12 AUSTRALASIAN ANTARCTIC EXPEDITION. 



the percentage of hornblende. There are, however, exceptions both at Cape Denison 

 and Mackellar Island, but these can be explained by varying metamorphic conditions. 

 The chemical composition of the epidote biotite schists differs distinctly from that of 

 the amphibolite, though both possess the general characteristics of a basic igneous 

 rock. The chemical composition of the biotite amphibolites is calculated to be near 

 the mean of the two end members of the series. 



Extraordinary examples of metamorphosed xenoliths have been found in an 

 amphibolite dyke at Cape Denison, and include both the cognate and accidental types. 

 These bodies were caught up by the invading dyke and set in position before the develop- 

 ment of the amphibolite characters by metamorphism. In some instances they still 

 preserve a very remarkable angular outline, but they are now an integral part of the 

 metamorphic rock, and the foliation passes through them irrespective of their outline. 

 The cognate xenoliths now possess the same recrystallisation products as the dyke. 

 The accidental xenoliths are undoubtedly foreign to the primary dyke, and bear relation 

 to the enclosing granodiorite gneiss. 



Quite distinct from these xenolithic bodies are certain varying rock types which 

 are enclosed in the amphibolite dykes. These include epidosites, chlorite rock, biotite 

 hornblende schists, and biotite felspar schists. These rocks bear definite relation to 

 the development of the metamorphic characters, and can only be adequately explained 

 by an hypothesis depending on the metamorphism. Their mineral composition shows 

 that their chemical composition is quite distinct from that of the amphibolite from which 

 they were derived. The proved difference in chemical composition involves the migra- 

 tion and segregation of material under metamorphic conditions, and is called metamorphic 

 differentiation. 



In addition to the sharply-walled dykes at Cape Denison there are isolated patches 

 of amphibolite completely enclosed by the granodiorite gneiss. Argument is deduced 

 to show that these amphibolites, differing only in grain size, have once been part of 

 the dyke series. This means that these " inclusions " are younger than the enclosing 

 gneiss, and the dyke channels have been rendered discontinuous during the meta- 

 morphism. In some cases the boundary between amphibolite and granodiorite gneiss 

 has been destroyed and replaced by a gradual transition. The destruction of a pre- 

 existing boundary by the migration of material across it during metamorphism is called 

 metamorphic diffusion. In all cases of transition there is no evidence of refusion. 



The processes of metamorphic differentiation and diffusion are very limited in 

 range and produce the exceptional types. Nevertheless, they have been found to be 

 applicable to localized portions of many widespread areas of crystalline schists. In 

 several cases they produce results directly opposed to previous conclusions. The 

 assumed evidence, for example, on which is based the conclusion that amphibolites 

 can be produced by the extreme metamorphism of limestones in Canada, is rendered 

 invalid. The evidence demands the reconsideration of all conclusions in areas of 

 crystalline schists which are based on the evidence of transition. 



