BY H. C. RICHARDS. 145 



The rhyolite from Ottaba. which is relatively low in 

 alkali contents, has an excess of soda over potash, also the 

 lime content is considerably higher than in any of the 

 other rhyolitic rocks. 



(6) Alkaline Rhyolites. — These are all very acid 

 rocks, and they differ from the sub-alkaline types in having 

 a slightly lower alumina, higher total iron-oxides, much 

 lower lime, and much higher total alkalies. The rocks 

 are all sodipotassic with potash in excess of the soda. It 

 is also noticed that normative acmite exists in two of the 

 rocks. 



The average of 6 comendites given in Column IV. 

 agrees remarkably well with the Mount Conowrin rock. 



Acid Pyroclastic Rocks. 

 These occur as breccias, agglomerates and tuffs, and 

 as previously indicated have an extensive development in 

 the southern portion of the area. Quite apart from the 

 main development which is probably of iliddle Cainozoic 

 age, there are the Brisbane tuffs. These were first deter- 

 mined as made up of volcanic ash by W. H. Rands^* in 

 1887. He gave a good account of their occurrence and also 

 showed them to be at the base of the Ipswich formation, 

 so that they are of early Trias-Jura age. The Brisbane 

 tuff is really a rhyolite-tuff and it consists of very fine- 

 grained felspathic matrix with blebs of quartz, orthoelase 

 and plagioclase. It has been very much silicified in most 

 places, and it has undergone a great deal of alteration. 



Under crossed nicols the base seems to be made up 

 of very fine-grained particles of felspar and quartz, but 

 in ordinary light the original shapes of the particles which 

 were originally glass but are now quite altered and 

 crystallised are seen, and show the characteristic drawn- 

 out shapes with frequent concave faces. Inclusions are 

 numerous, and one finds fragments of the underlying 

 schist and also carbonised wood. In certain places, but 

 particularly on the northern side of the Brisbane River, 



" Q 'land Geol. Surv., Pub. 34, 1887. 



