THE METAMORPH1C ROCKS OF ADELIE LAND. ST1LLWELL. 71 



under special conditions of metamorphism. If such migration occurs across a pre- 

 existing junction there must, a priori, be a strong tendency to efface that junction. 

 If we imagine a diffusion of some of the amphibolite material into the granodiorite 

 gneiss, or some of the gneissic material into the amphibolite, we would get the former 

 junction replaced by the gradual transition observed. The transition types would 

 be mixtures of amphibolites and granodiorite gneiss, and would correspond to the types 

 No. 13 and No. 10. Such a theory is in agreement with the observations, and we will 

 speak of the process, for convenience, as metamorphic diffusion. Diffusion products, 

 like the hornblende and biotite gneisses, are, therefore, looked upon as metamorphic 

 hybrid rocks. 



Solid diffusion has been suggested before to account for the perfectly gradual 

 passage of granitoid rocks into surrounding schists. Greenly* endeavoured to compare 

 such phenomena with the laboratory experiments of Roberts-Austen on the diffusion 

 of gold into lead. Greenly, however, postulates a mixing of a granite magma and the 

 neighbouring sedimentary rocks, a conception which involves not true solid diffusion, but 

 merely a mechanical percolation of the surrounding schists by highly fluid magma. Deschf 

 therefore pointed out that the term " diffusion " had been loosely employed. Another 

 claim for solid diffusion is mentioned by ElsdenJ in the observations of Trener on the 

 contact phenomena of Cima d'Asta, but the same objection again holds. While it 

 has been usual in these cases to suppose that the mixing takes place at the time of 

 intrusion, I do not know of evidence to show that a degree of mixing has not occurred 

 after complete consolidation ; and, if this is so, these cases may be examples of solid 

 diffusion. 



The difficulty lies in the proof of the solid nature of the rocks before the mixing. 

 At Cape Denison the granodiorite must have been solid before it could be fractured 

 and penetrated by the primary dolerite dykes ; and the presence of the meta-xenoliths 

 in the amphibolites indicates the consolidation of the dykes before their metamorphism. 

 Further, we cannot suppose that a thin sheet of dyke magma would remain fluid for 

 a sufficient length of time to permit the mechanical percolation that is possible in the 

 case of a large, deep-seated, slowly-cooling plutonic mass. Hence at Cape Denison we 

 consider that solid diffusion, in the strict sense of the term, has operated. The term 

 " metamorphic diffusion " implies that diffusion has occurred in the solid state. 



Metamorphic diffusion is not restricted to the amphibolite gneiss junction at Cape 

 Denison. It also appears along the junction of the aplitic gneisses with the granodiorite 

 gneiss. The examples quoted tend to show that quartz is a mineral that is readily 

 diffused, but other mineral molecules like hornblende and biotite can be so transferred. 



Basic segregations are common in many granitic masses and may be relatively 

 rich in either biotite or hornblende. If these were recrystalh'sed under conditions 



" Diffusion of Granite into Schuta," Greenly, GeoL Mg., vol. 10, dec. 4, N.S., p. 207. 

 t "Report on Diffusion in Solid*," C. H. Detoh, Brit. AM. Report (Dundee, 1912), p. 348. 

 t " Principle* of Chemical Geology," J. V. EUden, p. 2. 



