466 T. L. WATSON AVEATHERING OP ALLANITE 



rocks, in some of which it forms an important accessory mineral. Others 

 have since extended the geographic distribution of allanite as a subordi- 

 nate original constituent of igneous rocks, but have added little or nothing 

 in the way of additional rock types. Localities in the United States 

 where the mineral has been noted as a rock constituent are numerous and 

 widely separated. When formed directly as a product from the consoli- 

 dation of a molten magma, the mineral is almost invariably in small 

 grains and crystals, frequently of microscopic dimensions only. 



The principal occurrence of allanite in massive crystalline! form of 

 large but varying size and in more or less quantity is in pegmatite bodies 

 of chiefly granitic composition. For those occurrences studied the min- 

 eral is regarded as an original constituent of the pegmatite. It is some- 

 times associated with epidote, and in 1890 Brogger* called attention to 

 allanite-epidote intergrowths in certain pegmatites of Arendal, I^orway, 

 in which he regarded the allanite as primary. A similar association of 

 allanite and epidote in igneous rocks has been reported by W. H. Hobbs,^ 

 F. D. Adams,^ A. Lacroix,"^ G. H. Williams,^ and others. The fresh and 

 weathered allanite which forms the basis of this study was derived from 

 pegmatitic occurrences in various localities in the Atlantic States. 



The authentic occurrences of reported allanite in contact metamorphic 

 deposits are apparently rare. Probably one of the best known examples 

 is that described by Professor Emerson in Pelham, Massachusetts, where 

 a macroscopic "reaction rim" was produced between a great dike of black 

 olivine-enstatite rock and the inclosing gneiss. Professor Emerson says :^ 



"Against the olivine rock is a broad band, characterized by very basic min- 

 erals — thick bands of biotite, containing apatite, and fine large corundum crys- 

 tals. Then comes anorthite full of orthite, rutile, and corundum in large 

 masses, etcetera. 



"A crystal of corundum was shown — largely fine blue sapphire, in which 

 was a crystal of allanite a half inch across, which had coerced the corundum 

 into a fine radiated puckering nearly an inch wide, outside of which the fine 

 cleavage of the corundum asserted itself. The same puckering surrounds the 

 allanite in the massive anorthite." 



Allanite has been reported as a product of dynamo-regional metamor- 

 phism. According to Emmons and Calkins,^^ allanite is widely distrib- 



4 W. C. Brogger : Zeitschr. fur Kryst., Bd. xvi, 1890, p. 99. 



5 W. II. Hobbs : Am. Jour. Sci., 3d ser., vol. xxxviii, 1889, pp. 223-228 ; Tschermak's 

 Min. und Petrog. Mittb., Bd. xi, 1890, p. 1. 



8 F. D. Adams : Canadian Rec. Sci., vol. 4, 1891, p. 344. 



' A. Lacroix : Bull. Soc. Min. de France, t. xii, 1889, pp. 138, 157, 210. 



8 G. H. Williams : Bull. 62, U. S. Geol. Survey, 1890. 



9 B. K. Emerson : Bull. Geol. Soc. Am., vol. 6, 1895, p. 474. 



10 W. H. Emmons and F. C. Calkins : Professional Paper 78, U. S. Geol. Survey, 1913, 

 pp. 97, 159. 



