266 IRVING. 



thin mantle of jegirine, but have been invaded by the latter 

 along lines of fracture. 



The pleochroism varies with the amount of the aegirine mole- 

 cule present. In the larger and more perfect crystals, as in the 

 fine tinguaite from Englewood, it is light pea-green parallel to 

 c, also parallel to b ; yellowish green, parallel to a. Absorption 

 a= h > c. 



Inclusions occur in great numbers in the aegirine-augites but 

 with the exception of biotite and titanite, which are almost in- 

 variably found in the centers of the crystals, they are confined 

 to the later-added border of aegirine. The shredded crystals 

 rarely contain inclusions. 



Pyroxene of the Second Variety. — From this more conspic- 

 uous variety of aegirine we may pass to that of a later genera- 

 tion, by very gradual degrees. This is invariably parallel in 

 extinction and varies considerably in form. It has most often 

 shredded extremities, but in many cases terminations can be 

 seen. These are prevailingly oP (ooi). It is more strongly 

 developed in the tinguaites where it forms a reticulated, inter- 

 locked maze. As we pass toward the trachytoid varieties 

 of phonolite, as in the rock from Raum's Drill, these smaller 

 shredded segirines give place to a much larger development of 

 segirine-augite, which is sometimes arranged in a seeming flow- 

 structure around the phenocrysts. The smaller aegirines pen- 

 etrate the groundmass in all directions, and are often included 

 in the feldspar, and are arranged without orientation of any 

 kind. When nepheline is present in automorphic crystals, it is 

 formed earlier than these aegirines, for it occurs embedded in 

 them or extends far into them from the border. In the rock 

 described by Caswell from Black Butte, the aegirine forms inter- 

 stitial masses like the augite in a diabase, the role of the feld- 

 spars being played by the nepheline. This is true also to a less 

 extent in the phonolite from the Badger Shaft, and in that from 

 the railroad cut at Maurice. 



The pleochroism of the aegirine is very strong, and can be 

 :best observed when the crystals are large. It is blue-green to 

 ■emerald green. 



