ON THE GEOLOGY OF WEST MORETON, QUEENSLAND. 153 



and calcite. This latter is undoubtedly an inclusion of 

 calcareous sandstone and the isolated grains of quartz 

 appear to have a similar origin. The whole rock is much 

 impregnated with calcite. 



It is probable that the coarse and fine portions represent 

 "schlieren" in a non-homogeneous magma* 



Foot of Mount Greville. — Porphyritic rock with an even- 

 granular base averaging about 2 mm. The base contains 

 quartz and orthoclase in about equal amounts together 

 with alkaline amphiboles in considerable abundance and 

 apparently of several varieties. Among these amphiboles 

 we have : — 



i. Ophitic patches of strongly pleochroic dark green to 

 brilliant indigo riebeckite. This encloses idiomorphic 

 quartz and felspar grains. 



ii. Irregular prisms with ragged ends and sides, pleochroic 

 in dark brown to bluish-green tints. It is most difficult to 

 obtain suitable sections for optical examination. The 

 extinction is nearly straight, but, in the larger grains, is 

 not very perfect owing to strong dispersion. The elonga- 

 tion is negative, but the mineral is too opaque to yield 

 figures in convergent light. It is probably arf vedsonite 

 and belongs certainly to an older generation than the 

 ophitic patches of riebeckite above described. 



iii. Prisms and patches of a somewhat pleochroic brown 

 to yellow mineral, possibly cossyrite. This sometimes 

 forms the centre of a thin prism, the outer zones of which 

 consist of the green-brown mineral (arfvedsonite ?). 



Phenocrysts of quartz (1 mm.) and of sligtly decomposed 

 orthoclase (2 mm.) are not very abundant. There is a good 

 deal of haematite staining throughout the rock. 



Summit of Mount Greville. — A very similar rock to the 

 last, but decidedly more trachytic in character, the felspars 



