606 THE GEOLOGY OF THE WARKUMBUNGLE MOUNTAINS, 



W.30. Log.: The Forked Mountain. 



In this rock the £egirine-augite phenocrysts are still more 

 plentiful, and haematite less so than in W.32. 

 Name : ^girine-Olivine Trachy-Andesite. 



The Nandi, Forked Mountain, and other rocks from the buttes 

 around Coonabarabran must be ranked as trachyandesites on 

 account of their peculiar mineralogical composition. They are 

 intermediate between the trachyandesites (and phonolitic 

 trachytes) of Timor Ledges, Naman Ledges, etc., and the sodalite 

 or analcite basalts of Tonduron (The Spire) and Wombalong 

 (Mount Exmouth). They are apparently the volcanic equiva- 

 lents of an essexite magma. Variations in the relative propor- 

 tions of gegirine, haematite, olivine, and pseudobrookite (?) seem 

 to have been controlled essentially by variations in the titanium 

 percentage in different portions {cj?. Analyses W.l, W. 2 2; and 

 petrological descriptions W.l 17, W.9, W.22, W.32, W.30). 



Minerals of Trachyaridesites, Trachydolerites and Sodalite Basalts. 

 The minerals contained in common by these rocks are : — 



(a) Plagioclase Felspar. This mineral occurs in phenocrysts 

 which show Carlsbad, Albite, and Pericline twinning. Its 

 extinction angle in symmetrical sections varies from 10° to 25°. 

 Presumably the varieties albite, oligoclase, andesite, and labra- 

 dorite are all present. In the ground-mass the felspar has the 

 form of needles, and fine laths whose refractive index is lower 

 than that of Canada balsam and whose extinction angles are very 

 low. It is probably albite or anorthoclase. 



(b) Orthoclase Felspar occurs both as fragmentary phenocrysts 

 and as fine laths (sanidine) in the base. It is probably soda- 

 bearing, and often graduates into albite or anorthoclase. 



(c) The olivine is a clear colourless variety which occurs as 

 highly corroded phenocrysts with serpentinous cracks. It is the 

 chief mineral found included in the felspar phenocrysts. 



(d) Several varieties of augite occur. The chief is a light 

 brownish or copper-coloured, titaniferous, slightly pleochroic 

 diallage. This kind is of two generations, the first occasionally 



