348 dr. e. gkeenly On the succession and [vol. lxxix, 



have evidence, not merely of a foliation posterior to consolidation, 

 and of its being a consequence of deformation, but also of true 

 dynamic anamorphism, for a new mineral has appeared along a 

 plane of maximum deformation, subsequent to which the foliation 

 was itself re-folded. Certainly, in such sections as those of fig. 4 

 (p. 344), there cannot have been shearing stress with differential 

 movement, for the pegmatite is not disrupted. But in other 

 sections the pegmatites are seen to have been affected by the 

 agency which induced the foliation, for they are penetrated, and 

 sometimes crossed, by the foliated hornblendes of their encase- 

 ments ; besides which, where the encasements are sharply folded 

 (as at Cefn-du in the Aethwy region) the pegmatites are sheared, 

 and acquire a foliation of their own. Finally, there are cases 

 (fig. 6, p. 346) where, around a pegmatite of the second generation, 

 the foliation, both of its own encasement and of the adjacent gneiss, 

 is markedly deflected ; which could not have taken place until 

 after the pegmatite (if, indeed, it were ever fluid) had thoroughly 

 consolidated. 1 Yet the first pegmatitic generation, and still more 

 the gneiss itself, had been solid rock long before the separation 

 of this pegmatite. Our second question, accordingly, is answered. 

 Stresses were operating within the basic gneiss, perhaps continu- 

 ously, but certainly at intervals ranging from a late stage of the 

 consolidation of the differentiated magma until after its conrplete 

 consolidation, and continuing even after the separation of the 

 second generation of pegmatite. 



The evidence as to folial genesis in these gneisses may be sum- 

 marized as follows : — In no case is it necessary to postulate move- 

 ment anterior to consolidation. There are cases where foliation- 

 structure might be anterior or posterior to consolidation. But, 

 where definite evidence is available, it shows that folial structures 

 have developed within solid rock, that stresses were in operation at 

 that time, that in some cases at any rate the foliation must be 

 ascribed to them, and that it was accompanied by mineral meta- 

 morphism. 2 



Posterior to consolidation, however, is not necessarily posterior 

 to cooling. And there is decisive evidence that, in this case, the 

 rocks were at a high temperature throughout ; for there are no 

 chilled selvages anywhere, not even to the third generation of 

 pegmatites, which are very coarse. Now, it is well-known that 

 the adaptability of crystals, both to pressure and to metamorphism, 



1 This deflection might be ascribed to the force exerted by crystalline 

 growth in the pegmatite itself (a principle to which attention has been drawn 

 by Dr. Alfred Harker). But, if so, such a deflection would be the rule around 

 these pegmatites, which is not the case. 



2 That mineral metamorphism can rarely be demonstrated in these rocks is 

 easily understood, if we reflect that their magma crystallized originally, not 

 as dolerite or gabbro, but as diorite, not as a pyroxenic but as an amphibolic 

 rock ; and that amphiboles, being common products of dynamic metamorphism, 

 woidd naturally be stable under such conditions. 



