72 AUSTRALASIAN ANTARCTIC EXPEDITION. 



permitting metamorphic diffusion it may be imagined that hybrid rocks, similar to the 

 hornblende and biotite gneisses, might arise. Consequently such types may possibly 

 be discovered in isolated masses in regions where there are no traces of the existence 

 of dykes. At Cape Denison, however, the evidence seems clear that they are connected 

 with dykes. 



It will be subsequently shown that the most intricate dyke structures can be 

 preserved during the metamorphism at Cape Gray, where the recrystallisation has 

 occurred under kata zone conditions, in which the pressure factor is chiefly hydrostatic. 

 At Cape Denison the pressure factor in the metamorphism is chiefly stress, and hence 

 the destruction of the dyke structures and the migration of material is to be connected 

 with the dominating stress. 



11. FURTHER EXAMPLES OP METAMORPHIC DIFFUSION. 

 Junction Specimens. 



The above interpretation of the biotite gneisses as metamorphic hybrid rocks, 

 produced by an intermingling of two diverse rock types by solid diffusion, is upheld 

 by the examination of specimen No. 372, found on the moraines at Cape Denison. This 

 specimen was collected as a diagrammatic example of the normal " sharp " junction 

 between the amphibolite and the gneiss. One-half of the specimen (Plate XII., fig. 5) 

 is black amphibolite, and the other half is grey granitic gneiss. The junction, however, 

 is not sharp, and how far this applies to all the dyke junctions at Cape Denison is not 

 known. As the specimen was not found in situ it is not possible to assert that the 

 amphibolite represents a portion of a dyke originally intrusive into the granitic gneiss. 

 But as both the amphibolite and the gneiss are analogous to specimens found in situ 

 it is very probable that such is the case. 



In a section of the granitic portion of the specimen it is found that the cataclasis, 

 so marked in most of the typical granodiorite gneisses at Cape Denison, is absent. The 

 epi zone metamorphism, however, is signified by the amount of saussuritised or 

 sericitised felspar and by the chloritisation of the biotite. The amount of 

 ferromagnesian minerals is less than in the typical example No. 11, but it is not 

 noticeably less than in other examples from Cape Denison. In addition to the cloudy 

 felspar there is a considerable quantity of clear, recrystallised felspar which, with the 

 quartz, possesses the crystalloblastic structure. Some of the clear felspar is untwinned, 

 but some of the twinned crystals have been determined to be oligoclase-andesine. The 

 biotite is brown, but is largely altered to green chlorite. There is in addition scattered 

 epidote, allanite, lawsonite, pyrite, and apatite. The general characters and the 

 composition of the felspar indicate the relation to the granodiorite gneisses. 



A section across the junction reveals the presence of a zone of biotite gneiss, 

 approximately 1 c.m. wide, between the amphibolite and the granodiorite gneiss. The 



