GEOLOGY AND PETROLOGY OF ENOGGERA GRANITE. 129 



by Analysis E2 into two parts such as those we see in 

 analyses El and E2. Such an hypothesis as the second 

 might explain the intermediate chemical composition, but 

 does not explain the mixed mineralogieal nature of the 

 rock. 



The only alternative remaining is that this curious 

 rock is produced by admixture of two distinct rock types. 

 It seems to be what Harker terms ' ' a hybrid. ' ' 



By weighting the Analyses El and E4 in the proportion 

 3:1 (a ratio already mentioned as that of the outcrops of 

 the two phases in the somewhat deeply dissected Enoggera 

 area) we obtain the hypothetical rock which would be 

 produced by the admixture of the Pink and Grey Phases in 

 the proportions in which we believe them to exist. The 

 chemical composition of such a rock is represented by 

 Analysis A, which is seen to agree quite closely in many 

 respects with Analysis E2. 



In comparing the calculated Analysis A with the actual 

 Kock Analysis E2, it is seen that while the value of the 

 calculated CaO is somewhat less than the actual, both the 

 alkalies are considerably in excess, although the proportion 

 KjO : NagO is much the same. This is not unexpected, for 

 Harker" in discussing the calculation of the chemical com- 

 position of hybrid rocks points out that while in bulk the 

 hybrid rock must be a linear variation of the two unmixed 

 rocks, differences may be expected in the chemical composi- 

 tion of the rock specimens anal.ysed. The example quoted 

 by this eminent authority resembles the present case in 

 that the CaO found in the hybrid rock is less than the 

 calculated, the difference being that in Harker 's case the 

 discrepancy is considerably greater. This is explained by 

 Harker as probably due to difference in the rate of diffu- 

 sion of CaO on the one hand and of the alkalies on the 

 other. Hence the divergences to be expected are precisely 

 those found to exist in the present case. The calculated 

 value of CaO will be less than that of the actual, while that 

 of the alkalies will be greater and further as the rates of 

 diffusion of the potash and soda molecules are very nearly 

 identical, the proportion KoO : NaoO as calculated should 

 agree very nearly with the proportion found by actual rock 

 analysis. 



° ' ' The Natural History of Igneous Eocks, ' ' p. 358, 



