294 Weed and Pirsson — Bearjpaw Mountains, Montana. 



method shows that they have a higher index of refraction 

 than the anorthoclase in which they lie. They are, therefore, 

 oligoclase. Others much more rarely extinguish at angles 

 which reach a maximum of 16° (in sections perpendicular to 

 010) ; one of these that gave this maximum showed the exit 

 of a negative bisectrix, and the plane of the optic axes stood 

 at 74° to the trace of 010 ; the section showed both carlsbad 

 and albite twinning, and both sets extinguish and illuminate 

 nearly simultaneously. This is consequently albite, and Becke's 

 method shows its refraction to be higher than the anorthoclase. 

 Thus these microlites vary in composition from albite to oligo- 

 clase, the majority being the latter. A closely related occur- 

 rence with albite microlites is described by Brogger.* It is 

 thought, in the present case, that the phenocrysts having 

 originally formed at considerable depth were afterwards mostly 

 resorbed, as some of them plainly show by their rounded and 

 corroded forms. An example is to be seen in fig. 3. In the 

 later process of crystallization these were restored and a simul- 

 taneous crystallization of the oligoclase took place, the two 

 arranging themselves as seen in the figure. 



Fro 3.— Syenite Porphyry of Gray Butte, x 14, actual field 4 mm. 



The ground-mass consists of very small allotriomorphic 

 grains of feldspar with occasional ones of pyroxene. The small 

 feldspar grains are pretty turbid from kaolinization ; they do 

 not show any twinning, out with very high powers between 

 crossed nicois they show a mottled, zoned, or flamed appear- 

 ance in tones of gray and white. Becke's process shows that 

 their refraction is always less than that of quartz, and hence 



* Grorudit-Tinguait Serie, p. 11, 1894. 



