622 THE GKOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



namely, the akerites and many monzonites, are of an entirely 

 different physical appearance and habit (compare "Akerite," 

 N.15, Nandewar Mountains, H. I. Jensen in litteris). The 

 minerals of the norm are in close agreement with those actually 

 observed in the mode, a fact due to orthoclase and albite of a 

 trachytic magma having mixed with augite, olivine, and basic 

 felspar of a partially fused doleritic magma whereupon the whole 

 mass has consolidated. 



W.40. Corundum Basalt is a rock which contains minerals 

 which by optical means alone could not be determined with 

 accuracy. These were — (1) A black, lustreless, usually opaque 

 mineral, which in very thin slices appeared occasionally to be 

 translucent and bluish. It was wholly isotropic. The analysis 

 shows that ir, cannot be spinel or magnetite, and that it must be 

 corundum and sapphire. This determination bears out the Rev. 

 J. M. Curran's theory that our sapphires are derived from basalt. 

 (2) Yellow needles terminated by pyramids. This mineral w^as 

 considered to be wbhlerite or laavenite. The high TiO._, per- 

 centage and the presence of ZrOo make the occurrence of 

 laavenite highly probable, and the titanic acid is so high that 

 the allied minerals rosenbuschite and rinkite are probably also 

 present. 



The norm differs very considerably from the mode. The 

 ground-mass is very readily gelatinised with dilute acid, indicat- 

 ing the presence of a felspathoid which probably contains most 

 of the K2O. 



In the trachy-andesites, W.l and W.22, we again notice that 

 the mode is very different from the norm. This is, of course, 

 because the hypersthene and diopside molecules are incorporated 

 in the segirine-augite. In W.22 the TiOo percentage is higher 

 than in W.l, a fact which verities the determination of pseudo- 

 brookite in the rocks of Nandi Mountain and The Forked 

 Mountain. The norm of the trachyte-andesites calculates to 

 monzonose. Rosenbusch (in ' Gesteinlehre') describes such rocks 

 under the name keratophyre. This designation is, however,. 



