UNUSUAL RHYOLITE FROM THE BLACKALL RANGE, 203 



If one compares the Montville analysis ^vith either the 

 average rhyolite analysis given by R. A. Daly or with the 

 several analyses of Southern Queensland rhyolites,^ of which 

 the one from Springbrook Plateau is typical, it will be seen 

 that the variation lies in the silica, alumina, and alkalies. 



There is undoubted evidence of " secondary "' silica in 

 the Montville rhyolite, and even if the silica percentage were 

 increased 10 per cent, by its introduction one finds difficulty 

 to account for the very low alumina. What removed the 

 alumina ? In what form was it removed ? 



The alkalies are much lower than in the ordinary rhj^olites 

 •of Southern Queensland, particularly in respect to the potash. 

 Delesse^*^ insisted " strongly on the connexion between an 

 excessive proportion of silica and the development of 'globular ' 

 or coarse spheruhtic structure," and it is interesting to note 

 that much of the Montville rhyolite is composed almost entirely 

 of spherulites and many of them are decidedh' coarse. 



The norm of the rhyolite shows 27 '30 per cent, of felspar, 

 of which approximately two-thirds is albite and one-thir^ 

 orthoclase, while there is very httle anorthite and no corundum. 

 The norm of the spheruhte from Quail Is., Lyttleton Harbour, 

 lias 17 04 per cent, only of felspar, of which anorthite is the 

 most abundant, but in addition the norm shows 6' 12 per cent, 

 of corundum. 



The paucity of the alkalies in the latter rock accounts for 

 the difference in the felspar content and the need for forming 

 corundum in the norm, so that one might assume that the 

 Montville rhyolite has not been so much affected by loss of 

 alkahes. 



The Montville rhyoUte is almost certainly Middle Cainozoic 

 in age and is certainly post-Triassic, so that, although of the 

 pjTomeride type, it cannot be regarded as an ancient rhyolite. 

 It shows devitrification to a certain extent, its felspars 

 especially in many of the spherulites and lithophysae have been 

 kaolinised, and " secondary " silica occurs abundantly in 

 much of the rhyolite. 



Mr. Speight has regarded hot water charged slightly with 

 magnesian salts as the agent causing the alteration in the 

 spherulite from Quail Island, and assigns the origin of the 

 water to the basaltic lavas poured out subsequently. 



9 H. C. Richards, Proc. Roy. Soc. Qld. xxvii., 1916, p. 142. 

 ^^ Memoires de la Soc. Geol de France. 2 me serie, tome iv., pp. 325., 

 etc. Q.J.G.S. xlii., p. 188. 



