GEOLOGY AND PETROLOGY OF ENOGGERA GRANITE. IBS' 



Name. — Granite (Granodiorite?) Porphyry. 



Associated with the Piuk Phase, but a little later as 

 regards time of intrusion, are aplitic and granophyric rocks 

 which, however, do not merit detailed descriptions, since 

 they are quite normal in most respects. In the former, 

 however, flakes of molybdenite are occasionally found, while 

 the latter sometimes show verj^ beautiful micrographic 

 intergrowths of quartz and orthoclase. (See Plate II., 

 No. 3.) 



(h) The Grey Phase. 



Under this heading are included a number of rock 

 types which, though they ditfer in some respects, have in 

 common several characteristics which mark them off clearly 

 from the "Pink Phase." In general the rocks of this phase 

 are of a grey colour. Quartz is not so abundant as in the 

 other phase (pink orthoclase is absent), and the plagioclases 

 are more calcic, the phenoerysts usuallj' being acid 

 andesine and the felspars in the ground mass an inter- 

 mediate andesine. Hornblende is usually present, some- 

 times in great abundance. Pyrites is invariably present, 

 sometimes in considerable amount. 



The rocks of this phase vary between two types. Of 

 these the first is characterised by the very strong develop- 

 ment of numerous long prismatic and acicular crystals of 

 hornblende, which give the type quite a distinctive appear- 

 ance. This rock occurs as inclusions, and larger masses set 

 in and surrounded by the Pink Phase. They are evidently 

 the remnants of an older rock. Although they are marked 

 off from the enclosing mass by a well defined boundary, 

 there is evidence that the hornblendic rock has been in some 

 cases somewhat modified by the enclosing acid magma. In 

 particular, one very interesting rock has resulted from a 

 partial mixing of the two types. In the hand specimen 

 it suggests mechanical mixing rather than chemical inter- 

 mingling, although evidence of chemical interaction is seen 

 on a closer inspection. The specimen examined was 

 obtained from Portion 373, parish of Enoggera, and pre- 

 sents a curious appearance, irregular pink patches being 

 scattered through the dark hornblendic rock. Numerous 

 rounded quartz blebs occur sporadically throughout the 

 rock, and each is surrounded by a very distinct corona of 

 ferro-magnesian minerals. This suggests that at least some 

 chemical interaction took place. 



