176 AUSTKALASIAN ANTARCTIC EXPEDITION. 



garnets, and the part of felspar near the contact with augite is more sodic than the 

 central portion. The relic augite is bordered by the secondary hornblende and this 

 may have prevented the interaction along this junction. 



The proportions of the minerals garnet, biotite, and hornblende vary in different 

 slides with the varying amounts of recrystallisation. 



If these varying alterations had not occurred the rock would consist chiefly of 

 augite, labradorite, and ilmenite, and, perhaps, some biotite. The primary ophitic 

 structure has been detected, and there can be no doubt at all that the rock is a meta- 

 morphosed dolerite. Yet it has been reported as a band with ill-defined boundaries. 

 Such ill-defined borders are probably to be explained by some such process as meta- 

 morphic diffusion. The formation of the garnet in this case is most certainly not due 

 to any absorption of any sedimentary material, as has been suggested by Cole for the 

 origin of certain garnet amphibolites in Ireland.* 



The rock may be called a garnet plagioclase pyroxene gneiss. 



No. 953. Several specimens of a garnet amphibolite have been collected from 

 Stillwell Island, where a dark basic band becomes definitely banded. Specimen No. 

 953 is a moderately coarse-grained rock with noticeable schistosity showing pink garnet, 

 black hornblende, and biotite. 



In section, the rock is remarkable for its percentage of hornblende, garnet, and 

 quartz ; and a casual study would suggest that the rock possesses a composition different 

 from that of the more obvious dyke rocks. The chemical analysis shows that this is 

 not so. The percentage mineral composition has been determined as follows : 



Felspar and quartz 24-1 



Pyroxene 2-4 



Hornblende 38-7 



Garnet 19-7 



Ilmenite 6-7 



Biotite 7-9 



Apatite and sphene 0-5 



The green hornblende is the most abundant constituent, and at times seems to be 

 wrapped round a garnet crystal in the manner suggested in No. 942. The grains are 

 much embayed and sometimes poikiloblastic. The pink garnet is often crowded with 

 small inclusions. The grains are mostly rounded, but exert their form against the felspar 

 and quartz, and tend to do so against the hornblende. While quartz forms the bulk 

 of the colourless constituents, a garnet crystal is always set in a felspar base an 

 association which clearly has genetic meaning. A twinned felspar has been found to 

 give an extinction angle of 33 measured from the lamellae, and to possess a refractive 



" On the Growth of Crystals in the Contact Zone of Granite and Amphibolite," G. A. J. Cole, Proe. Roy. Irish Acad., 

 vol. 25, sect. B, 1905, p. 117. 



