56 AUSTRALASIAN ANTARCTIC EXPEDITION. 



seen in the Bendigo mines, and a broken surface outcrop in a metamorphic series is not 

 unfavorable when such can appear in the unaltered series at Bendigo. The detached 

 fragments of the dykes which are encircled by gneiss, and which could be mistaken 

 for inclusions caught up by the invading magma, are undoubtedly related to and belong 

 to the dyke magma. 



Xenolith Evidence. The discovery of metamorphosed xenoliths in one outcropping 

 band of amphibolite is very important evidence of igneous and intrusive origin. The 

 fragments of gneiss with sharp boundaries and with marked likeness to the surrounding 

 gneiss possess a composition fundamentally different from that of the amphibolite host. 

 The saussuritic type of meta-xenolith is one that might be expected to come from the 

 magma reservoir from which the dykes issued. Knowing the granitic -nature of the 

 surrounding gneiss, it is impossible to conceive these xenoliths as undigested fragments 

 of an igneous or of any other pre-existing rock. 



Structural Evidence. No relic of any kind of sedimentary structure is to be found. 

 The typical granoblastic structure is in this case more suggestive of igneous origin. 



Mineralogical Evidence. It would be difficult to account for the suite of minerals, 

 particularly the abundant saussuritised felspar, the relic felspar, and some well-formed 

 apatite crystals, on any other hypothesis than that of igneous origin. The uniform 

 variation in mineralogical composition of the different members of the series, which is 

 illustrated in Table I., and which reflects uniform variation in chemical composition, 

 indicates an igneous differentiated rock series. 



Chemical Evidence. The chemical analyses bring forward strong evidence of 

 derivation from doleritic rocks. The analyses are similar in all essential points with 

 analyses of diabases or dolerites. The definite grouping, on quantitative data, among 

 the amphibolite group of the crystalline schists, is further evidence when we recall that 

 many members of this group have arisen from diabasic dykes*. 



The total evidence is thus conclusive that this suite of rocks from Cape Denison, 

 conformable to the general foliation of the country, is the metamorphosed equivalent 

 of a system of parallel igneous dykes. The dykes have intruded the granite prior to 

 the development of the foliation. The granodiorite and dykes have then suffered the 

 same metamorphic conditions with varying amounts of recrystallisation. The 

 surrounding granodiorite excludes any possibility of the amphibolites representing 

 altered bedded tuffs. 



The nature of the primary dyke corresponds with a diabase or a dolerite whose 

 mineral composition has been calcic felspar (labradorite), pyroxene, biotite, ilmenite, 

 and apatite. No trace of serpentine is found, and, as serpentine can be preserved 

 under epi zone conditions, it is concluded that no olivine was present in the primary. 



* " Die Kristallinen Schiefer," vol. II., p. 94. " Data of Geochemistry," F. W. Clarke, Bull. 330, U.S.A. Geol. Surv., 



p. 508. 



