Weed and Pirsson — Bearjpaw Mountains, Montana. 353 



In thin section under the microscope the following minerals 

 are found : alkali feldspar, augite, iron ore, quartz, a very 

 little biotite, hornblende, and tilanite. 



The augite is a clear green diopside. It occurs in small 

 anhedral grains and there is very little of it. The iron ore is 

 about equal in amount; of the biotite only a few shreds have 

 been seen. Titanite is rare. 



The feldspar forming the main constituent is entirely of the 

 alkaline series ; no plagioclase feldspars have been found. In 

 the main it has a rude, thick, tabular habit on 5(010), giving 

 square or rectangular cross sections whose interstices are filled 

 by smaller grains and quartz. It shows throughout a pro- 

 nounced and beautiful microperthite structure, more developed 

 on the boundaries and thus fringing the sections with palisade- 

 like edges. The remaining feldspar consists also of varying 

 mixtures of the albite and orthoclase molecules, giving spotted, 

 cloudy effects in polarized light. At times the interlaminated 

 lamellae are so fine as to be seen only with high powers. The 

 feldspars in fact present most perfectly the varying effects so 

 well described by Brdgger and by Rosenbusch in their articles 

 on the alkali feldspars. The numerous contacts with quartz 

 show that they have always, in all positions, a lesser index of 

 refraction than the quartz. In a section parallel to 5(010) an 

 obtuse positive bisectrix emerges, and in some areas measured 

 from the trace of the good cleavage c(001) the extinction-angle 

 is 7° plus; these are of orthoclase. In other areas it rises to 

 19° which are albite ; these are marked by a higher birefrac- 

 tion. Others give varying angles between them and are mix- 

 tures ; on this face the albite interlaminations follow the direc- 

 tion of a very steep dome. In sections perpendicular to 5(010) 

 the albite patches show often the albite twinning whose maxi- 

 mum extinction-angle was measured at 15° on either side of 

 the twinning line. We believe that these feldspars were once 

 homogeneous anorthoclase and have split up into these variable 

 masses through secondary processes. The average composition 

 is given after the analysis. 



The quartz fills small spaces left between the other com- 

 ponents. It frequently contains vast quantities of slender 

 hair-like interpositions which are indeterminable, but are 

 believed to be of rutile like those found in certain granites. 



The analysis of the rock by Dr. H. N. Stokes yielded the 

 results shown on the following page. 



