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AUSTRALASIAN ANTARCTIC EXPEDITION, 



The modal mineralogical contents of the rocks (determined by use of the Leitz 

 Integration Table) are shown in the following table where they are compared with that 

 of the reference specimen, No. 794 (determined by Dr. Stillwell). 



It will be seen that the general tenor of the rocks is the same, yet, notwithstanding 

 this, notable variations occur. The felspar ranges from a minimum of 36-3 per cent, 

 in No. 792 to a maximum of 55-0 per cent, in No. 775. The pyroxene of No. 775 is 

 least, viz., 32-4 per cent, (the same amount as in No. 783) and that of No. 974 is most, 

 viz., 45-5 per cent. A remarkable variation in the amount of hornblende is shown by 

 the table. It is practically absent from No. 783 and yet in No. 792 it forms over one-fifth 

 of the rock. A correlation between the inverse of felspar and hornblende can be drawn. 

 Considerable variation is also shown in the biotite content, it reaches 13-9 per cent, in 

 No. 783 and practically disappears in Nos. 788 and 794. The iron ore content is nearly 

 uniform, although No. 792 differs somewhat from the rest. 



A triangular graph (Fig. 16) showing percentages of felspar, F, pyroxene, P, and 

 metamorphic matter, M (viz., hornblende + biotite + iron ores) is given. A serial 

 relationship from C to E to A to B to D is seen. This shows the progression from 

 minimum metamorphic matter in C to the maximum in D. (The letters A, B, C, D, B 

 refer respectively to rock specimens Nos. 775, 783, 788, 792, 794.) 



The diagram suggests that part of both the original felspar and pyroxene is taken 

 up in the formation of these metamorphic products (hornblende, biotite, and iron ore), 

 but this aspect of the matter will be treated more fully in part (c) below. 



(b) A Discussion of the Mineral Characters of the Rocks. 



As the Rosiwal analysis shows there are five main minerals present, felspar, 

 pyroxene, amphibole, biotite, and iron ore. Accessory minerals include apatite and 

 zircon. 



The felspar present in all four rocks is plagioclase, although a little antiperthite 

 is found in them all. 



The natures of the various plagioclases present have been determined by 

 refractive index methods and by the maximum extinction angles in the symmetrical 

 zone. 



