THE MBTAMORPHIC ROCKS OF ADELIE LAND. 8TILLWELL. 65 



We cannot at present indicate the conditions which permit metamorphic 

 differentiation in localised portions of certain bands, while the majority of bands remain 

 undifferentiated. The conditions may possibly arise from some combination of solid 

 diffusion with that force of crystallisation of the specific mineral which determines 

 its position in the crystalloblastic order. If it be objected that solid diffusion is a process 

 of much too limited range, and of too infinitesimal a rate, then it must be remembered 

 that the whole record of geological time is available. The presence of water may be 

 an assisting factor it is necessary material in the formation of epidote and chlorite 

 but we cannot assume mere migration by solution while we insist that the formations 

 occurred under the influence of strong stress. We can only surmise that the points of 

 metamorphic differentiation have in some way been the focus of special conditions of 

 stress or uniform pressure which, combined with other special mineral forming 

 conditions, have caused an abnormal development of that special mineral. A condition 

 of relatively low hydrostatic pressure and stress might, on the one hand, permit the more 

 ready transfer of molecules ; but, on the other hand, a condition of low hydrostatic 

 pressure and strong stress might favor the process of solid diffusion. 



Metamorphic diffusion and differentiation are essentially processes of limited range. 

 They occur in a rock which is, to all intents and purposes, solid, and molecular movement 

 is hindered. Their products can never attain the dimensions of the products of 

 magmatic differentiation. 



9. DESCRIPTION OP THE COARSELY CRYSTALLINE BASIC PATCHES IN THE 



GRANODIORITE GNEISS. 



Apart from the well-defined series of metamorphosed dykes that have just been 

 described, there exist a number of outcrops of hornblendic rock whose origin has only 

 become evident on investigation. In the field the dykes are distinct in that they have 

 maintained their sharp junctions and their linear trend, even though their surfact: out- 

 crop may be broken. The hornblendic rocks now under consideration present a contrast 

 and have scarcely any definite shape, and appear as irregular dark-coloured clots in the 

 grey gneiss. They possess a rough lenticular outline and tail out in the direction of 

 foliation, but the boundaries may be indefinite when the dark rock passes gradually 

 out into the grey rock. These dark rocks are often characterised by a uniformly coarser 

 grain and the average diameter of the mineral grains may reach one and a half times 

 that in the normal amphibolites. The rock type is not constant, and one may find 

 patches of almost pure hornblende rock, or massive amphibolite or hornblende and biotite 

 gneisses, which may pass through varying stages into the normal granodiorite gneiss. 

 Sometimes one can macroscopically distinguish brown sphene crystals up to |in. long 

 as well as pyrite or magnetite. 



The indefinite boundary, the coarse granularity, and a relatively massive texture 

 suggested in the field that they would yield evidence of primary consolidation of the 

 same nature as the granitic rock. Later study, alone, has shown that the coarse 



SeriM A, VoL m.. Prt 1 B 



