THE METAMORPHIC ROCKS OF ADELIE LAND BTILLWEI.L 109 



permanent change of form without conspicuous fracture. This permanent change is 

 supposed to be accomplished by interior readjustments of rock substances by chemical, 

 mineral, and mechanical changes, and produces the slaty cleavage and schistose 

 structures. The latter are included by Leith under the one term " flow cleavage "*. 



We find, however, that there are objections to the use of these terms in a treatment 

 of metamorphic rocks. It is desirable to analyse and distinguish more carefully the 

 interior readjustments which are combined in rock flowage. We need to distinguish 

 between slaty cleavage and crystalline schistosity, which are separated by the degree 

 of the all-important recrystallisation, while all the physico-chemical conditions of 

 recrystallisation are not included in the zone of rock flowage. A zone of rock flowage 

 implies a zone where there is a dominating stress combined with a hydrostatic pressure, 

 and it will not include a zone of very high hydrostatic pressure with less important 

 stress ; and under such conditions we picture certain recrystallisations. As a general 

 term " rock flowage " includes too little and as a restricted term too much. 



Primary gneissic banding or primary pressure banding has been frequently recorded 

 on the margins of igneous masses. They are terms which are frequently supposed to 

 involve movement in the rock magma. We consider, however, that the primary pressure 

 banding, apart from injection banding, can be considered as a metamorphic texture 

 without any appeal to fluxion or moving magma, and that it is identical with the 

 schistose structure produced by recrystallisation under stress. The coarse granularity 

 and holo-crystalline character of even-grained plutonic rocks indicate that there has 

 been no movement in the magma during its consolidation. Large crystals do not grow 

 uniformly in moving solutions, and it is difficult to see how the symmetrical arrangement 

 of the mineral constituents in a schistose margin can be produced by movement in a 

 viscous, semi-solid rock. For the injection of rock magma against the weight of the 

 overburden of enveloping rocks we must postulate big orogenic forces. If these forces 

 continue after the magma has been brought to rest they are distributed through the 

 magma only as a hydrostatic or uniform pressure. Cooling and consolidation proceed 

 under the uniform pressure as in any normal case. Though the crystallisation may 

 be uniform throughout the whole mass, we imagine that the outer margin will become 

 solid before the centre. If, after the development of this solid crust, the pressure causing 

 intrusion be still maintained in the molten portion, the hydrostatic pressure in the molten 

 portion will be exerted normally on all parts of this crust, which is then affected as if 

 subjected to a stress (non-uniform pressure) or a squeeze (fig. 10). This stress, combined 

 with the other factors of temperature, etc., produce, not movement, but the stable 

 molecular rearrangement and the gneissic banding in the manner most recently 

 expounded by Grubenmannf, Johnston, and NiggliJ, and proved experimentally by 

 Wright. This stress may also possibly be produced by the expansion caused by the 



* " Structural Geology," C. K. Leith, Constable & Co., 1914, p. 76. 



f Op. cit., vol. I., p. 42. 



t Op. cit., p. 610. 



{ " Schistosity by Crystallisation," F. E. Wright, Amer. Journ. Sci., vol. 22, p. 224. 



