264 AUSTEALASIAN ANTAECTIO EXPEDITION. 







of the garnet. In section, the rock has a coarsely-crystalline schistose structure, and 

 the brown biotite is the most abundant mineral along some of the schistose bands. 

 The plagioclase is mostly andesine, and varies towards labradorite. It is mostly 

 clear, and only occasional grains show alteration. Pyroxene occurs in large crystals 

 along one schistose band, and in smaller crystals in other parts of the rock. It 

 includes hypersthene, augite, and fibrous diallage, from which ilmenite has 

 separated out. Only occasional crystals of green hornblende are present. Garnet 

 occurs in large pink crystals up to 6 mm. in diameter, with numerous inclusions, and 

 also in small idioblastic crystals. A little quartz is present, and there are large 

 accessory crystals of apatite and zircon. The rock may be called a garnet-biotite- 

 plagioclase-gneiss. It differs from members of the eclogite family in the large develop- 

 ment of biotite in place of pyroxene. 



MESO DIVISION. 



The conditions of the Meso zone metamorphism are considered to become more 

 important in the formation of the augite-amphibolites. Several examples, including 

 Nos. 380, 351, and 212, exist in the collection; and the percentage of hornblende in 

 these is in large excess of the percentage of pyroxene. They resemble the pyroxene 

 amphibolites described from the Cape Pigeon Rocks. 1 Of these specimens No. 212 

 is remarkable in possessing large, dark -green porphyroblasts of a very calcic felspar. 

 Some of these crystals are over an inch in width, and possess well-defined crystal 

 boundaries. The texture of the rock is massive, and the structure is typically 

 granoblastic and porphyroblastic. The mineral composition of the rock, excluding 

 the porphyroblasts and the accessory minerals apatite and ilmenite, is : 



Felspar 31-4 per cent. 



Hornblende 64-8 ,, 



Pyroxene ..3-8 ,, 



The proportion of felspar to the ferromagnesian in this case is practically the 

 same as in the Cape Denison amphibolites. The felspar is quite clear and unaltered. 

 Most grains show lamellar twinning and are highly calcic. The hornblende is green 

 and fresh, without the tinge of blue colour noticeable in many of the amphibolites 

 with Epi zone features. The pyroxene includes both augite and hypersthene. 



The porphyroblasts possess crystal outlines, and, like the felspar in the base, 

 are perfectly clear and free from traces of decomposition. They show complex 

 twinning, and, in addition to simple twinning, there are at least two sets of lamellar 

 twinning. They contain small inclusions of pyroxene and hornblende. The pyroxene 

 inclusions tend to occur in rounded grains, while the hornblende inclusions develop 

 their crystalline form against the plagiolcase. The inclusions in many places show a 

 linear arrangement along cleavage lines and twinning planes. The presence of these 

 inclusions and the fresh character of the plagioclase in the base of the rock indicate 



1 Op. oit., p. 180. 



