288 



University of California. 



[Vol. i. 



remarkable parallel with that of the best preserved of the rocks 

 described in this paper. 



Wolff* describes a rock which he terms a tephrite, from the 

 Crazy Mountains, Montana, in which the nepheline is said to lie 

 between the feldspars in imperfect crystals or to fill triangular spaces. 

 In another publication! he describes acmite trachytes from the 

 same region in which nepheline occurs, filling angular areas between 

 the other components. 



The third important condition under which the analcite occurs, 

 that of replacing the feldspars, has already been described. The 

 change to analcite does not seem to have taken place from the 

 wedge-shaped areas between the feldspars, but to have gone on 

 independently in various portions of the crystals. The appearance 

 in many places, where the isotropic areas are so sharply defined, is 

 that of a paramorphic rather than a chemical change. This replace- 

 ment of feldspar by analcite Jias been noted in all the rocks of this 

 class, as well as in the eleolite syenites of Arkansas and the nephe- 

 line rocks of Montana, and does not seem to depend on the percent- 

 age of soda in the feldspar. This phenomenon seems to be mostly 

 confined to rocks rich in nepheline and related minerals, or to those 

 in which they are supposed to have existed. 



The analcite occupying the wedge-shaped areas is undergoing 

 decomposition to a green fibrous aggregate. The fibers extend 

 inward from the outer edge, often aggregated in radial tufts. They 

 are non-pleochroic, polarize quite brilliantly in greenish to reddish 

 tints and extinguish parallel to their long diameters. These fibers 

 often exhibit an undulate form and terminate in the analcite in 

 slender branching needles. They are almost colorless toward their 

 extremities, but being doubly refracting are strongly marked from 

 the analcite under crossed nicols. After treatment with hydro- 

 chloric acid and fuchsine, these fibers were stained a deep purple. 

 A specific determination was found to be impossible on account of 

 the small dimensions and lack of crystal outlines. 



In many of the sections, especially those from the smaller dikes, 



*Notes on the Petrography of the Crazy Mountains. North. Trans. Survey, 

 t Acmite Trachyte from the Crazy Mountains, Montana. Bull. Museum 

 Comp. Zoology, Harvard College, Vol. XVI. 



