186 AUSTKALASIAN ANTAKCTIC EXPEDITION. 



SUMMARY. 



In all exposures on the Cape Gray Promontory basic gneisses are found associated 

 with the garnet gneisses. With one exception, these basic gneisses are found in dyke 

 form which is so definite that the field examination convinced the observers of the 

 igneous origin. Microscopic and chemical examination have confirmed this observation 

 and interesting mineralogical changes have been traced. 



At Cape Gray the outlines of primary felspar laths and augite crystals can be 

 determined and a blastophitic structure is found. The primary felspars are now repre- 

 sented by aggregates of interlocking, clear, secondary felspar. The primary augite, 

 recognisable in all cases by the presence of minute ilmenite inclusions, becomes trans- 

 formed into clear, granular, secondary augite and hypersthene, with associated ilmenite. 

 A varying amount of hornblendisation of the pyroxene occurs. The basic rocks at 

 Cape Gray bear evidence of kata zone metamorphism like the surrounding cordierite 

 garnet gneiss. 



At Stillwell Island massive types occur and further changes are traced. A coarsely 

 crystalline rock (No. 951), which is probably a completely recrystallised dolerite, consists 

 of granular crystals of clear pyroxene and clear felspar. It shows the incipient changes 

 of modification in a rim of diablastic intergrowth of pyroxene and felspar which surrounds 

 crystals of pyroxene, biotite, and ilmenite. This rim is looked upon as the incipient 

 stage of reaction between pyroxene or biotite and felspar ; which produces garnet and 

 quartz or garnet and orthoclase. 



This reaction is advanced in another example (No. 935), and a well developed rim 

 of garnet can be traced around pyroxene areas. The aggregation of the small garnets 

 which form the garnet rim may produce the larger garnet crystals of the garnet 

 amphibolite. This origin explains why the garnet crystals are always set in a felspar 

 base a constant association which must have genetic meaning. If hornblendisation 

 of the remaining pyroxene occurs, we get the garnet amphibolite, of which No. 953 is 

 an example. In some cases the hornblendisation of the pyroxene occurs and a normal 

 amphibolite, No. 952, is found. 



At the Cape Pigeon Eocks the large dyke is found to be a hornblende plagioclase 

 pyroxene gneiss. Garnet is present and illustrates the relation with some of the Stillwell 

 Island gneisses. Hornblendisation is prominent but not sufficient to mask the relation 

 of the gneiss to the plagioclase pyroxene gneisses. The narrower dykes on this area 

 show a much greater degree of hornblendisation than the large dyke. The percentage 

 of pyroxene decreases from 26-8 per cent, to 7-6 per cent, in No. 782, and is zero 

 in others. These last are amphibolites. A coarser amphibolite comes from this area 

 which did not maintain the dyke form in the field. It is a rock which is clearly related 

 to the dyke rocks, and the relation is considered to be the same as that between the 

 coarse-grained amphibolite patches at Cape Denison (No. 9 type) and the well-defined 

 dyke bands. 



