52 AUSTRALASIAN ANTARCTIC EXPEDITION. 



examples have a pure white colour, and others, again, have a vitreous grey colour 

 which is suggestive of a colour change during recrystallisation. 



The shape, in many instances, is clearly angular and fragmental, and the corners 

 may be well preserved. Frequently the gneissic fragment is drawn out into a lenticular 

 shape in the direction of the schistosity (Plate X., fig. 1). In this example the lenticular 

 bodies are not symmetrical to the schistose plane. A side view of the same specimen 

 is shown (Plate X., fig. 2). Here a meta-xenohth at the upper right hand corner is almost 

 triangular in outline, yet the schistosity of the rock can be distinctly seen to follow 

 through the inclusion from the amphibolite irrespective of its shape. Hence the 

 amphibolite and the fragment must have formed a single unit before the reception 

 of the metamorphic impress. In fewer cases the cross section is elliptical and, therefore, 

 symmetrical to the schistosity ; in such examples recrystallisation and rearrangement 

 are evident. The back and front views of another specimen are illustrated on Plate 

 X., figs. 3 and 4, where the gneissic meta-xenoliths are not lenticular but possess an 

 angular and variable shape. 



The outline of the gneissic inclusion is often clear and sharp, though we may find 

 it slightly embayed. There are instances, however, where the entire boundary is lost 

 and replaced by a transition between the amphibolite and the white gneiss. An 

 inclusion is also observed where part of the boundary is sharp and part indistinct. This 

 indefinite boundary might be accounted for by postulating chemical action between 

 the xenolith and the host before the metamorphism. Such, however, is not a necessary 

 hypothesis, because evidence will be produced later which leads us to discount the normal 

 face value of transitions in metamorphic rocks, and to believe that such transitions 

 can arise during the progress of the metamorphism. That there has been an adjustment 

 of molecular equilibrium along the junction during metamorphism seems evidenced 

 by the lines of amphibolite which may be sometimes seen threading their way from the 

 host in the direction of the schistosity of the inclusion. Though the junctions may be 

 sharp there is perfect crystalline continuity and an interlocking of crystals across them. 



In thin section (No. 628-3) the fragments are found to be clear granoblastic 

 aggregates of quartz and felspar (Plate II., figs. 1 and 2). The grains have a tendency 

 to be rounded or elliptical, and are of moderately even size, averaging about -16mm. 

 in diameter. Actually each grain has irregular outline, is much embayed, and always 

 interlocks with its neighbour. Undulose extinction is marked in the quartz and some- 

 times in the felspar. The felspar often possesses lamellar twinning, and as its refractive 

 index is near that of quartz, and sometimes above, it is identified as andesine. Small 

 crystals of biotite and chlorite are distributed through the mass and show a tendency 

 to parallel arrangement. Hornblende is present in rather larger crystals, and epidote, 

 clinozosite, and other saussuritic products are scattered in groups with a tendency to 

 linear distribution. Magnetite and pyrite are accessories. 



In other cases (No. 628-6) porphyroblasts of quartz and felspar are found. They 

 possess the lenticular cross section, and are relics of the primary minerals. The quartz 



