54 AUSTRALASIAN ANTARCTIC EXPEDITION. 



of saussurite show that we are not dealing with a porphyritic dyke rock, while the 

 irregular and local distribution is strong evidence of cognate xenoliths. No one can 

 suppose that the meta-xenolith in Plate X., fig. 5, could be anything but a fragment 

 of a pre-existing felspar crystal. Large calcic felspars do develop in amphibolites 

 under the metamorphic conditions of the kata zone or the lower meso zone. An 

 example of this nature (No. 212) was found among the boulders on the moraine and 

 the porphyroblasts (Plate IX., fig. 5) do not bear a trace of decomposition in thin 

 section. Such crystals would become saussuritised if subjected for a sufficient length 

 of time to the conditions of the epi zone. We have found no evidence to suggest that 

 any of the amphibolites found in situ on Cape Denison have been subjected to the 

 kata zone conditions, and no such hypothesis would explain the extraordinary local, 

 irregular, and unsymmetrical distribution. 



The composite variety of saussuritic meta-xenolith is also best explained as a 

 metamorphosed cognate xenolith. It is not likely that they are unabsorbed residuals 

 of primary rock which has survived the metamorphism. Their boundary is too definite 

 and they actually bear the same metamorphic impress as the amphibolite itself. They 

 are fragments of a rock of the same composition as the amphibolite, but of much 

 coarser grain than the primary dolerite. They have been cognate xenoliths brought up 

 from the magma reservoir, and probably represent differentiation products produced 

 by crystallisation in that reservoir. 



2. The Gneissic Type. It has been shown that this type of inclusion is foreign to 

 the enclosing amphibolite, but that it is related to the granodiorite gneiss which 

 surrounds the amphibolite. Their unsymmetrical character and arrangement can be 

 accepted as definite evidence that they attained their present situation before the 

 reception of metamorphic characters. There is no alternative but to consider them 

 as "accidental xenoliths" or fragments which have no genetic relation to the enclosing 

 amphibolite, and which have been caught up during the injection of the primary dolerite 

 dyke. Xenoliths have been frequently reported in the basaltic and doleritic dykes, and 

 such was the original nature of this amphibolite host. 



Significance of the Meta-xenoliths. 



The consideration of these different kinds of meta-xenoliths collected from the 

 same small area shows conclusively that their host is an igneous rock intrusive into 

 the surrounding granodiorite. There can be no question of bedded tuff. 



The marked angularity of some of the fragments means that the xenoliths did not 

 travel far along the dyke channel. The saussuritic type must have come from the 

 magma reservoir, and therefore the present surface must be close to the original magma 

 reservoir. The gneissic xenoliths may have been fractured from the walls of the dyke 

 channel and not necessarily from the roof of the magma reservoir. 



