part 8] MfiTAMORPHISM IN Tfii, MONA COMPLEX. 347 



their bounding-planes. But where (as in the left-hand part of 

 tig. 4) their parent pegmatites turn round so as to cut across the 

 banding, then, although they accompany their pegmatites, their 

 foliation does not turn round, but retains the same direction as 

 before, and so it is now transverse to their trend and to their own 

 bounding-surfaces. Moreover, where they accompany pegmatites 

 of the somewhat irregular kind shown in fig. 5 (p. 345), the same 

 is the case. In general, we are able to discern a law, which is 

 that, whatever may be their own direction in relation to the 

 gneissic structures, their foliation steadily conforms to that of the 

 gneiss, with which it is, indeed, identical. 



But we have seen that the gneiss had solidified before the 

 segregation of the first generation of pegmatite. It follows that 

 the foliation of the ultrabasic encasements must have developed 

 within a solid rock. 



By what agency was it developed? Let us consider, first, 

 whether the new hornblendes are simply enlargements of pre- 

 existing crystals that were already foliated, the direction of the 

 vertical axes of which would determine the direction of growth. 

 But there has also been an increase in the number of the crystals, 

 and this implies that new ones must have grown in such a direction 

 as to give rise to foliation. Can we then avoid looking to the now 

 familiar agency of strain ? The two questions which call for 

 answer are, first, whether effects such as we see here can be pro- 

 duced by stresses operating within a solid rock ; and, secondty, 

 whether there be any evidence that such stresses were actually 

 operating within these gneisses. To the first question, another 

 part of this paper supplies, as it happens, a decided answer. For 

 we have seen that in the hornblende-schists of the Penmynydd 

 Zone there are ultrabasic encasements of similar origin, and that 

 the}' are foliated. Now, in the Penmynydd Zone, there is no 

 doubt whatever (' Gr. of A.' pp. 118-28) that the rocks were solid, 

 and that the foliations are effects of dynamic metamorphism. To 

 the second question, the phenomena now described supply an 

 answer. In the fix'st place, we have seen that the banding of the 

 basic gneisses is clearly traceable to stresses, and that these were 

 in operation at a stage when consolidation of the differentiated 

 magma was, at any rate, very far advanced. These banded rocks 

 are, on Craig-allor and other places, folded, and the first generation 

 of pegmatites with encasements, which we know to have been 

 posterior to consolidation, is folded with them. Nor is this due to 

 the fact that they merely followed pre-existent folds, for the 

 foliation of their encasements is also folded. Further, the folds 

 are, in places, torn out into thrusts. Such rupture is, in itself, an 

 evidence of solidity. But we also find that the gneiss is much 

 more strongly foliated and ' spun-out ' along the limbs (especially 

 the middle limbs) of the folds than at their apices, and that 

 foliation attains a maximum along the thrust-planes. Along one 

 such thrust there is a quarter-inch seam of hornblende-schist with 

 micas, and this seam is itself minutely folded. Here, then, we 



