THE METAMORPHIC ROCK8 OF ADELIE LAND. 8T1LLWELL. 169 



however, shows that foliation can be traced in part of the dykes. Most of the dyke 

 specimens look like dense fine-grained basalt, except that a vitreous appearance is 

 more noticeable on the fractured surface. Microscopic examination shows that they 

 are not normal igneous rocks, and we find a definite metamorphism of a varying kind. 



No. 773. No. 773 is an example of the massive rock. The structure is finely 

 granoblastic and relic structures can be seen. The outlines of the felspar laths of the 

 primary dolerite are plainly visible (Plate VI., fig. 5), and are marked by lines of granular 

 augite producing a blastophitic structure. Diablastic structure is produced by an 

 intergrowth of augite and felspar. The mineral composition is as follows : 



Felspar 40-5 



Pyroxene 45-3 



Hornblende 3-4 



Ilmenite 6-6 



Biotite 4-2 



The original felspar laths are replaced by a granoblastic aggregate of clear secondary 

 felspar, which becomes evident in polarised light (Plate VI., fig. 6). The average absolute 

 grain size of the aggregates is 0-05mm. In some places relic felspar is found ; it is always 

 dusty with minute inclusions, and, therefore, appears in contrast to the clear secondary 

 felspar. The relic felspar is both simple twinned and lamellar twinned, and an extinction 

 angle of 29 has been measured from the trace of the lamellae. This felspar is, therefore, 

 labradorite. The relic felspar is usually surrounded by a zone of granular clear felspar, 

 which may contain vermicular grains of pyroxene. The refractive index of the 

 secondary felspar is less than that of the labradorite, but the difference is not great as it 

 is not noticeable under low power objectives. The maximum extinction angle obtained 

 in pieces showing twinning is 18, and hence we consider it to be an andesine. The 

 pyroxene forms 45-3 per cent, of the rock and includes hypersthene and augite and relic 

 augite. The relic augite is dusty through numerous minute inclusions of ilmenite. 

 These inclusions are more or less regularly arranged and may be called schiller inclusions. 

 Plates of this dusty augite have been found which have been ophitically inlaid with 

 laths of relic felspar, now represented by strings of secondary felspar. The recrystaliisa- 

 tion of the primary dusty augite has produced a granular aggregate of clear secondary 

 pyroxene, while the minute dusty inclusions have coalesced and now form a number 

 of small ilmenite crystals. The clear granular pyroxene sometimes forms a zone around 

 the dusty augite, but it may appear as a parasitic aggregate enclosed within the primary 

 dusty plate. Some of this granular pyroxene is certainly hypersthene with its pink 

 to green pleochroism and its straight extinction ; but it is impossible to determine its 

 proportions to the secondary augite. 



The development of the secondary augite and hypersthene from the primary augite 

 means that the secondary augite will be more aluminous than the primary augite. A 

 high value for alumina in this secondary augite provides a point of resemblance to the 



