G. W. Knight — Occurrence of Pseudo-Leucite. 291 



which average l cm in diameter. They consist of an allotrio- 

 morphic mosaic of ortlioclase and nephelite. "In some cases 

 the outer edge of the section is composed of ortlioclase crystals 

 with their longer axes perpendicular to the crystal faces of the 

 original leucite." A thin section of the Bearpaw pseudo-leu- 

 cite examined by the writer contains in its center an isotropic 

 kernel, whose outline resembles the figure 8. Part of the feld- 

 spar surrounding the kernel radiates out from this material, 

 giving a spherulitic structure to this part of the slide. It may 

 be that the ortlioclase and nephelite have been formed directly 

 from the isometric kernel, or, in other words, this isometric 

 substance might be regarded as a remnant of the original leu- 

 cite crystal. Prof. Pirsson, however, has proved by chemical 

 tests that the material is either sodalite or analcite and not any 

 original unchanged leucite substance. The material examined 

 by the writer was too small to permit the use of microchemi- 

 cal tests. 



Prof. Wolff" studied a leucite-tinguaite from Beemerville, 

 JS\ J., which contains pseudo-leucites similar to those described 

 above. They are made up "of an aggregate of ortlioclase, 

 partly in irregular grains, more often in radiating prisms." 

 Between the ortlioclase a comparatively small amount of 

 nephelite occurs. The centre of the pseudo-crystals contains 

 an isotropic mineral determined as analcite. Near the same 

 district Prof. Kempf examined a basic dike consisting of 

 biotite and pyroxene set in a ground-mass that is chiefly 

 analcite. In portions some curious, spheroidal inclusions 

 appear having a diameter up to 10 mm ; they are made up 

 chiefly of analcite and are always rounded and without 

 crystal form. In the fresh material Prof. Kemp was able 

 to show that the spheroids consisted partly of leucite. These 

 pseudo-crystals have, however, evidently been formed by 

 surface alteration and differ in this respect from the pseuclo- 

 leucite from Yukon territory and the other localities men- 

 tioned above. 



Prof. Pirsson % described pseudo-leucites from the High wood 

 mountains, Montana, consisting of alkali feldspars, nephelite 

 and small amounts of what is held to be analcite. They occur 

 in a "granular intrusive rock consisting of dominant leucite 

 with subordinate augite. " These pseudo-crystals are similar 

 to those already described. 



In order to study the chemical composition of the pseudo- 

 leucites from the Yukon territory, and also to compare them 



* Harv. Coll. Mus. Comp. Zool. Bull., vol. xxxviii,p. 273-277, 1902. 

 f This Journal, 3d series, vol. xlv, p. 298-305, 1895 ; also vol. xlvii, May 

 1894. 



% Bull. 237, U. S. Geol. Surv. p. 83. 



