246 IRVING. 



to the ground-mass. In the other types quartz appears in large 

 and numerous phenocrysts, and as this constitutes a difference 

 between these types and any rock heretofore described the 

 name quartz-aegirite -porphyry has been employed. These rocks 

 average about 72 %Si02. Six types are described. 



PJionolite family. 



The phonolite family includes an extended and varied series 

 of rocks rich in soda and forming the more basic phase of the 

 quartz-aegirite-porphyries. They are composed of orthoclase 

 (and probably some anorthoclase) microcline, segirine-augite, 

 aegirine, nepheline, nosean, with accessory hauyne, biotite, 

 magnetite, titanite and melanite garnet. The accessory mine- 

 rals do not occur in the same specimen but appear separately 

 in single types. 



The rocks of this family have been divided for convenience of 

 description into three groups. The tinguaites are those in which 

 an unusually marked interlacing of aegirine needles is present in 

 the ground-mass. Nepheline as a rule can be detected in them 

 only by gelatinization and never occurs as phenocrysts. They 

 contain little or no aegirine-augite and are prevailingly fine- 

 grained. The phonolites contain more or less abundant crystals 

 of nepheline easily identified by optical methods and are mostly 

 rich in nosean. The interlacing network of aegirine needles is 

 not pronounced. The trachytoid-phonolites show a great in- 

 crease in abundance of orthoclase, are comparatively coarse- 

 grained, show large crystals of aegirine-augite but little aegirine. 

 Nepheline is present only in small quantities and is then in the 

 ground-mass as isolated interstitial masses. They indicate a 

 transition toward the trachytes. 



Rliyolite family. 



The quartz-porphyries of this division are a series of rocks 

 with very varying texture. A fine-grained ground-mass of 

 quartz and feldspar with phenocrysts of orthoclase, plagioclase 

 and quartz are characteristic. In one type quartz was confined 

 to the ground-mass. The dark silicates are usually too de- 

 composed for identification, but when recognizable are horn- 



