Z. V. Pirsson — Phonolitic Rocks from Black Hills. 343 



Neglecting the small amount of water and ferric oxide — the 

 latter due to minute amounts of limonite infiltrated into cleav- 

 age cracks — the ratios are those of a normal feldspar in which 

 Or : Ab : : 1 : 2£. 



Attempts made to measure the cleavage angle of c(001)^ 

 #(010) were not successful as the cleavages were too imperfect to 

 reflect light well and measurements could not be made within 

 1° with accuracy. The cleavage parallel to 5(010) is much 

 better than that parallel to c(001). Such cleavage plates rest- 

 ing on c(001) between crossed nicols extinguish parallel to the 

 edge of c(001) on 5(010) or so nearly that no appreciable devia- 

 tion could be seen with the Bertrand ocular or in monochro- 

 matic light ; resting on 5(010) they have a positive extinction 

 angle of about 9° and in convergent light show the obtuse 

 bisectrix c about perpendicular to the field. The feldspar is 

 therefore a soda-orthoclase. 



From these results it would seem that soda-orthoclase or 

 anorthoclase is probably a more common constituent of phono- 

 litic and trachytic rocks than might be supposed from their 

 petrographic description. The concealment of the triclinic 

 character, produced very probably by sub-microscopic or even 

 intimate molecular twinning according to the albite law, would 

 naturally lead to confusion with sanidine while examination of 

 cleavage plates of such material, giving only a slightly in- 

 creased angle in the positive sense on 5(010) and a somewhat 

 higher specific gravity, would scarcely yield sufficient data for 

 reliable conclusions and it is evident that only chemical analy- 

 sis combined with these can safely establish the precise nature 

 of the mineral.* 



Pyroxene. — This is segirine-augite with characteristic color, 

 pleochroism and extinction. Dispersion of the optic axes is 

 marked. It is zonally built, the asgirine molecule increasing 

 toward the periphery which is finally surrounded by a deep 

 green segirine mantle. This was evidently growing during 

 the final stage of consolidation as the minerals of the ground- 

 mass are idiomorphic against it or else actually imbedded in it. 

 Titanite occurs in characteristic lozenge shaped cuts which are 

 almost invariably twinned. The groundmass is made up 

 mostly of small lath-like feldspars with Carlsbad twinning 

 arranged in trachytic structure and scattered amidst them are 

 shreds and rods of segirine and very small idiomorphic nephe- 

 lites. These latter are very clear and colorless and often 

 bounded by the segirine shreds as is common in phonolites. 

 They are easily told by their hexagonal basal cuts, dark be- 

 tween crossed nicols and short rectangular prismatic ones with 



* Conf. also, article by Wolff and Tarr, Bull. Mus. Comp. Zool. Cambridge, 

 vol. xvi, 1893, pp. 230-231). 



