166 VOLCANIC ARKA OF -EAST MORETON, ETC., DISTRICTS, Q., 



Coolum trachyte has flowed over and enveloped a mass of this 

 granophyric porphyrite is good evidence of relative age. 

 The following is the succession obtained : — 



(a) Porphyrite, tonalite (quartz-diorite), and monzonite 

 intruded at Pt. Arkwright, Noosa Head, and Cooroy. Simul- 

 taneously there were eruptions of closely related soda-andesites 

 and quartz-andesites in the Maroochy District : (Cretaceous 1). 



(b) Eruptions of trachyte, trachy-rhyolites, and rhyolites, 

 mostly highly alkaline, in all the area examined from Mt. Byron 

 (keratophyre) to Mt. Cooran (comendite). These eruptions had 

 their maximum effect in the Glass House Mountain region: 

 (Eocene 1). 



<c) Eruptions of dacite (Glass House Mountain type*) in the 

 Glass House Mountains and the Maroochy district: (late Eocene 1 ?); 

 also andesites of this age. 



(d) Basaltic outpourings: (Pliocene?). 



It was remarked by me previously that in the Glass House 

 Mountains Richtofen's law of volcanic succession did not hold in 

 all respects. Taking the wider area dealt with in this paper, we' 

 find that the eruptions followed, generally speaking, in the order 

 of Richtofen's law. An examination of the foregoing penological 

 work brings one to the conclusion that there were two great 

 periods of volcanic activity. The first was in Pre-Gympie times, 

 and the magma erupted was essentially calcic in composition, 

 giving rise to lavas now altered to greenstone, anthophyllite 

 schist and amphibolite. A sodic and more acid differentiation 

 product of this magma gave rise to the rocks now represented 

 by epidote-glaucophane schist, quartz-glaucophane schist, and 

 glaucophane-albite schists on Mt. Mee. 



The great granitic intrusions followed in Carboniferous or 

 Permo-Carboniferous time, and were of a calcic nature; but the 

 predominence of plagioclase over orthoclase in these masses 

 (tonalite, quartz diorite, etc.) hints at the alkaline period to 

 follow. 



The second great volcanic period came in Post-Triassic times, 

 and gave lavas of an alkaline nature. At the same time great 



* See my previous paper, these Proceedings, 1903. 



