356 Weed and Pirsson — Bearpaw Mountains, Montana. 



andesine to oligoclase. Sometimes the crystals are not zonal 

 and these may be of labradorite. Thus a section in the zone 

 001 on 100 perpendicular to 010 gives for the albite twins in 

 one carlsbad half 35°, 35°, in the other half 17° and 19°, then 

 S = 17°, and the feldspar is a labradorite between AfyAUj and 

 Ab 3 An 4 . The index of refraction is also higher than the 

 orthoclase, in which the laths lie. Its period of formation laps 

 that of the biotite. 



The orthoclase, which was the last mineral to crystallize, fills 

 the interspaces and encloses the other components in a poiki- 

 litic manner over considerable areas. Its determination as 

 orthoclase rests on the fact that in numerous sections perpendi- 

 cular to an obtuse positive bisectrix the angle of extinction 

 varied from 5 to 7 degrees from the trace of the cleavage 001 

 in the obtuse angle /2, the section being oriented by this cleav- 

 age and by inclusions which give the direction of the vertical 

 axis as well as a parting which appears to be parallel to the 

 prismatic faces. Thus in such a section the angle was measured 

 63 and /9 = 63° 54'. The extinction measured with the Ber- 

 trand ocular is 7° 30' from the trace of the good cleavage 001 

 in the obtuse angle. The albite molecule is surely present 

 in it to a certain extent, and some examples contain patches 

 of interlaminated albite. 



All of the minerals are very fresh, and the structure is hyp- 

 idiomorphic, typical of granular abyssal rocks. The differences 

 in the mineral composition of this rock and the syenite just 

 described are of the same kind as those noted between the two 

 types described from Yogo Peak, but the contrast is even 

 greater, as will be apparent from the chemical analysis. 



An analysis of this type, made in the laboratory of the U. S. 

 Geological Survey by Dr. H. N. Stokes, is given in the first 

 column of the table on the next page. 



Not knowing the exact proportions in which the elements 

 enter into the various minerals present, it is not possible to 

 calculate their relative percentages, but it is clear that the 

 albite molecule must be largely present in the alkali feldspars. 



It will be seen that the agreement between this type and the 

 one from Yogo Peak is very satisfactory, as shown by the small 

 differences of only about 1 per cent in Si0 2 , A1 2 3 , and MgO. 

 The mineral composition and structure are also the same. 



Since our paper on Yogo Peak was published the first half of the 

 new edition of Kosenbusch's "Mikroskopische Physiographic" 

 has come into our hands, and almost simultaneously a notable 

 work by Brogger chiefly on the rocks of Predazzo in Tyrol. 

 The latter shows that the granular rocks there exposed form a 

 transition group between the alkali feldspar and plagioclase 

 groups with all grades from those rich in feldspar to those free 



