270 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Specimen No. 317 is a coarse-grained rock, with a mottled black-and-white 

 appearance, due to glomeroplasmatic areas of hornblende and felspar of average width 

 of 3 to 4 mm. Each dark hornblende mass resolves itself on examination into clusters 

 of small crystals, associated with crystals of chlorite, ilmenite, and apatite. This 

 ilmenite may have been discharged from the original pyroxene in the manner 

 recorded in the garnet-plagioclase-pyroxene-gneiss, No. 935, from Stillwell Island 1 . 

 Biotite is replaced by chlorite and epidote. In the white areas are large saussuritised 

 felspars, in which sericite, white mica, epidote, chlorite, and zoisite are found, 

 in addition to clear albite. There are also areas of clear labradorite, and a few 

 scattered fragments of garnet. The rock may be distinguished as a saussurite-gabbro- 

 gneiss. 



Specimen No. 728 is another variety of saussurite-gabbro-gneiss. Its felspar is 

 completely saussuritised, and there is a little garnet and a small development of 

 secondary quartz in the hornblende, probably associated with the development of 

 epidote and chlorite. The specimen is not schistose, and the outline of the original 

 felspar crystals is discernible in the hand specimen, averaging about 6-7 mm. long 

 and 2-3 mm. wide. The form of the felspar is even better preserved in No. 861, where 

 it is sufficiently obvious to suggest that the hand specimen is a hornblende-felspar- 

 porphyry. There is, however, complete saussuritisation of the felspar, while the 

 biotite and some of the hornblende is converted into epidote and chlorite. Scattered 

 garnets occur around the fringe of some of the hornblende clusters, and are suggestive 

 of the garnet roms around the pyroxene in the garnet-pyroxene-gneisses of Stillwell 

 Island. 



The conversion of hornblende into chlorite is further advanced in specimen 

 No. 515, another saussurite-gabbro-gneiss. The chloritisation of the hornblende 

 proceeds from the edges and cleavage cracks. The section is crossed by branching 

 veins of lawsonite (PL XL, fig. 2), which have developed during the saussuritisation of 

 the felspar. These veins cross hornblende as well as felspar areas. Zoisite occurs 

 with the epidote in the altered areas. There are, however, considerable areas of 

 unaltered basic felspar. 



Apart from the gabbro gneisses and the amphibolites that have been described, 

 there are a number of coarse-grained hornblendic rocks. With the segregation of the 

 lighter-coloured constituents, these rocks become representatives of the hornblende- 

 plagioclase-gneisses of Group III. With the segregation of the darker constituents, 

 they grade towards the hornblende schists of Group V. Specimen No. 721 (PI. XLI, 

 fig. 4) illustrates a segregation assuming the form of a vein. The vein contains large 

 crystals of hornblende, some of which are 3 cm. long, together with felspar and a little 

 quartz. A similar vein occurs on the edge of another specimen, No. 25, in which there 

 is a noticeable increase in grain size of the amphibolite as it approaches the edge of the 

 vein. 



1 Op. oit., p. 174. 



