CHARACTER OF WEATHERED PRODUCTS 485 



County, Virginia, which were not studied microscopically, yielded similar 

 results. For this reason the microscopic data obtained on the powder of 

 the weathered product from the various localities in the Atlantic States 

 are not tabulated separately, but are here summarized in order to avoid 

 unnecessary repetition. 



Microscopically the powdered crust is reddish brown in color, waxy in 

 luster, and distinctly granular rather than earthy in character. It is 

 heterogeneous in mineral composition, composed partly of birefracting 

 grains and partly of isotropic ones, with probably the latter predominant. 

 Both the birefracting and the isotropic grains are variable in physical 

 properties and probably in composition. 



Pleochroism, if developed, is extremely weak and not important; only 

 noticeable in an occasional strongly colored grain. Index of refraction 

 variable within wide limits, a number of grains ranging as low as 1.60 

 and a few above 1.85, with the average probably around 1.75. These fig- 

 ures apply both to the birefracting and to the isotropic grains. For the 

 anisotropic grains the double refraction is very weak, but variable. Some 

 grains exhibited entirely sharp and distinct extinction, and several indi- 

 cated that they were probably optically positive ( + ). Occasional grains 

 of quartz were identified in the powder of a number of specimens of the 

 decayed product from different localities. 



The evidence gained from both a microscopic and a. chemical study of 

 the powdered weathered product (crust) conclusively proves its hetero- 

 geneous character. Attempts to effect a separation of the birefracting 

 grains from the isotropic ones for separate chemical analyses of each were 

 unsuccessful. However, should a separation prove successful, chemical 

 analyses of either the birefracting or isotropic grains would probably re- 

 veal inconstancy of composition in each, since the microscopic study 

 reasonably establishes the heterogeneous character of both. 



CHEMICAL COMPOSITION 



For purposes of comparison analyses of the weathered product of al- 

 lanite crust are brought together in the table below in the order of increas- 

 ing silica : 



Analyses of the iceathered Product of Allanite from Virginia 



I II III IV 



SiO^ 8.05 8.48 18.66 21.37 



TiO^ 2.16 



AI2O3 16.83 2.57 23.28 20.66 



Ce^Oa 7.13 6.44 1.30 21.90 



