118 AUSTRALASIAN ANTARCTIC EXPEDITION. 



Epidosites, that are found in association with the Landewednack schists and with the 

 green schists and granulites of the old Lizard Head series, can be considered as 

 metamorphic differentiation products. 



At present there is no evidence to discriminate in age between the Landewednack 

 hornblende schists and the Traboe hornblende schists. The observed differences 

 can be explained by a varying temperature factor during the metamorphism and 

 by metamorphic diffusion. Caution is necessary in order that the rocks termed 

 " gabbro " are not misinterpreted. The flaser gabbro and the gabbro schist are 

 decidedly metamorphic rocks. It is also possible to consider the " normal gabbro " 

 as a slightly metamorphosed rock, but if it be maintained that the " normal gabbro " 

 is a true igneous rock it must be recognised that it is not the normal rock of the area 

 but a relic gabbro. In the latter case it is a remnant of the original gabbro that has 

 escaped metamorphism. 



The foliation of the dykes and sills of the Kennack gneisses can receive the same 

 explanation as the foliation of the gabbro dykes. Serpentine inclusions in the sills are 

 not evidence that serpentinisation occurred before the intrusion of the gneisses. They 

 are evidence that the gneissic sills are the metamorphosed equivalents of dyke sheets 

 that invaded the original peridotite. Finally, talc and tremolite are not to be considered 

 pneumatolytic products and associated with the intrusion of the granite gneiss into the 

 serpentine. They are metamorphic products, and must receive a metamorphic 

 explanation. 



6- THE VALUE OF CHEMICAL CRITERIA IN IDENTIFYING THE ORIGIN OF METAMORPHIC 



ROCKS. 



We may perhaps be permitted to add to the discussion on the value of chemical 

 criteria in determining the origin of metamorphic rocks. Bastin* has quoted authori- 

 tative opinion on the value of these criteria, and has summarised and discussed the 

 characteristics of fresh foliated rocks of sedimentary origin. The features indicating 

 sedimentary origin in a chemical analysis are set out by Bastin thus : 



1. Dominance of MgO over CaO is strong evidence. 



2. Dominance of K 2 over Na 2 has lesser critical value, but is suggestive. 



3. Presence of considerable excess of A1 2 S over and above the 1 to 1 ratio 



necessary to satisfy lime and alkalies. 



4. High silica may be indicative when supported by other criteria. 



When three or all of these relationships hold good the evidence for sedimentary 

 origin may be regarded as practically conclusive. 



With the appearance of Adam's and Barlow's memoir on the Haliburton-Bancroft 

 area, the value of these criteria seemed to be lessened, inasmuch as Adams and Barlow 



" Chemical Composition as a Criterion in Identifying Metamorphosed Sediments," E. S, Bastin, Journ. Geol., vol. 

 XVII., 1909, p. 445. 



