138 VOLCANIC AREA OF EAST MORETON, ETC., DISTRICTS, Q., 



near Wardrop's selection, north-east of Nindherry. The Mountain 

 forms the northern end of the Toolburra Range. 



Under the microscope this rock is seen to be of variable 

 texture; one band is microfelsitic; others are porphyritic, having 

 phenocrysts of orthoclase and of corroded elongated quartz 

 crystals lying in a cryptocrystalline base. The quartzes include 

 stunted laths of a basic felspar in a micropcecilitic manner. The 

 base is chiefly felspar. Ferromagnesian minerals are now only 

 represented by hematite and limonite. Beautiful cubes of mag- 

 netite were also present in the original rock, now represented by 

 hematite pseudomorphs. 



Name : Rhyolite. 



The foregoing rocks present certain features in common. The 

 handspecimens have considerable porosity, a fact which indicates 

 that the lava was very hydrous when erupted. Under the micro- 

 scope we recognize two generations of felspar and quartz, which 

 crystallised in the order mentioned. A re-heating of the magma 

 after the phenocrysts had formed, led to the refusion of the 

 quartz and its extreme corrosion. The re-melted viscous quartz 

 was drawn out into long streaks and, when sufficiently liquid, 

 small crystals of felspar were often enclosed in the quartz in a 

 micropcecilitic manner. These changes took place at the time of 

 eruption and flow. 



The scarcity of ferromagnesian minerals in these rocks is 

 suggestive of quartz-keratophyres; owing to the minuteness of 

 the crystallites composing the base, and the decomposition which 

 has set in, it is not possible to decide whether they are true 

 cryptocrystalline rhyolites or quartz-keratophyres. 



From the state in which we find the quartz phenocrysts we can 

 conclude that the lava was erupted at a temperature of between 

 8 0° and 1200°. 



Sp. No.15. Loc: Railway cutting at 66| m. between Yandina and 

 Nambour (Text fig. 11 a, pt. i.). 



Weathered handspecimen not unlike trachyte; on fractured 

 fresh surfaces it has, however, a dull vitreous appearance, resem- 

 bling chert or fine-grained felsitic tuff. It occurs in highly 



