30 AUSTRALASIAN ANTARCTIC EXPEDITION. 



We can notice in general that the biotite-rich varieties belong to the western side 

 of Cape Denison and that the hornblende-rich varieties belong to the eastern side. These 

 two groups are separated geographically, with the exception of No. 5, by bands in which 

 biotite and hornblende are both important. The exception, No. 5, loses its importance 

 when we reflect that it is not an example of a typical band. This uniform mineral 

 variation will be considered later to be a reflection of the variation in chemical 

 composition. The geographical nature of these variations is important, and can have 

 but two possible explanations. The explanation may be a metamorphic one, and 

 we may consider the reason to be a uniformly varying set of metamorphic conditions 

 across Cape Denison combined with a uniform degree of migration of material 

 corresponding with differences in chemical composition. The detailed examination 

 indicates that we really get considerable variation in the conditions in localised patches, 

 and thus scarcely supports this explanation. The alternative consists in regarding the 

 whole as a differentiated series of primary basic dykes. In either case it does signify 

 that we are not dealing with bands that have been repeated by folding. 



With a high percentage of mica there is always found a high percentage of epidote. 

 The only important exception to this rule is again No. 5. No. 720 is a partial exception, 

 but a relatively low percentage of epidote is compensated by a high percentage of hydrous 

 lime silicate, lawsonite. Nos. 720 and 634A possess a percentage of ferromagnesian 

 minerals very considerably above the average while the felspar percentage is 

 correspondingly low. It is possible that they are chance specimens which are not 

 strictly normal of the bands they represent. Unfortunately, there is no systematic 

 set of specimens collected for the purpose of studying the longitudinal variation in 

 any particular band. Only in exceptional cases is the variation noticeable in the hand 

 specimen. In cases where more than one specimen has been examined from one band 

 some degree of variation is always noticeable. 



The table renders it evident that the majority of the examples are rich in sphene, 

 but the accessories show considerable variation. Apatite is relatively high in some 

 cases and practically absent in others. The iron ores show much greater variation, and 

 vary from 6 per cent, to instances where it is almost negligible. The complete discussion 

 of this table of mineral constituents must await the presentation of the microscopical 

 and chemical characters. 



Microscopical Characters. 



No. 153. This rock is rather fine grained and schistose, but it is not so particularly 

 fissile as some of the hornblende varieties. The glint of the mica is obvious on the 

 cleavage faces. In thin section the constituents conform to a linear arrangement 

 and produce the crystallisation schistosity (Plate I., fig. 3). The biotite is strongly 

 pleochroic in sections showing cleavage from a light straw to a deep brown colour ; 

 it is reddish brown in basal sections. Pleochroic haloes are sometimes found around 

 inclusions which appear to be sphene. Hornblende is sometimes included in the biotite 



