196 



VOLCANIC ROCKS OF SOUTH-EASTERN QUEENSLAND 



sub-alkaline and alkaline ones. Serial differentiation then 

 went on in these magmas, and as a result the various rocks 

 were separated out. 



"With the sub-alkaline magma, the order of separation 

 and extrusion of the material seems to have been as follows : 

 — Basic, then sub-basic and basic, extensive acid, still more 

 extensive basic and sub-basic, and finally basic. While 

 the sub-alkaline magma was extruding the acid material 

 the alkali magmas were sending off acid and sub-acid 

 material in the different centres, and these were exhausted 

 before the sub-alkaline magma again extruded the basic 

 and sub-basic material. The magmas of intermediate com- 

 position began extruding basic material previous to the acid 

 sub-alkaline extrusions, and while the latter were being 

 exhausted, acid and sub-acid material of an intermediate 

 nature was separated off and extruded; some of the most 

 recent flows in the area were the last extrusions of a basic 

 nature from these magmas. 



Table X. 



I. — Average Composition of Granites of the World, (a) 

 II. — Average Composition of Basalts of the World, (a) 

 III. — Mean of I. and II. 

 IV. — Average Composition of Volcanic Rocks of the Area. 



(a) Daly, Igneous Rocks. 



The estimated average composition of the parent magma 

 bears a close relationship to the mean of the world 's average 



