TTNUSITAL RHYOLITE FROM THE BLACKALL RANGE. 195 



An Unusual Rhyolite from the Blackall 

 Range, South-Eastern Queensland. 



By H. C. Richards, D.8c., Professor of Geology and Mineralogy, 

 University of Queenslanrl 



(Plates v.- VI.) 



{Read before The Royal Society of Queensland, 30th October, 1922.) 



INTRODUCTION. 



In a recent communication to this Society the author 

 described an anorthoclase basalt — the first of its kind in 

 Queensland — from Mapleton,i at the northern end of the 

 Blackall Range in South-Eastern Queensland. The rock con- 

 sidered in this paper is from the same area, but is of a much 

 rarer type and has many peculiarities, both structurally and 

 chemically. It is a rhyolite with an extremely rich silica 

 content and low alumina value ; it occurs near MontvDle, about 

 the centre of the Blackall Range, and outcrops over an area 

 of several square miles. 



On account of its extreme values for silica and alumina 

 and its interesting spherulitic and lithophysal characters it has 

 a considerable intrinsic interest. Two recent scientific publica- 

 tions, however, give it a much wider interest. 



Mr. R. Speight, M.A., in describing the rhyolites of Banks 

 Peninsula, 2 in New Zealand, shows by analyses the character 

 of the spherulites in comparison with the rhyolite, and accounts 

 for the higher silica and magnesia and lower alumina of the 

 former by the action of heated waters charged slightly with 

 magnesian salts derived from basaltic lavas subsequently 

 effused. 



Spherulitic rhyolite, even more extreme chemically than the 

 New Zealand one, is considered here, and the writer accounts 

 for the pecularities by the action of heated waters and vapours 

 accompanying the rhyolitic effusions themselves, so that the 

 alteration and modification are not the result of ordinary 

 atmospheric weathering, but rather due to agents of a magmatic 

 origin coeval with the effusion of the rhyolites — the deuteric 

 reactions of Sederholm.^ 



1 Proc. Roy. Soc. Qld. xxxiv., 1922. 



- Rec. Cant. Mus. N.Z., Vol. ii., 1922, pp. 77-89. 



3 Bull. Geol. Soc. Amer., Vol. 33, 1922, p. 237. 



