PETROLOGY 313 



essentially absent from them, although it occurs in great profusion in the 

 substance of the other alteration products derived from the augite. 



Biotite is developed as a minor constituent in small crystals and flakes, 

 with the usual pronounced pleochroism. Primary or original magnetite 

 in scattered crystals and occasional pyrite make up the other accessory 

 minerals. 



Chemical composition and classification in the quantitative system. — 

 The chemical composition of the quartz-gabbro (norite) is shown in the 

 subjoined analysis made by Mr. J. G-. Dinwiddie. The large percentages 

 of H 2 and C0 2 given in the analysis clearly show that the rock was not 

 fresh, although it had the appearance of such in the hand specimen. 



Analysis and Norm of Quartz-Gaobro, 1.5 Miles South of Milnesville, Augusta 



County, Virginia 



(J. G. Dinwiddie, analyst) 



Norm 



Per cent 



Q 15.781 



Or 8.90 ! 



Ab 24.63 J 7650 



An 27. 24 J 



Di 2.071 



Hy 11.97 I 



Mt : 3.25 i-19.18 



II 



Ap 



H 2 



C0 2 ! 



100.83 





Analysis 







Per cent 



Ratios 



Si0 2 



57.62 



.960 



ALOo 



16.44 



.161 



Fe,0, 



2.34 



.014 



FeO 



4.89 



.068 



MgO 



2.53 



.064 



CaO 



6. 40 



.114 



Na 2 



2.91 



.047 



K 2 



1.52 



.016 



H 2 - . . . 



0.20 





H 2 + . . . 



1.15 





Ti0 2 



0.64 



.008 



MnO 



0.27 



.004 



PA 



0.29 



.002 



C0 2 



3.75 





1 



,22 



0. 



,67 



1 



.35 



3 



,75 



100.95 



The position of the rock in the quantitative system may be expressed 

 as follows : 



Ratios 



Class, — — = — — = 3.99 = IT, dosalane. 

 Fern 19.18 



Q 15.78 



Order, — = = 0.26 = 4, austrare. 



F 60.77 



K 2 + Na, 2 63 



Rang, — — — =— = 0.64 = 3(4), tonalase. 



*' CaO 98 



Subrang, —^——.— = 0.34 = 4, tonalose. 

 Na 2 47 



Symbol Name 



II. 4. 3(4). 4. Tonalose. 



