146 Weed and Pirsson — Bearijaw Mountains^ Montana. 



periphery it would furnish a section like g ; these are very 

 rare; more often it is represented by ^, which shows it in a 

 ruder, less perfect stage. The section i shows an anomalous 

 form observed several times and in ^ is seen a cut half way 

 between the center and edge of a crystal with slender glass 

 rods as in f. The location of each trapezohedral face is 

 marked by the cross section of its inclusion. These also occur 

 in various degrees of perfection. 



Studied with very high powers, it is seen that the inclusions 

 in these leucites have not remained a homogeneous glass, but 

 have partly crystallized and contain globuKtic material and 

 granules of iron ore. 



The base which surrounds the leucites and cements them 

 together appears, with low powers, a dark brownish glass : with 

 high powers, it is a confused mixture of ore grains, glol3ulitic 

 material, and minute indeterminable microlites that act faintly 

 on polarized light, all cemented together with glass. Proba- 

 bly the microlites are in part a pyroxene. 



The structure and proportion of the minerals to each other 

 is shown in fig. 2, which presents the rock in a somewhat 

 diagrammatic way. 



Fig. 2, — Leucitite of Bearpaw Peak. Augite, iron ore, 

 leucite. Actual field 1 mm. x 50. 



The analysis of this interesting lava is given in ]^o. 1 of 

 the following table. It was made for us by Dr. H. N. 

 Stokes of the chemical laboratory of the U. S. Geological 

 Survey. 



