BY H. C. RICHARDS. 195 



along this line before, it is difficult to say. However, we 

 have along this particular line both the upper and lower 

 sub-alkaline extrusions with the alkaline extrusions between, 

 so that it is difficult to see how Barker's generalisations 

 hold here. The available evidence certainly does not sup- 

 port the generalisations made by Harker. Similar con- 

 clusions have been arrived at for the Victorian Cainozoic 

 rocks by Professor Skeats and Dr. Summers.*^" 



In all probability, further evidence of faulting will be 

 forthcoming in the southern portion of the area, but there 

 is nothing to suggest that folding except on a very minor 

 scale has happened anywhere in the area since the Palteo- 

 zoic era. 



VIII. ORIGIN OF THE VOLCANIC ROCKS. 



It has been shown that different portions and also in 

 some cases the same portions of the area have been charac- 

 terised by different series of volcanic rocks at various 

 times. This one assumes to be the result of magmatic 

 differences, and further that these different magmas have 

 been derived from the parent magma. Certain characteris- 

 tics which are common to the different series strengthen the 

 belief that they are magmatically related. These charac- 

 teristics have been frequently cited earlier, and are the 

 general paucity in alumina and corresponding richness in 

 iron-oxides, together with a slight lack of lime and magnesia 

 and a slight excess of the alkalies. 



An estimate of the general composition of this parent 

 magma has been made, and it is very close to that of the 

 average andesite. 



That such a rock magma could exist is shown by the 

 similarity of its composition with that of the andesite from 

 the southern portion of Tamborine Plateau. 



The parent magma split up in some way into a large 

 sub-alkaline magma, and several smaller alkaline magmas, 

 and magmas with a composition intermediate between the 



=»Geol. Snrv., Victoria, Bull. 24; and Proc. Eoy. Soc, Vic, 1914. 

 p. 289. 



