GEOLOGY AND PETROLOGY OF ENOGGERA GRANITE. 151 



Variation diagrams, Plate I., Figures 1 and 2, have been 

 carefully prepared, which show a very definite chemical 

 relationship between the "Pink Phase" and "Acid" 

 granites on the one hand, and the "Gray Phase" and 

 "Basic" granites on the other. 



If reference be made to Figure 1, in which the per- 

 centage weights of K2O and NaoO have been plotted against 

 that of SiO., it will be noticed" that the "Grey Phase" of 

 the Enoggera granite (E.4) is connected with the "Grey- 

 mare" granite (S.4) on the one hand and the "Basic 

 Granites" of New England (N.7 and N.8) on the other by 

 variation lines which show that Na^O is present in consider- 

 able excess of K2O in each case. While at first glance these 

 lines seem parallel, a closer inspection shows that there is 

 a gradual convergence as one proceeds from the more basic 

 to the more acid rocks. In other words, the ratio 

 NaoO : K2O varies inversely with the acidity. The Ballan- 

 dean granite (S.3) has been linked up with these curves 

 for obvious reasons, although Saint-Smith refers to the 

 specimen from which the analysis was made as one type of 

 the "Stanthorpe" granite — ""a medium-grained variety 

 from Ballandean. ' '-* 



If these "curves" be compared with the corresponding- 

 curves joining the "Pink Phase" and the "Stanthorpe" 

 and "Sandy" granites, some very decided differences will 

 be observed. A very close approximation in the values of 

 K2O and Na20 is apparent, the curves for these oxides 

 interweaving and remaining close together. One would 

 naturally expect that the "Acid" granites and "Eurites"' 

 of New England, which have been so definitely correlated 

 with the "Stanthorpe" and "Sandy" granites respectively, 

 would approach them so closely in chemical composition as 

 to be readily reconciled to one variation curve for the two 

 series. In the KoO and Na^O values such is not the case. 

 The K2O is somewhat higher in the New England rocks and 

 the Na20 correspondingly lower. Consequently the ratio 

 KoO : Na20 is very much greater than in the Stanthorpe 

 rocks. Curiously enough, in the chain of chemical evidence 

 connecting the Pink Phase, the acid Stanthorpe granites, 

 and the acid granites of New England, the latter is the 

 weaker link in spite of the fact that the former connects 

 outcrops which are separated by approximately 100 miles, 



" Op. Cit., p. 43. 



