180 AUSTRALASIAN ANTARCTIC EXPEDITION. 



are occasional large crystals of saussuritised felspar and neither pyroxene nor garnet 

 is present. The rock is a typical amphibolite. 



A shear zone, developed subsequently to the formation of the hornblende, can be 

 detected in this rock as in No. 767. Without the microscope the shear plane looks like 

 a thin vein running through the slide. Under the microscope it is again marked by a 

 line of decolourised hornblende, saussuritised felspar, and some fine, highly polarising 

 aggregates. The broken strings of pyrite do not appear in this case, but specks of 

 this mineral are found in this zone. 



These specimens of amphibolite, Nos. 782 and 771, were collected from the narrow 

 dykes on the Cape Pigeon Rocks, whereas the very broad dyke produces a hornblende 

 plagioclase pyroxene gneiss. We have insufficient data to determine whether this is 

 generally the case. It may be so, and it is quite possible that thin dyke sheets may tend 

 to become shear planes during the compression of a composite rock body, in which case 

 the thin dykes may be subjected to metamorphic conditions of the meso or epi zone 

 rather than those of the kata zone. 



No. 786. Specimen No. 786 was collected as an amphibolite associated with the 

 gneiss. It did not appear in the field as a definite dyke-like band. It is much coarser 

 grained than the other amphibolites, and felspar and hornblende are plainly visible 

 in the hand specimen. 



Under the microscope, however, it is found to be similar in kind to the altered 

 dyke rocks. The same type of green hornblende is again the most abundant mineral 

 and its development from the pale green pyroxene is apparent. The hornblende some- 

 times contains inclusions of sphene. Both quartz and felspar make up the colourless 

 components of the rock. There is a considerable amount of quartz which does not 

 show cataclasis or undulose extinction. Part of the felspar is aaussuritised and part 

 is quite clear. Labradorite has been recognised, but as some pieces of felspar have a 

 lower refractive index than basal quartz, there is some andesine or oligoclase as well. 

 Fragments of garnet are occasionally set in the felspar areas. Ilmenite, sphene, and 

 apatite are accessory minerals. 



The rock may be described as an augite amphibolite. 



We are inclined to think that this rock is related to the dyke bands at the Cape 

 Pigeon Rocks, in the same manner that the coarse-grained amphibolites (No. 9) at Cape 

 Denison are related to the corresponding amphibolite dykes. This example differs 

 from the coarse amphibolites of Cape Denison in the possession of augite and garnet ; 

 but in a like manner the altered dykes at the Cape Pigeon Rocks differ from the Cape 

 Denison series in the possession of augite and garnet. 



The history of the coarse-grained patches at Cape Denison is considered to be 

 probably associated with great stress which has rendered former dyke channels dis- 

 continuous. It is interesting to note that the area near No. 786 at the Cape Pigeon 

 Rocks has suffered intense crumpling (Plate XXV., fig. 2). 



