THE MBTAMORPHIC ROCKS OF ADELIE LAND 8TILLWELL. 83 



large mass of amphibolite. The same general metamorphic conditions have been applied 

 to each, and now the general foliation is diverted by the amphibolite block and not by 

 the gneissic block. There is some disparity in size between the two typical cases, but 

 we cannot see that any such disparity can provide adequate explanation. 



It has been stated that the gneissic xenoliths seem to show a greater degree of 

 recrystallisation than the granodiorite or aplite gneisses. This is probably to be 

 explained by the degree of recrystallisation of the host. We obtain the following data 

 from Van Hise * : The change of augite to hornblende is exothermic, and, for an 

 assumed average composition, the increase in volume is 4-30 per cent., provided all 

 the resulting compounds are solid ; the change of augite to biotite is exothermic and, 

 for an assumed average composition, the calculated increase in volume is 17-26 per cent. ; 

 the change of felspar into each component of saussurite is exothermic and involves 

 expansion of volume. Hence we can be quite certain that the recrystallisation of the 

 dolerite which involves these changes has been accompanied by an expansion of volume 

 and a liberation of heat. The recrystallisation of the granodiorite is not so complete 

 as the recrystallisation of the dolerite. We may, then, imagine that the small gneissic 

 xenolith enclosed in the relatively large mass of amphibolite has been exposed to greater 

 pressure and higher temperature than the main mass of the granodiorite gneiss. As a 

 result the small gneissic xenolith shows a different degree of recrystallisation than the 

 granodiorite gneiss. 



If a small mass of rock be enclosed within a larger mass of another type and the 

 whole subjected to metamorphic conditions, then I think it would be generally expected 

 that the foliation would travel independently through the two types as has happened 

 in the case of the gneissic xenolith. We would, therefore, be inclined to view the diverted 

 foliation as the abnormal case, even though it has been more commonly observed. As 

 the general metamorphic conditions are the same in both cases, the only important 

 difference lies in the greater expansion in volume of the amphibolite which is directly 

 due to the chemical rearrangement. In this expansive effect we are forced to conclude 

 must lie the cause of the diverted foliation. 



* " Treatise on Metamorphism," C. R. Van Hise, pp. 277, 278. 



