624 



THE GEOLOGY OF THE WAKRUMBUNGLE MOUNTAINS, 



The phouolites analysed, \V.222 and W.l 40, are very high in 

 alkali, as was expected from the amount of their felspathoid 

 minerals, which form about 10 % of the mass. The determina- 

 tion of pseudoleucite appears from the chemical results to be 

 correct, inasmuch as there is insufficient SO3 to give so much 

 nosean. In both of these rocks and also in the trachytes (W.l 6, 

 W..38) the norm differs from the mode in that all the diopside 

 and hypersthene molecules have gone to form segirine augite and 

 arfvedsonite. As these minerals are richer in silica than augite 

 there has been insufficient silica left for the formation of felspar, 

 hence felspathoids have formed. The trachytes and phonolites 

 analysed fall in the divisions phlegrose and nordniarkose o<" the 

 chemical classification, usually near the border line between the 

 two subrangs. 



The analysis of trachyte from Wantialable Creek (D, by J. C. 

 H, Mingaye*) is interesting as being of very similar composition 

 to the orthophj'-ric pantellarite from Ngun-Ngun, Glass House 

 Mountains, Queensland. The Mount Flinders trachyte contains 

 barkevicite instead of arfvedsonite and is slightly more calcic 

 than the Warrumbungle trachytes, and as a result falls in a 

 different subrang. The Canoblas trachyte is much more like the 

 orthophyric comendites of the Glass House Mountains than any 

 of the Warrumbungle rocks. 



* This rock has since been found to be a silicified trachyte tuff. 



