156 AUSTEALABIAN ANTARCTIC EXPEDITION. 



In the hand specimen it is a coarse brownish-coloured rock, showing quartz, felspar, 

 and hypersthene, and a little biotite. In section it is a coarse granoblastic aggregate. 

 The quartz shows a little granulitisation. The felspar consists of both twinned and 

 untwinned varieties. Peg structure and diablastic structure are common in the felspar. 

 The development of the diablastic structure by the dissociation of plagioclase crystals 

 in situ is very plain in some instances. Lamellar twinned felspar has again a refractive 

 index occasionally higher than that of the basal quartz, and it is probably andesine. 



A brown biotite is the most abundant ferromagnesian mineral. It may be 

 associated with ilmenite, and it is found in the hypersthenic areas. The hypersthene 

 in the section is largely decomposed to the serpentinous greenish alteration product. 

 Biotite is intimately associated with the serpentine and seems to develop from it in a 

 pale-green form. Sometimes the hypersthene loses its iron content, becomes colourless, 

 assumes lower polarisation colours, and changes into enstatite. In places a very pale- 

 green biotite is associated with the hypersthene, and this seems again to be an 

 intermediate stage between brown biotite and the green delessite. Occasionally large 

 crystals of apatite and zircon are present. Grains of ilmenite and fragmentary garnet 

 are near the hypersthene. The garnet may form fragmentary rims around the biotite, 

 and occasionally the biotite is grouped in radial sprays. 



In a second slide of No. 979, cut from the opposite end of the specimen, the sprays 

 of radial biotite are more prominent surrounding the ilmenite nuclei (Plate V., fig. 3). 

 Some of the biotite flakes are associated with fan-shaped felspar vermiculse, as in Plate 

 V., fig. 4. The association of quartz with the biotite sprays is also noticeable, especially 

 in the aggregates of small basal biotites and quartz, which are in some cases cross sections 

 of biotite sprays. This slide also contains a plate of basal biotite which is surrounded 

 by a rim of later biotite straws set in quartz, which in turn has a thin coating of iron 

 ore (Plate VI., fig. 4). The garnet is more abundant in this slide and may be detected 

 as fragmentary rims around ilmenite as well as biotite. These features will be found 

 to be better developed in the next example, No. 947. 



In this case we have also noticed large crystals of untwinned plagioclase with 

 inclusions of orthoclase distributed in the same manner as the schiller inclusions in 

 olivine in peridotite from the Isle of Rum*. The appearance is not unlike a graphic 

 structure, but it is distinct from the vermicular intergrowths in the same slide. The 

 inclusions of orthoclase have a considerably lower refractive index than the untwinned 

 plagioclase (andesine), and the larger pieces contain minute fusiform inclusions of a 

 felspar with higher refractive index. The crystal plate of plagioclase possesses cleavage 

 which extends in places across the orthoclase inclusions, and the extinction angle, 

 measured from the cleavage, is 4. When the stage is rotated in the opposite direction 

 an extinction angle of 3 is measured in the orthoclase inclusions. The complete 

 recrystallisation of this rock, together with the abundant metamorphic felspar 

 intergrowths, indicates that this graphic-like structure is also of metamorphic origin, 

 and possibly connected with diffusion phenomena. 



* " Natural History of Igneous Rocks," A. Barker, p. 258. 



