THE METAMORPHIC ROCKS OF ADELIE LAND. 8TILLWELL. 91 



features. Microcline and perthite individuals are present, but there is a greater amount 

 of sericite. Relic plagioclase lamellae can be recognised in large sericitic masses. The 

 development of sericite can be found along shear planes in microcline crystals. There 

 is also some kaolinisation of the orthoclase. Fracturing and granulation of the quartz 

 and felspar is more prominent than in the preceding example, and so also is mortar 

 structure. The diablastic structure is not common and mostly in incipient stages. 

 The mica content is small and includes green chlorite, green biotite, and white muscovite. 

 The chlorite and biotite are often associated in one individual. Grains of epidote are 

 associated with the mica. A little calcite is present, and magnetite, apatite, monazite, 

 and allanite are again accessories. The epi zone characters are here considered dominant, 

 and the example is described as an epi alkali felspar gneiss, poor in mica, or as an epi- 

 aplite gneiss. 



In further examples the amount of sericite may increase sufficiently to yield sericite 

 gneiss. Occasionally a relic garnet is found with considerable development of green 

 chloritic products along the cracks. The percentage of ferromagnesian minerals increases 

 towards the margin of the bosses as we pass outward through metamorphic diffusion 

 types into the granodiorite gneiss. 



3. INTERPRETATION OF CERTAIN VARIATIONS IN THE 

 GRANODIORITE GNEISS. 



Since the boundaries of the aplitic masses with the granodiorite are in places 

 destroyed and replaced by metamorphic diffusion types, there is no a priori reason why 

 such diffusion types should not, under favourable circumstances, extend across the whole 

 width of the vein. In such contingencies the vein will completely lose its identity 

 and become part of the main gneissic mass. A study of metamorphic diffusion 

 specimens indicates that this has actually taken place. Rocks which were collected 

 in the field as varieties of the granodiorite gneiss are now considered to be diffusion 

 types. This is particularly the case with gneisses collected from the locality by the 

 magnetograph house, which is the precise locality from which the conception of meta- 

 morphic diffusion is developed in the case of the amphibolites. 



In this case the gneiss has a lighter colour, due to the absorption of some quartzo- 

 felspathic material. The composition and the granularity are variable and the texture 

 is usually more massive. A feature of the locality is the abundance of monazite and 

 allanite (Plate XI., fig. 6). The allanite is found in exactly the same manner as noted 

 in the aplitic gneiss, i.e., frequently with a radial arrangement of felspar around it. No 

 definite gneissic vein is recorded from the precise point where the examples were collected, 

 and they are very similar to the metamorphic diffusion products on other parts of Cape 

 Denison. 



Specimen No. 60 is an example of the pale grey gneiss of this type. The chief 

 constituents are quartz, orthoclase, perthite, and microcline. The ferromagnesian 



