CONCLUSIONS 499 



decay, many examples of which are on record^^ and need not be discussed 

 here. 



Conclusions 



The following conclusions seem warranted from this investigation: 



1. Allanite has wide distribution as a primary minor accessory mineral 

 in many kinds of igneous rocks. 



2. Of the several modes of occurrence, the most important as regards 

 size and quantity of mineral is in pegmatite bodies. 



3. The ordinary black vitreous allanite is apparently not homogeneous 

 in composition, but is made up of at least two types, the proportions of 

 which are subject to wide variation in different specimens examined. 

 One is isotropic; the other is birefracting and derived from the former 

 by either alteration with or without appreciable change in chemical com- 

 position or by inversion, but probably by alteration. 



4. When found at or near the surface, the allanite masses are usually 

 partially or entirely encrusted with a reddish brown alteration product, 

 sometimes of lighter color, closely resembling in general appearance some 

 form of hydrous iron oxide, which is clearly the result of weathering 

 (decomposition). 



5. The weathered product of allanite is not homogeneous except in the 

 sense of many colloidal materials, but is definitely shown microscopically 

 to be heterogeneous in character, composed of a variant mixture of iso- 

 tropic and weakly birefracting grains, each type of which varies consid- 

 erably in physical properties and probably in chemical composition. The 

 bulk of the decomposition product, however, is apparently composed of 

 an isotropic substance in colloidal or metacoUoidal form of variable com- 

 position. 



6. Chemical analyses of the weathered product likewise disclose its 

 variable nature, and when compared with analyses of the supposed fresh 

 mineral the processes involved in the change are chemical, the principal 

 reactions being hydration, oxidation, carbonation, and solution, or those 

 common to the belt of weathering. For the stage of decay represented 

 the transformation from fresh to decomposed mineral has been accom- 

 panied by a total loss from leaching of approximately 75 per cent of the 

 original material, as calculated on a ferric oxide constant basis. 



7. AVhen chemical analyses of the decayed product are compared, the 

 difference in composition is found to be one not of unlike constituents, 



s'^ G. p. Merrill : Rocks, Rock-weathering, and Soils. The Macmillan Co., N. Y,, 1906, 

 400 pages. 



