THE METAMORPHIC ROCKS OF ADELIE LAND.-STILLWELL. 



153 



The outstanding feature of these analyses is the high percentage of alumina, a 

 considerable excess over the 1 to 1 ratio necessary to satisfy the lime and alkalies. In 

 addition there is, in both cases, a dominance of MgO over CaO, and of K 2 over Na20. 

 Bastin's criteria of sedimentary origin* are therefore satisfied. 



The total alkali percentage of No. 772 is high, and finds mineral expression in the 

 abundance of biotite and of felspar. The felspar is less important in No. 784, and there 

 is a greater dominance of K 2 over Na 8 than in No. 772. The great dominance of 

 alkalies over CaO in each case is an important factor when considered with the silica 

 percentage. The low CaO percentage of No. 784 means that there can be little lime 

 in the abundant garnet and probably, also, little CaO in the plagioclase. In No. 772, 

 where the plagioclase is more important, there is three times as much CaO as in No. 784, 

 but there is still the large excess of MgO. 



The high alumina percentage is reflected in the Group Value T, and the values 

 for T are again indicative of sedimentary origin. T is lower in No. 772 because the 

 higher alkali percentage absorbs more A1 2 8 . The value F, which expresses the 

 ferromagnesian constituents, is very high in both cases. As a consequence the projection 

 values are dominated by the excessive value of f. No. 784 occupies a position (fig. 12) 



8 C 



Fig. 12. 



II. Mean Position of Group II., the Aluminium Silicate Gneisses. 



III. Mean Position of Group III., the Plagioclase Gneisses. 



784. Garnet Cordierite Gneiss, Cape Gray. 



772. Cyan te Biotite Gneiss, Garnet Point. 



- Chemical Composition a* a Criterion in Identifying Metamorphoaed Sediment*," E. S. Baitin, Joum. Geol., 1909 

 Tol. 17, p. 445. 



