70 AU8TEALASIAN ANTARCTIC EXPEDITION. 



If we assume, as in the preceding case, that silica is the chief mineral addition 

 to the original dyke rock, and the mineral composition be recalculated to 100 per cent, 

 after neglecting the quartz, we obtain the figures in column 10A. These figures bear 

 some resemblance to the composition of No. 153, in Table I. The proportion of felspar 

 to ferromagnesian is much the same in both cases ; but the felspar of No. 10 is nearly 

 all saussurite, whereas the felspar of No. 153 is perfectly clear. No. 10 thus appears 

 to be related to the epidote biotite schists in the same way that the hornblende gneiss 

 No. 13 is related to the normal amphibolites. 



10. ORIGIN OF THE COARSELY CRYSTALLINE BASIC PATCHES. 



The origin of these dark hornblendic and biotitic rocks which are enveloped in 

 the granodiorite gneiss, and which have been designated the coarse-grained types, 

 is a very interesting question. It has been shown that the massive amphibolite from 

 these patches is identical, except for larger grain size, with the amphibolites which have 

 been established as altered dykes. It is also plain that there is true transition from 

 this amphibolite through hornblende gneiss or biotite gneiss to the granodiorite gneiss. 

 Accepting the face value of these gradual transitions, we might say that these " basic " 

 patches have been derived out of the granodiorite itself. We might conceive of a 

 magmatic differentiation which was initiated in the granodiorite magma which became 

 frozen before the differentiation process was complete. A sudden cessation of the 

 differentiation forces has left a gradual apparent transition between the amphibolite 

 and the granodiorite. Such an argument completely ignores the observed similarity 

 of the textural, structural, mineralogical, and chemical relations of related rock types 

 at Cape Denison, and at the same time we miss the recognition of a true metamorphic 

 process. 



Since the coarse-grained type is so precisely similar to the amphibolite dyke series 

 which has been proved to be the metamorphosed equivalent of diabasic dykes, it is 

 extremely likely that the No. 9 type of amphibolite has been derived from a primary 

 rock of similar nature. They occur approximately along the extension of well defined 

 dykes, and hence it is extremely likely, and as definite as it is possible to be, that the 

 primary rock of the No. 9 type was part of the intruded series of dykes. We have 

 described fragments of the dyke series proper which have been torn away from the 

 dyke channel and now appear completely enclosed in the gneiss. Discontinuity of the 

 hornblendic clots, irregularity or isolation are, therefore, matters of little weight. These 

 detached fragments of the established dykes have escaped the more intense meta- 

 morphism which produced the larger grain size of No. 9, and they have been able to 

 preserve their sharp outline against the gneiss. 



There is no special reason, however, why a pre-existing junction between two rock 

 types must be preserved during metamorphism. We have maintained in our hypothesis 

 of metamorphic differentiation that a limited migration may occur in the solid rock 



