206 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND, 



At Montville there have been andesitic and basaltic 

 outpourings subsequent to the effusion of the rhyolite, but while 

 one is inclined to assign the cause of much of the alteration 

 and sihcification of the rhyolite to heated waters and vapours, 

 there does not seem to be any need, especially in this case 

 where the magnesia content is not increased, to invoke the 

 succeeding basic flows as the source. 



It is believed (a) that heated waters containing silica in 

 solution and also carbonic acid gas and other constituents 

 have been responsible for the alteration and " weathering " of 

 the rhyolite, (6) that this has gone on in the last stages of the 

 cooling down of the lavas, and (c) that the source of these 

 agents is magmatic and not atmospheric. 



Dr. Harkerii has stated that there is every reason to 

 believe that in the tertiary basalts of the British area many 

 minerals, such as various zeolites with chloritic minerals, etc., 

 have been formed from the partial decomposition of minerals 

 in basalt as a result of water and gases of magmatic origin, 

 while J. J. Sederholmi^ has given the term " deuteric " to those 

 reactions of the very closing stages of crystallisation and 

 dependent on the mineralisers or " juice." 



Deuteric action of this type is therefore regarded as the 

 cause of the peculiar chemical and mineralogical composition 

 of the Montville rhyolite. 



COMPARISON OF THE CHEMICAL CHARACTERS OF 

 THE RHYOLITE WITH THOSE OF THE BROKEN 

 HILL " QUARTZITES." 



In view of the difference of opinioni^ as to the origin of 

 the Broken Hill " quartzites," the chemical analysis of this 

 rhyolite has much interest. These rocks have been studied at 

 much length by Mr. E. C. Andrews, Dr. W. R. Browne, and 

 Dr. F. L. Stillwell. The two latter, who are highly skilled 

 petrologists, have made special petrological studies of these 

 metamorphic rocks in the field and laboratory, and while 

 Dr. Stillwell advocates a sedimentary origin, Dr. Browne 

 prefers an igneous origin. 



11 Nat. Hist, of Igneous Rocks, p. 308. 



12 J. F. Kemp, Bull. Geol. Soc. Amer., Vol. 33, 1922, p. 237. 



" Geology of the Broken Hill District. Mem. GeoL Surv. if.S.W 

 Geology No. 8, 1922, p. 107. 



