42 AUSTRALASIAN ANTARCTIC EXPEDITION. 



No. 153. No. 629. 



BaO 0-05 . . nil 



Li 2 trace . . trace 



= C1 0-02 .. 0-01 



= 8 . 0-01 0-02 



Total . 99-63 .. 99-86 



Specific gravity 2-953 . . 3-030 



These two analyses bear important resemblances. The silica percentages are 

 relatively low and bear approximately the same ratio to the alumina. The total iron 

 is almost identical in the two cases. Both percentages of magnesia are lower than the 

 percentages of lime, and, further, the ratio of the magnesia to the lime is the same in 

 each case. The soda percentages are not far different, and the water content is similar. 

 Both are rich in titanium, and, in general, the similarity is- sufficiently strong to emphasise 

 the field observation that the two samples are of common origin. 



At the same time the differences are interesting and important when compared 

 with the relative mineral compositions expressed in Table 1. The high mica percentage 

 in No. 153 involves a higher silica percentage and a noticeably higher percentage of 

 total alkalies with potash in greater amount. The high hornblende percentage in No. 

 629 involves the correspondingly lower silica, the much higher percentages of magnesia 

 and lime, and the much lower alkali total. The amounts of felspar are approximately 

 the same in each case, and hence the alkali percentage of No. 629 gives approximately 

 the amount of alkali in the felspar, and the extra amount in No. 153 can be attributed 

 to the mica. There is considerably greater quantity of iron ore in No. 153, and its 

 amount of Fe 2 3 is correspondingly greater. There is no corresponding variation 

 with FeO, as varying quantities of FeO are required for the ferromagnesian constituent. 

 The larger amount of sphene in No. 153 is also partly responsible for its higher titanium 

 percentage, but the differing percentages of P 2 5 are precisely reflected by the differing 

 percentages of apatite. The chlorine is probably associated with the apatite, and 

 thus appears in greater amount in No. 153. The sulphur is derived from the very 

 occasional grains of pyrite. Cr 2 3 , MnO, NiO, and CoO are no doubt contained in 

 the ferromagnesian. Finally, No. 153 is notable for its definite percentage of barium. 



Since No. 153 expresses the composition of the bands with high mica content on 

 the western side of Cape Denison and No. 629 gives the composition of the amphibolites 

 on the east, the intervening bands, containing varying proportions of biotite and 

 hornblende, can confidently be expected to possess a chemical composition within the 

 limits of these two extremes. The actual variation could, indeed, be approximately 

 estimated from the mineral content expressed in Table 1 e.g., the greater the mica 

 percentage the nearer will the silica percentage approach that of No. 153. The percentage 

 of the minor constituents, like Ti0 2 and P 2 5 , will vary in much the same manner 

 as the corresponding accessory minerals. 



