THE METAMORPHIC ROCKS OF ADELIE LAND. 8TILLVVELL. 157 



The rock may be described as a meso-hypersthene felspar gneiss. We may conclude 

 that the hypersthene is less in quantity than in No. 949, because more of it has reacted 

 with the felspar to produce biotite and ilmenite. It is certain that the type is more 

 closely related to No. 949 than to No. 917, the garnet felspar gneiss. It has only been 

 reported as indefinite bands associated with the garnet gneisses, which are considered 

 to be sedimentary in origin, but it possesses undoubted affinities to igneous rocks and 

 to No. 949, which occurs as a dyke. It is quite likely, therefore, to be the metamorphosed 

 equivalent of an acid dyke whose identity has been wholly or partially lost by the 

 operation of metamorphic diffusion. 



No. 947. Another example of the hypersthenic gneiss was obtained from the 

 summit of the island. It is a coarse rock similar in outward appearance to No. 979. 

 The brownish colouration of the rock is a little more prominent, and, at the same time, 

 more like the brownish coloured rocks of Madigan Nunatak. While related to the 

 preceding example, No. 979, it differs from it in possessing much more garnet, more 

 pyroxene, less biotite, and very little quartz. Yet a rough determination of the specific 

 gravity gave 2-74 in the. first case (No. 979) and 2-76 in the second (No. 947). 



The place of the quartz in No. 979 is taken by areas of untwinned felspar (orthoclase) 

 with cryptoperthitic inclusions. Augite, as well as hypersthene, is present. The pink 

 garnet appears in two ways : it may appear first as large crystals with felspar, biotite, 

 and ilmenite inclusions, or it may appear as granular garnet surrounding ilmenite, biotite, 

 and hypersthene. The garnet rims around the ilmenite and biotite clearly follow closely 

 all the irregularities in shape of the ilmenite and biotite nuclei (Plate V., figs. 5 and 6). 

 Biotite and ilmenite are often associated with the pyroxene, and have no doubt been 

 formed from it in part in the familiar reaction with felspar. The biotite may partly 

 enclose the pyroxene and it may fill cracks and indentations in the pyroxene crystals. 

 Further, as the garnet rims around the pyroxene may be in part separated from the 

 pyroxene by biotite, we can fairly safely conclude that an explanation of the garnet 

 around the biotite will provide an explanation of the garnet rim around the pyroxene. 

 Again, we can find crystals of ilmenite symmetrically enclosed by a biotite zone, and, 

 if this biotite zone were replaced by a garnet zone, we should get the garnet rims 

 around ilmenite as are observed. Consequently, an explanation of the garnet-biotite 

 reaction will also provide an explanation of the garnet zone around ilmenite. This 

 conclusion is supported by the discovery of an aggregate in which an ilmenite crystal is 

 surrounded by biotite, and this in its turn is practically surrounded by garnet, while 

 pyroxene crystals jut against it in part. 



It is therefore clear that biotite is on one side of the equation and garnet on the 

 other. Sometimes outside the garnet rim a change in character of the plagioclase is 

 quite evident. Hence the plagioclase may be considered to take part in the reaction 

 and to supply a lime molecule which may enter the garnet. The following equation 

 shows that for average compositions of biotite and garnet the change is chemically 



