188 VOLCANIC ROCKS OP SOUTH-EASTERN QUEENSLAND 



curves for these combined oxides which have been adapted 

 from Barker's curves. The only rock that does not conform 

 well is the Toowoomba basalt, and the low alumina value 

 is accompanied by a high magnesia value ; this rock is 

 extremely rich in olivine. 



Thus while the alumina is certainly low and the total 

 iron-oxides as ferrous oxide certainly high, the combined 

 oxides are thoroughly normal. 



The curves for lime are all good ones, and when com- 

 pared with the generalised curves of Harker it is seen that 

 both the sub-alkaline and alkaline series are slightly lower 

 than normal, while the third series occupies an intermediate 

 position. 



The curves for magnesia are, generally speaking, good, 

 although the Burleigh Heads andesitic-basalt has a low 

 value, and the Toowoomba basalt a high value. The Bur- 

 leigh Heads rock in other respects is. however, nearly 

 normal. The Toowoomba basalt is extremely rich in olivine, 

 and this accounts for the high value for magnesia, and it 

 is to be noted that the alumina value for this rock is corres- 

 pondingly low. The curve for the sub-alkaline series is 

 almost normal although a little low, but that for the alka- 

 line series is considerably lower than the normal alkaline 

 curve. The third series gives a very good curve which 

 takes up an intermediate position between the other two. 



The curves for soda are reasonably good, but in com- 

 parison with the generalised curves of Harker the alkaline 

 curve is seen to be considerably higher, and the sub-alkaline 

 curve, while nearly normal, is a shade high towards the 

 basic end. The curve for the third series is again in an 

 intermediate position. The trachyte from the summit of 

 Mount Flinders is considerably below the alkaline curve, 

 but in the potash diagram it is seen to be somewhat above. 

 The rocks as a whole are a little above the normal in soda. 



The curves for potash are not as good as those for soda, 

 and show the alkaline series to be slightly above the normal, 

 and the sub-alkaline series rather more so, except towards 

 the basic end. The rhyolite from Glen Rock is considerably 

 off the curve for the third series. The rocks as a whole 

 are little above the normal in their potash value. 



