260 IRVING. 



Trachytoid Phonolites. 



These have prevalent orthoclase and little or no nepheline 

 that can be identified without resort to gelatinization, and few of 

 the fine crystals of segirine that characterize the tinguaites and 

 many of the phonolites proper. Much anorthoclase is probably 

 present. The phenocrysts are large and the groundmass coarser 

 than in the first two varieties. A trachytic flow structure of the 

 feldspar of the groundmass is often observable. The rocks de- 

 scribed from the Judith mountains by Weed and Pierson, as 

 aegirite-syenite -porphyries are probably analogous to the coarser 

 members of this series. 



All three of these divisions are chemically similar, as will ap- 

 pear from the analyses on a later page, and the division is made 

 more for convenience in petrographic description than for any 

 other reason. For ordinary purposes the name phonolite is 

 amply sufficient to cover the entire series. 



Petrographic Description of Phonolite Family. 



Megascopic Appearances. — The rocks of the phonolite family 

 range in color from deep almost brilliant green to dark olive, 

 grayish-green, dark bluish-gray, dove colored, light gray and 

 almost white. The texture of the tinguaitic varieties is in some 

 cases almost aphanitic as is notably that of the brilliant green 

 variety from the hill to the southwest of Englewood. From 

 this it becomes more porphyritic with both sanidines and aegi- 

 rine minerals as phenocrysts. In relation to the groundmass the 

 phenocrysts vary greatly in size and abundance, being now 

 large and closely crowded, and again of small size and sparsely 

 scattered through the rock. The phonolite described by Cas- 

 well from Black Butte, of which a specimen was collected 

 and studied shows a dark brown color, with dark mottlings of 

 a deep blue-green, giving it a singular poikilitic appearance to- 

 tally different from that of any other phonolite in the region. 

 It has, moreover, in an unusually marked degree the greasy 

 lustre so frequently remarked in nepheline rocks, but which 

 is completely lacking in many, if not most, of the phonolites 

 collected by the writer, and which disappears altogether as we 



