264 IRVING. 



acid plagioclase. The latter attains a maximum development 

 in the more trachytoid varieties. 



Interstitial masses of what seems to be nepheline are present 

 in greater or less abundance, as also are recognizable crystals 

 of this mineral, but they do not occur in as great abundance in 

 the trachytic, as in the more granular groundmass. 



Penetrating these feldspars in every direction are minute 

 needles of aegirine which are present in great abundance in the 

 tinguaites, but show a decreasing development as we approach 

 the trachytes. 



The other variety of feldspathic groundmass differs only in 

 the degree of perfection of the constituent feldspars. It is best 

 illustrated in the biotite phonolite from below Maurice (sepa- 

 rately described on page 269) in which the feldspars do not shov/ 

 so strong a tendency toward automorphism as in many other 

 of the rocks. In the tinguaites both the trachytic and hypidio- 

 morphic types of groundmass occur. 



Microcline . — Microcline is present sometimes as the core of 

 the sanidine crystals previously mentioned, and again as distinct 

 phenocrysts. The phonolite from the bottom of the Badger 

 Shaft shows beautiful phenocrysts of microcline. 



^gerine-augite. — Soda-pyroxene is to be considered typical 

 of these rocks, above any other mineral. It occurs in the most 

 basic phonolites in great abundance, is a persistent feature up 

 to the very most acidic types, and beyond these is still found in 

 the very acidic quartz-aegirite-porphyries. 



The pyroxene occurs in three distinctly separable varieties. 



First: Large automorphic crystals of aegirine-augite ; Second: 

 Smaller elongated needles of aegirine which have sometimes 

 shredded extremities and sometimes abrupt crystal terminations 

 and which are subsequent to the larger crystals in age of 

 formation. They often occur as outer rims in parallel or- 

 ientation with the latter. Third: Extremely fine needles of 

 aegirine, long and sharp, but so very small in cross section that 

 it is difficult to detect any pleochroism. 



These have been briefly described under the quartz-aegirite- 

 porphyries, but the more complete description is inserted here. 



