Weed and Pirsson — Bearioaic Mountains^ Montana. 141 



In thin sections the following minerals are seen : apatite^ 

 iron ore^ aitgite^ olivine, hiotite, nephelite, alkali felspar, and 

 sodalite. 



The augite phenocrysts are perfectly idiom orpine and 

 bounded by the usual planes m{liO), <2(100), and ^(111) ; the 

 mineral is a very pale olive brown, almost colorless in the sec- 

 tion, and has all the characteristics of basaltic augite. It is 

 sometimes filhid with small inclusions of glass having irregu- 

 lar shapes. It shows no trace of alteration. 



The olivine is remarkably fresh and unaltered, and is perfectly 

 colorless and limpid ; it is not so idiomorphic as the pyroxene 

 and is much less common. It contains inclusions of iron ore, 

 glass, and occasional grains of pyroxene ; smaller crystals are 

 free from these inclusions. 



The hiotite is far less common than either of the minerals 

 noted and in bulk amount does not compare with them. It 

 does not occur in well formed tablets as is usual with this min- 

 eral, but in formless, embayed, ragged patches and very com- 

 monly a number of these lying near together have a similar 

 crystallographic orientation and extinguish simultaneously, 

 thus resembling a poikilitic structure. It quite vividly recalls 

 the embayed, skeleton brown hornblendes and biotites in the 

 nephelite tephrite from Langehanskiippel by Poppenhausen in 

 the Rhongebirge (buchonite of Sandberger), and may like 

 them be only the remains of formerl}' larger resorbed crystals ; 

 it does not, however, show any opacite borders. It has the 

 rich red-brown color that so frequently distinguishes biotite in 

 rocks of this class and in the theralites. It is appai-ently uni- 

 axial. Where it occurs it is noticeable that the minute grains 

 of iron ore which dot and pepper the groundmass are for con- 

 siderable areas around it almost entirely lacking — the iron 

 evidently being consumed in its production. It contains inclu- 

 sions of apatite, iron ore, and grains of pyroxene, the last two 

 more rarely. 



Apatite occurs in very small prisms and in moderate 

 amount ; is very often in the mica. 



The groundmass, in which these minerals lie, is clear and col- 

 orless, dotted with small anhedral grains of pyroxene and 

 minute octahedra of iron ore — a little of the iron ore is pres- 

 ent in larger grains — , the pyroxenes and iron ore being a 

 second generation. While the amount of pyroxene is consid- 

 erable, it is by no nieans so preponderant an ingredient as is 

 commonly the case in the nephelite basalts of Germany, 

 where, especially in the finer grained forms, the anhedral 

 grains of augite are so thickly crowded together in the ground- 

 mass. In ordinary light there appears in this rock a large 

 proportion of the colorless groundmass, and seen in this way 



