184 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND' 



Mean Average Composition of the Volcanic Rocks 

 of the Area. 



All three divisions are represented in each of the 

 important accumulations of extruded material, and the 

 average thicknesses are 1,000, 500 and 100 feet respectively 

 for the upper, middle and lower divisions. 



The actual areal distribution of each division is 

 approximately the same, as they are all piled one on top 

 of the other, so that a general ratio of 10 : 5 : 1 is arrived 

 at for the respective volumes of the upper, middle and 

 lower divisions. 



By combining the averages for each of these divisions of 

 rocks in these proportions, the following average composi- 

 tion as shown in Table VIII. of the whole of the volcanic 

 rocks of the area is obtained. For the purposes of these 

 calculations, all the rocks were assumed to have the same 

 density, as the roughness of the mapping did not warrant 

 one taking differences of density into account. The 

 average analysis is seen to be close to the average andesite 

 given by Daly,*° and also to the andesite from the southern 

 end of Tamborine Plateau. It differs from the average 

 andesite analysis mainly in alumina and iron-oxide value, 

 though on totalling these a close approximation is ob- 

 tained. The magnesia is rather higher, the lime slightly 

 lower, and the potash a little higher than in the average 

 andesite analysis. 



In comparing the average obtained with the Tam- 

 borine andesite it is seen that the former is about 1 per 

 cent, higher in silica, the ferric oxide is considerably lower, 

 thfe magnesia is rather higher, and the potash and also the 

 titania a little lower. 



Igneous Eoeks and their Origin, p. 26. 



