150 AUSTRALASIAN ANTARCTIC EXPEDITION. 



through serialisation and partly through saussuritisation. Scattered through these 

 felspathic areas are small and large crystals of monazite, while in local patches there 

 are numerous small crystals of corundum. The corundum is more or less rounded in 

 cross sections, while longitudinal sections are long prisms with evidence of basal cleavage. 

 The rock may be called a garnet felspar gneiss. 



The Junction of the Cyanite Biotite Gneiss with Amphibolite. Specimens showing the 

 junction of this type of gneiss with the amphibolite are in the collection. Macroscopi- 

 cally there is a short and rapid transition from the gneiss to the amphibolite. The 

 line of junction is straight, and by no means indented as it would be if the gneiss had 

 been eaten away by the invading amphibolite. The dykes are relatively small and the 

 transition can not readily be explained by assimilation. 



Under the microscope there is perfect crystalline continuity across the junction, 

 and hornblende appears and increases in quantity with the diminution of biotite and 

 garnet (Plate III., figs. 5 and 6). The cyanite and quartz seem to travel further into 

 the amphibolite than the biotite and garnet. 



The gneiss in the specimens is similar to the biotite cyanite gneiss No. 772, except 

 that the garnet is more abundant and the character of the cyanite is different. The 

 cyanite possesses here pronounced lamellar twinning, and its polarisation colours reach 

 the lower part of the second order colours. Quartz in the section never shows a higher 

 polarisation colour than a very pale yellowish white, and hence the double refraction 

 of this cyanite must reach at least 0-019. The highest recorded value for cyanite 

 is 0-016. Yet it seems necessary to associate this mineral with cyanite. It 

 is colourless, with a cleavage parallel to the twin lamellae. Cross sections, which show 

 indistinct twinning, show two cleavages, both of which are oblique to the direction 

 of the twin lamellae. Crystal outlines are completely absent and it appears in irregular 

 plates with marked sieve structure. The abundant inclusions consist of biotite, garnet, 

 quartz, felspar, and ilmenite. Its refractive index is about the same as the other cyanite. 

 Extinction angles have been measured up to 28 from the lamellae, but they are often 

 less. In the region of hornblende it is found to pass by direct transition into hornblende. 

 Part of an individual crystal may be green hornblende, and part the colourless cyanite, 

 and, further, the cleavage continues indiscriminately through the green and colourless 

 portion. The hornblende usually has a smaller angle of extinction, measured from 

 the trace of the lamellae. The regular arrangement of hornblende with the cyanite 

 in the hornblendic part of the section is in contrast with the irregular inclusions of 

 biotite in the cyanite on the biotite part of the slide. 



The cyanite extends some distance out into the amphibolite, and some of it may 

 be found in most sections of the amphibolite. Unfortunately, the number of specimens 

 is limited, and the collection is too incomplete for us to be able to deal fully with this 

 case of migration. Nevertheless, it is certain that the amphibolite dyke intruded the 



