VIRGINIA LOCALITIES 495 



alkaline; and reacts faintly for carbonates. Several tests with hot acid 

 were made on the lighter colored crnst, which resulted in yielding little 

 or no gelatinous silica^ but an insoluble residue remained composed chiefly 

 of silica. A portion of the powdered red crust yielded Prof. F. P. Dun- 

 nington 15.40 per cent total SiOs, 12.50 per cent of which was insoluble 

 in a 5 per cent solution of sodium carbonate, the difference, 2.9 per cent, 

 being accounted amorphous silica. Similar determinations made of total 

 soluble and insoluble SiOs on the weathered crust from Eoanoke County 

 showed nearly 75 per cent of the total SiOo to be soluble in a 5 per cent 

 soda solution (see pages 491-492). 



Microscopic examination of several separate portions of the powdered 

 crust showed it to be identical with that from other localities. It was 

 composed partly of isotropic and partly of weak birefracting grains, each 

 type exhibiting considerable variation in physical properties. The light 

 colored powder contained a goodly sprinkle of minute colorless grains 

 that were double refracting and were identified as probably quartz. 



Chemical analyses and their discussion. — In the subjoined table are 

 given analyses of the fresh allanite in column I, and of the inner red 

 layer of weathered allanite in column II, and of the outer white layer in 

 column yi. Columns III, IV, and V give the gain and loss in constitu- 

 ents by comparing the analyses of the fresh and the weathered inner red 

 layer of the mineral on a ferric oxide constant basis. Likewise figures 

 showing gain and loss in constituents of the outer white layer when com- 

 pared with the fresh mineral on the same basis are given in columns VII, 

 VIII, and IX. 



Analyses of fresh and decomposed Allanite from Amherst County, Virginia 



I 



SiO^ 31.23 



AI2O3 16.45 



Ce^Os 11.24 



Di^Os 1 



LaA r-90 



Y2O3 1.65 



Fe^Oa 3.49 



FeO 13.67 



BeO 0.24 



CaO 8.69 



MgO 22 



H2O 2.28 





Weathered allanite, 





Weathered allanite, 





inner red layer 



A 







outer white layer 



A 





II 



III IV 



V 



VI 



VII 



VIII 



IX 



8.05 



2.42 97.58 



30.47 



21.37 



19.55 



80.45 



25.12 



16.83 



9.61 90.39 



14.87 



20.66 



35.88 



64.12 



10.55 



7.13 



5.98 94.02 



10.57 



21.90 



55.67 



44.33 



4.98 



none 



0.00 100.00 



9.90 



none 



0.00 



100.00 



9.90 



none 



0.00 100.00 



1.65 



none 



0.00 



100.00 



1.65 



37.14 



100.00 0.00 



0.00 



12.24 



100.00 



0.00 



0.00 



none 



h-ejeL^ 



0.00 



none 



0.00 



0.00 



0.00 



.94 



.15 



1.95 



23.21 



76.79 



.18 



none 



0.00 100.00 



8.69 



none 



0.00 



100.00 



8.69 



none 



0.00 100.00 



0.22 



none 



0.00 



100.00 



0.22 



29.55 



100.00 0.00 



0.00 



21.37 



100.00 



0.00 



0.00 



99.06 99.64 76.52 99.49 61.29 



XXXVII — Bull. Geol. Soc. Am., Vol. 28, 1916 



