chemical composition and. classification in the quantitative system. — 

 In order to show the composition of this rock, there is given in column I 

 in the subjoined table a chemical analysis made by Dr. W. F. Hillebrand 

 on a typical sample. For comparison, there are added two analyses of 

 related rocks — a trachyte (?) in column II and a granite-porphyry in 

 column III. As may be observed from an examination of the table, the 

 three rocks are remarkably similar in chemical composition. They fall 

 into the same position in the quantitative system 12 and are designated 

 by the subrang name toscanose. 



Analyses of Felsophyre and related Rocks 



(W. F. Hillebrand, analyst) 



I la II III 



Si0 2 69.56 1.159 69.52 69.68 



A1 2 3 15.52 .152 15.44 14.97 



Fe 2 3 1.67 .011 1.90 0.79 



FeO 1.19 .017 0.09 0.34 



MgO 0.41 .010 0.17 0.66 



CaO 1.20 .021 1.70 2.10 



Xa 2 4.46 .073 4.54 3.38 



K,0 4.68 .050 5.04 4.40 



H 2 0— 0.34 .... 0.33 1.10 



H 2 0+ 0.67 .... 0.27 0.92 



Ti0 2 0.31 .004 0.23 0.28 



P 2 5 0.08 .001 0.14 0.17 



MnO 0.07 .001 0.08 Trace 



BaO 0.10 .001 0.19 0.14 



SrO Trace 



Li 2 Trace .... 



CO, Xone 0.17 0.88 



S : Trace 



100.26 



99.90 



f»9.8<3 



12 H. S. Washington : Professional Paper No. 14, U. S. Geological Survey, 1903, pp. 

 160-161, 164-165. 





