76 AUSTRALASIAN ANTARCTIC EXPEDITION. 



the area of composite gneiss from which No. 144 was taken. It occurred close to the 

 junction plane of the dyke which was cut by a quartz segregation vein carrying large 

 crystals of epidote. This quartz vein may have carried felspar. The specimen has a 

 more noticeably brecciated appearance than No. 146, and whereas in the latter the 

 felspar is white or transparent, it is here pinkish or greenish white, a colour which 

 indicates saussurite. The outline of the crystals is not definite, and they approach 

 to lenticles in character. In section there is no special evidence of crushing, and the 

 large irregular felspar crystals are set in the amphibolite ground mass. The same 

 minerals are present as in the normal rock of the band, No. 412, though the large felspar 

 causes a preponderence of the colourless constituents. Some of the felspar is clear, 

 and approaches andesine in character, but its refractive index is close to, but less than, 

 1-551 (nitrobenzol). The bulk of the felspar in the section is saussuritised. A few 

 blebs of quartz are recognised, and there is perhaps a little more chlorite than in No. 

 412. As in No. 412, lawsonite is present in small amount. 



The altered nature of the large felspar in this case prevents the assertion that they 

 are metamorphic products. They may have been associated with the accompanying 

 epidote-bearing vein, or they may be allied to the xenoliths of saussurite described 

 from the band No. 629. 



12. FURTHER EXAMPLES OF METAMORPHIC DIFFERENTIATION AT CAPE DENISON. 



If the amphibolite patches, which may have either sharp or indistinct boundaries, 

 are to be included in the dyke series, we immediately find further samples of 

 metamorphic differentiation. In the description of these coarse-grained types, reference 

 has been made to the bands and lenses of pure hornblende associated in the field with 

 them. That these are also part of the original diabasic magma seems evident, because 

 we only find them in such association. We think, therefore, that we can consider these 

 patches of hornblende in the same way as we have considered the biotitic, chloritic, and 

 epidotic clots which are enclosed in the sharply- walled dykes. As the biotite, etc., 

 patches are metamorphic differentiation products, so also are the hornblende patches. 

 In the one case there has been long continued conditions for the formation of biotite, 

 and in the other case an analogous set of conditions for the formation of hornblende. 

 It has been noted throughout the series that the hornblende and the biotite appear as 

 equivalent zonal products, and if we get the differentiation of one we should reasonably 

 get the differentiation of the other. Indeed, we have already discovered this in the 

 biotite hornblende clot. It is true in the case of hornblende that its composition may 

 be similar to a xenolith of pyroxene crystals in the primary magma, or to an ultrabasic 

 magma, and a hornblende patch may conceivably develop by the metamorphism of 

 such a primary xenolith. If this were so, we should reasonably expect to find some 

 such altered xenoliths among the sharply-defined dykes. The distribution, however, 

 in layers conformable to the schistosity is sure evidence of at least some transference, 

 and the frequency and variation of shape, combined with symmetry to the plane of 



