Weed and Pirsson — Bearpaw Mountains of Montana. 191 



the acute bisectrix and is c ; the angle of the optic axes is 

 rather small, 2E = 75° — 80° as nearly as could be measured 

 with a micrometer ocular with movable thread ; in this case 

 the plane of the optic axes was 15° from one of the prismatic 

 cleavages ; the bisectrix was not exactly centered. What the 

 exact nature of these pyroxene-like cores is, must be largely 

 a matter of conjecture ; the analysis and the calculated min- 

 eral composition derived from it seem to indicate clearly that 

 they are a diopside-like product. 



The large phenocrysts of feldspar are orthoclase or rather 

 sanidine in some cases. They have an excellent cleavage par- 

 allel to (?(001), a less perfect one parallel to Z>(010). The angle 

 2E was measured about 80° with a micrometer ocular ; the 

 measurement is approximate, as the bisectrix was not perfectly 

 centered. The extinction on c(OOl) appears rigidly parallel to 

 the trace of ^(010); on 5(010) it is 7°. These were deter- 

 mined on cleavage fragments. In thin section it appears per- 

 fectly fresh, homogeneous and uniform. It contains quanti- 

 ties of segirite microlites as inclusions, and it is noticeable 

 that numbers of the feldspars have an interior wreath or shell 

 of these segirites, as if marking a definite period of renewed 

 growth. 



Another peculiarity is that the segirite prisms of all sizes 

 lying outside of these feldspars are all arranged parallel to the 

 sides of the feldspar and form a sort of coating or mantle 

 around them. They appear exactly as if, having been already 

 present, the growing and expanding feldspars had pushed 

 them along, exactly as a heap of scattered straws lying on a 

 table would be arranged in parallel position if swept to one 

 side by a book. That this phenomenon is not due to fluidal 

 movements of the molten rock is clearly shown by the fact 

 that the segirite prisms surround all the faces of the feldspar 

 in this way, and by the further fact that the flattened tablets 

 of feldspar themselves lie unoriented, scattered in all positions 

 in the rock. It thus really appears that these large pheno- 

 crysts are of later origin than the greater number of the 

 augites, though the latter are so much smaller, thus agreeing 

 with the megascopical characters previously mentioned. 



The groundmass is composed of the same minerals, with the 

 addition of formless grains of fresh nephelite and an occasional 

 one of sodalite and cancrinite. There is, indeed, no sharp line 

 of division, and the size of the components grades from the 

 largest to the smallest ; the aegirite eventually sinks to 

 extremely fine slender microlites ; it occurs only in the form 

 of slender prisms. In some cases the aegirites of largest size 

 appear to be altered to a finely fibrous aggregate of a deep 

 green color ; whether this is still segirite or a fine hornblende 

 cannot be certainly told. 



