300 Weed and Pirsson — Bearpaw Mountains, Montana. 



erally much altered, in which large crystals of white feldspar 

 occur in a reddish-yellow ground-mass. Good exposures of 

 the intrusive mass are rare even where the slopes are deeply 

 cut by drainage channels, for the prevalent covering of grass, 

 everywhere a feature of the mountains, often conceals even 

 the debris. The rock is a trachyte or syenite- porphyry which 

 is too greatly altered and decomposed to be of value for petro- 

 graphic study. It consists of a brownish, earthy, feldspathic, 

 ground-mass filled with limonite and with hollow cavities 

 caused by the weathering and decay of a former iron-bearing 

 mineral. This is thickly filled with feldspar phenocrysts of a 

 thick tabular or short columnar habit about l cm in greatest 

 length. Its determination as a rock of the alkali class rests 

 upon the character of its contact form. 



The contact form of the trachyte is a dark greenish-gray 

 rock thickly crowded with small idiomorphic feldspar pheno- 

 crysts of thick tabular form. There is a sprinkling of rather 

 dull, inconspicuous black dots which are either pyroxene or 

 micas. 



In the section the great quantity of feldspar phenocrysts is 

 very evident. The great majority are clear and homogeneous, 

 while some show microperthite intergrowths with albite lamel- 

 lae. In a section parallel to £(010) shown by its outline, high 

 birefraction and the exit of a positive obtuse bisectrix, the 

 angle /? was measured 65° and the extinction was'S 30' plus 

 in the obtuse angle /?. A section perpendicular to the acute 

 bisectrix a was examined in convergent light, but the hyper- 

 bolas were too* vague and shadowy for accurate measurement 

 — it was estimated that 2E was over 60° and under 75°. All 

 these facts show clearly that the feldspar is an alkali one, not 

 of the lime-soda group. No albite twinning occurs, but Carls- 

 bad and more rarely Baveno twins were seen. The measure- 

 ments would point to the orthoclase nature of the feldspar, 

 but it unquestionably contains the albite molecule to some 

 extent as shown by the extinction angle on 5(010). 



The dark-colored minerals are occasional small tablets of a 

 greenish lepidomelane-like mica and prisms of a colorless 

 augite of a wide extinction-angle. This latter is partly changed 

 in some places to serpentine and in others into a fibrous green 

 mineral that is believed to be hornblende. Occasional apatite 

 and iron ore occur. 



These lie in an excessively fine-grained ground-mass that 

 appears to be composed of fine feldspar granules. It is so fine 

 as to be almost cryptocrystalline, and even with very high 

 powers little can "be learned save that it appears noncrystalline, 

 of untwinned feldspar, and is filled with fine ore grains and 

 microlites of an extremely pale-green mineral occurring in 

 slender needles, and which is thought to be pyroxene. This 



