464 T. L. WATSON WEATHERING OF ALLANITE 



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Weathered product (crust) of allanite 483 



Megascopic characters 483 



Microscopic characters 484 



Chemical compositiou 485 



Weathering of allanite in Virginia localities 487 



Introductory statement 487 



Previous work 487 



Roanoke County 488 



Locality and mode of occurrence 48S 



Fresh allanite 489 



Weathered crust of allanite 489 



Chemical analyses and their discussion 490 



Nelson County 492 



Locality and mode of occurrence ; 492 



Fresh allanite 492 



Weathered allanite 493 



Chemical analyses and their discussion 493 



Amherst County 494 



Locality and mode of occurrence 494 



Fresh allanite 494 



Weathered allanite 494 



Chemical analyses and their discussion 495 



Discussion of results 496 



Conclusions , 499 



Introduction 



Many observers, both in this country and abroad, have noted the altera- 

 tion exteriorly of allanite to usually a reddish brown product whose 

 general appearance on casual examination seemed to be closely similar. 

 This alteration is frequently conspicuous in localities where allanite is 

 found at or near the surface in lumps and masses, and is shown especially 

 well at places in some of the Atlantic States, particularly in Virginia, in 

 the Carolinas, in Pennsylvania, and in Texas, and in specimens examined 

 by the writer from other localities in the United States and abroad. 



Some of the more important localities have been studied in the field 

 and collections made of the fresh allanite and its alteration product for 

 laboratory study. During the progress of the investigation certain fun- 

 damental questions arose which the writer had assumed as settled, but 

 which are now regarded by him to be at least doubtful. Chief among 

 these is the composition of the ordinary so-called fresh, black, and vitreous 

 allanite, which from preliminary work thus far accomplished suggests 

 that it is a heterogeneous mixture and not a homogeneous compound. 

 The problem, therefore, of allanite and its alteration can not be considered 



