Weed and Pirsson — Bearpaw Mountains, Montana. 293 



Under the microscope the rock is found to be composed of 

 apatite, oegirite-augite, alkali feldspars and quartz. 



The oegirite-augite has the usual characters of this mineral ; 

 a clear light grass-green color, pleochroism, though not strong, 

 into tones of yellow, an excellent cleavage, and is usually 

 quite idiomorphic. It is zonally built, increasing in the 

 aegirite molecule towards the periphery, and has in these parts 

 a corresponding decrease in the angle of extinction. It has in 

 many cases a pronounced dispersion of the optic axes, which 

 is probably owing to the presence of titanium. 



Since the mineral composition of the rock is, as indicated 

 above, so simple, we may reckon out of the bulk rock analysis 

 the chemical composition of this augite, which then has 

 approximately the following composition : 



Si0 2 



Fe 2 3 



FeO 



MgO 



CaO 



Na 2 



45-3 



22-1 



1-8 



8-6 



13*6 



8-6 = 100-0 



corresponding in round numbers to two molecules of diopside 

 to one of gegirite. 



The apatite has been seen only in a very few, rather large, 

 scattered grains ; the small amount of it is indicated in the 

 minute quantity of phosphoric anhydride shown in the 

 analysis. 



The large feldspar phenocrysts between crossed nicols 

 appear homogeneous and in general untwinned, except an 

 occasional carlsbad. In some places areas of kaolinization 

 occur, otherwise they are quite fresh. The cleavages parallel 

 to o (010) and c (001) are both excellent and furnish good 

 plates for optical examination : such plates parallel to c (001) 

 extinguish at about one degree from the trace of o (010) while 

 plates parallel to b (010) extinguish positively at 10° 30' to 11° 

 30' from the trace of c(001) and show in convergent light a 

 positive bisectrix nearly centered in the field. The feldspar is, 

 therefore, not orthoclase but anorthoclase, as must indeed be 

 expected from the rock analysis. It seems to be closely rela- 

 ted to the cryptoperthite described by Brogger.* A notice- 

 able feature of these phenocrysts is that they contain certain 

 quantities of slender microlites of a striated feldspar. While 

 these are apt to be scattered through the phenocrysts they are 

 especially found arranged in a zone around the edge and pro- 

 jected vertically from the plane faces inward, as illustrated in 

 fig. 3, which shows the phenocrysts and the character of the 

 rock in a diagrammatic way. The great majority of these 

 slender feldspar microlites, no matter how twinned, extinguish 

 parallel to the plane of the nicols or nearly so, and Becke's 



*Mmeralien der Syenit-pegmatit Gange, Zeit. fur Kryst, vol. xiv, p. 524, 

 1889. 



