ANALCITE GROUP OF IGNEOUS ROCKS 683 



properties of analcite, and must therefore be that mineral and 

 not a pitchstone glass, as had formerly been supposed. 



The sharpness of the ratios given above is an excellent 

 testimonial to the purity of the material, the care with which it 

 was se])arated, and to the analytical skill of Hunter. 



Thus the supposition that the base of these rocks was very 

 unlikely to be a glass, and the indications previously mentioned 

 that it was analcite, are most strikingly confirmed by these 

 results. It is not to be wondered at that a base of analcite 

 should have been mistaken for a glass by many petrographers, 

 including the author, since, the grains having everywhere 

 the same optical orientation and the same index of refraction, 

 there would be no means of distinguishing them in plain or in 

 polarized light, either from one another or from a continuous 

 isotropic substance like glass. In the original monchiquites 

 from Brazil, specimens and sections of which the author owes to 

 the kindness of Professor Rosenbusch and of Professor A. 

 Lacroix from material collected by Professor O. A. Derby, the 

 analcite often shows a tendency to cr3'stal form by the produc- 

 tion of areas which are free from the larger prisms of the ferro- 

 magnesian minerals, the latter being arranged around them in 

 wreaths. The areas thus resemble phenocrysts of leucite and 

 they are in reality phe?iociysts of analcite. They are sprinkled 

 full of the microlites of hornblende described by Rosenbusch, 

 which do not, however, show any tendency to the zonal arrange- 

 ment shown by such inclusions in leucite. 



It is a matter of or-reat interest to recall in this connection 



O 



that Lindgren' onl}' a few months previous to Hunter and 

 Rosenbusch had published an account of certain basaltic dikes 

 occurring in the Highwood Mountains of Montana. They were 

 shown to consist of augite, olivine, iron ore and analcite as 

 phenocrysts in a groundmass of magnetite grains, augite micro- 

 lites and a second generation of analcite. 



The analcite was separated and two analyses were made 

 which are given in I and II. 



'Proc. Cal. Acad. Sci., Series 2, Vol. Ill, July 1890. 



