THE METAMOKl'HIC ROCKS OF ADELIE LAND. STILL WELL. 113 



conditions. As a necessary consequence gneissic banding appears parallel to the trend 

 of the dyke. The metamorphic character of the so-called gabbro dykes is therefore 

 caused in the first place by the same external metamorphic forces which are producing 

 serpentinisation, and in the second place by the simultaneous internal metamorphic 

 forces that develop during serpentinisation. Irregularity is, therefore, to be expected, 

 and it becomes easy to understand why it should be commonly observed (p. 98) that 

 the gabbro dykes are wrenched off by irregular planes of movement. The general 

 theory would cause the impression recorded by Flett (p. 98) that these veins have been 

 caught up in powerful but irregular movement ; that they have been softer and more 

 plastic than the peridotite which surrounded them and have yielded to stresses ; that 

 there has been an internal shearing which has crushed the minerals and set up a rough 

 foliation. It also yields explanation why there should sometimes be a development of 

 foliation (p. 96) in the serpentine walls parallel to a dyke junction. This foliation 

 in the serpentine is always parallel to the foliation in the dyke, is very similar to it in 

 character, and in some cases becomes gradually lost as one passes outward from the 

 edge of the dyke. 



Apart from external pressure the expansion of the peridotite will be more or 

 less symmetrical in all directions from the centre of the mass, and so there will arise 

 approximately hydrostatic pressure in the centre and non-uniform pressure towards 

 the margins. Such will be more or less the case, but the external pressure will tend to 

 destroy the symmetry and move the region of hydrostatic pressure away from the 

 centre. There will therefore arise concentric zones of similar pressure throughout 

 the mass. Where the hydrostatic pressure prevails there can arise the coarsely 

 crystalline massive product, and where non-uniform pressure dominates there can 

 arise the schistose character. Consequently there may arise the approximately zonal 

 arrangement of tremolite serpentine around the bastite serpentine as noted by Flett 

 (p. 20). The dunite serpentine may be the result of a marginal facies in the primary 

 peridotite. It has a lower alumina and lime content than either the tremolite or bastite 

 serpentine. The microscope shows that it consists entirely of olivine and its alteration 

 products. As an absorption of heat is required for the formation of tremolite from 

 olivine, we have only to suppose that the heat factor was not sufficiently strong on 

 the margin to yield a tremolite schist, and serpentine without tremolite would be 

 found. If, then, we are to give the Lizard serpentine a metamorphic history of this 

 kind, we cannot accept Flett's statement (p. 68) that fluxion banding in the Lizard 

 serpentine is a very common phenomenon until the metamorphic processes have been 

 fully considered and the laminae which are marked by olivine, or by olivine and pyroxene, 

 or by olivine and tremolite, have been shown not to be comparable with Grubenmann's 

 crystallisation schistosity texture. It would be very remarkable if fluxion banding is 

 always parallel to the subsequently induced foliation which, in turn, is parallel both 

 to banding and foliation of the adjacent hornblende schists and to the actual line of 

 junction of the serpentine and the hornblende schists. Apart from fluxion banding 

 this parallelism is a necessary consequence of the suggested explanation. 



Seriee A, Vol. ra., Part 1 H 



