590 THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



Probably the mineral is a variety of arfvedsonite. It com- 

 menced to crystallise after the larger felspars (felspar phenocrysts) 

 had formed, and continued to crystallise until shortly before the 

 last felspar of the base had consolidated. Hence we find felspar 

 phenocrysts often enveloped by soda-amphibole and dendritic 

 intergrowths of felspar and hornblende of the base. 



This protraction of the period of crystallisation of the arfved- 

 sonite was undoubtedly due to the action of mineralising vapours 

 containing FjCljZrOg and TiOg- 



^^girine-augite. — In acicular crystals this is a very abundant 

 constituent of man}^ of the trachytes, particularly of the d;irk 

 and phonolitic varieties. It is the sole ferromagnesian con- 

 stituent of some rocks, but is associated with soda-amphibole in 

 some, and with other pyroxenic minerals and olivine in the 

 trachydolerites. It varies from highly pleochroic eegirine to 

 almost nonpleochroic segirine-augite. The latter is the dominant 

 variety in the dark green trachytes and trachyandesites (W.l 

 Timor Ledges, W.l 17 Naman Ledges, and W. 1 LS Tooraweanah 

 Mountain, etc.). In these rocks it compensates with a selenite 

 plate in a direction across the length, a being considerably 

 removed from crystallographic c; the extinction angle is mostly 

 oblique at angles from 15°-30°; occasionally straight; the pleo- 

 chroism is weak, in colours from yellowish -green to grass-green 

 (with an occasional tinge of blue). The double refraction {y-a) 

 is about 0-030. 



True pegirine and acmite are not well represented, but in most 

 slides there are some crystals which can be referred to these 

 types. 



Felspathoids. — The felspathoid minerals identified in the 

 trachytes are nosean and pseudoleucite. The chemical analyses 

 prove that nepheline must also be widely distributed in the more 

 basic types, although it was not at first noticed under the micro- 

 scope. Staining tests confirm its presence. A more detailed 

 description of these minerals will be given in the petrological 

 descriptions of the slides in which the}^ occur. 



