310 AUSTRALASIAN ANTARCTIC EXPEDITION 



Nos. 632, 633 and 635A are specimens of amphibolites from the metamorphosed 

 dyke series of Cape Denison. These specimens are like Nos. 634, 634A and 635 

 described by Stillwell in this connection. They have developed schistosity to a 

 considerable degree by a parallel arrangement of hornblende prisms. Both Nos. 632 

 and 633 are markedly fissile because of the perfection of this parallelism. 



2. MINER ALOGICAL CHARACTERS OF THE INDIVIDUAL XENOLITHS. 



Under the microscope Nos. 142, 421, and 431 fall into one mineralogical division 

 and Nos. 666 and 1240 in another. The differences are not so much in actual mineral 

 content as in the nature of the crystal development. In the first three of these rocks 

 the felspar, not only in the phenocrysts themselves, but throughout the whole 

 rock-mass is turbid and highly saussuritised. The amphibole present is pale green in 

 colour and is definitely actinolitic in character. The crystals are elongated but small. 

 They have not the massive, compact appearance of typical hornblende. Their colour, 

 as a matter of fact, tends to blue, suggesting that the glaucophane molecule has been 

 concentrated in them to some extent. No. 666 serves as a transition stage. It is marked 

 by the plagioclase intermingled with the amphibole crystals, as distinct from that in the 

 saussurite phenocrysts, being pellucid. This shows that the felspar of these regions has 

 completely recrystallised and either assumed again the matter rejected during 

 saussuritisation or expelled it by diffusion to be taken up in the composition of the 

 hornblende. The amphibole in this rock is not so actinolitic as in the first three, but it is 

 by no means so compact and green as in rock No. 1240. 



In this rock, No. 1240, the chief difference from the others mentioned above, is the 

 intense green and brownish green colour of the amphibole, which now is quite definitely 

 hornblende. Another striking difference is the almost entire absence of saussurite which 

 is present in the nuclei of two large phenocrysts only. 



Other mineralogical points of interest are first, the relationship of sphene and 

 nuclear ilmenite. The development of this is shown quite well in these rocks. In No. 142 

 the border of the sphene is quite narrow, sometimes less than 0-005 mm., while in the other 

 rocks there is a range of 0-05 mm., and perhaps even more. Secondly, a genetical 

 relationship between the three minerals ilmenite, sphene and zoisite (or clino-zoisite, or 

 epidote as the case may be) and felspar is shown by a similar serial arrangement. 



Other reactions which involve sphene are best seen in Nos. 142 and 421. In these 

 rocks a more fibrous biotite than usual, whose pleochroism is from Z a weak brown to 

 X practically colourless, is found associated with grains of sphene, hornblende and 

 nearly colourless chlorite (Plate XLV, fig. 1). The chlorite is derived from the biotite, 

 its development being a possible explanation of the bleaching of the colour of the mica. 

 The sphene seems to be forming chiefly at the expense of the hornblende which, as a 

 result, is poikiloblastic in texture and studded with small inclusions of the sphene. 

 But a similar relation exists, to a more limited extent, between the biotite and sphene. 



