600 THE GEOLOGY OF THE WARKUMBUNGLE xMOUNTAINS, 



in W.140) forms a connecting link between the light arfvedsonite 

 trachytes like W.116, and the dark fegirine-tracliytes (and phono- 

 litic trachy-andesites) like W.l, W.22, etc. En both physical 

 appearance and volcanic succession they are intermediate between 

 these two groups. 



W.l 24. Loc: Needle Mountain. 



1. Handspecimen of a greenish colour and shining lustre. It 

 is porphyritic in shining tabular felspars. 



2. Texture holocrystalline, uneven-grained, with phaneric 

 felspar phenocrysts, and trachytic fabric of the base. 



3. Composition : the constituents, in order of decreasing abun- 

 dance, are felspar, legirine-augite, ferrite, magnetite, and blue 

 amphibole. Only felspar and aegirine-augite are represented in 

 notable amount. The felspar phenocrysts are tabular, and show 

 Carlsbad twinning; from their cleavage angle, low refractive 

 index (less than canada balsam), and extinction angles they are 

 seen to be composed of albite. They include fegirine grains. 

 The felspar needles composing the base are probably also albite, 

 but may be partly sanidine or anorthoclase. The tegirine-augite 

 occurs in idiomorphic twinned phenocrysts (twinning plane 

 parallel to a (100), with moderate pleochroism). The extinction 

 angles on the cleavage in sections showing but one cleavage are 

 moderately high. It is therefore true tegirine-augite, and occurs 

 abundantly in minute idiomorphic grains. Magnetite and soda- 

 amphibole occur only in very minute proportions. 



4. Order of consolidation : 



1. Magnetite 



2. ^girine 



3. Albite (phenocrysts) 



4. Felspar of base 



5. Name : Trachytic ^girine Soda-Trachyte. 



Note : this rock is interesting as showing that an absence of 

 mineralising vapours in the soda-trachyte magma leads to the 

 crystallising out of the ferric constituents as magnetite, segirine 

 and ferrite (from the excess of Fe^Og), and these minerals under 

 ^uch conditions commence to form before the felspar. 



