part 3] METAMORPHISM IN THE MONA COMPLEX. 349 



is greatly facilitated by a high temperature ; which may enable us 

 to understand the apparent ease with which the ultrabasic selvages 

 acquired a foliation. 



VII. Chronology op the Gneisses. 



Banding and permeation. — It has been assumed (' G. of 

 A.' pp. 139-42) that permeation, granitoid gneiss, and gneissoid 

 granite, being higher stages of granitization than banding, are 

 also later stages. But re-examination of the section at Henblas 

 (' G. of A.' pi. xv) has revealed that a good deal of the rock into 

 which the bands were injected, lit par lit, though with trans- 

 gression at acute angles (' G. of A.' fig. 21), was already a 

 permeated biotite-gneiss. Banding is, therefore, in some cases at 

 any rate, later than permeation. Yet, as the bands are still sodium- 

 granites, the banding is to be regarded as a fresh invasion by the 

 same magma, although the conditions did not at this stage bring 

 about such intimate union as before. 



The basic and the acid gneisses. — On re-examination of 

 many sections in Allor and at Henblas, not a single case has been 

 found where granitoid permeation in biotite-gneiss is later than 

 any basic rock. The sillimanite-biotite-gneiss at Werthyr allu- 

 vium (<G. of A.' pp. 322-23; E 11378) is a lenticular xenolith 

 100 to 150 feet wide, within the hornblende-gneisses of Allor. No 

 decisive junction has been found, but the two rocks are seen 

 within 20 feet one of the other. The basic gneiss contains rather 

 fewer pegmatitic seams than usual, and they have the character- 

 istic ultrabasic selvages ; foliation is often rude, and the aspect of 

 the rock dioritic. So soon as we pass into the bio ti tic gneiss, 

 granitoid permeation (which is really a sort of granitoid albitiza- 

 tion) becomes intense. Had the basic gneiss existed when this 

 permeation was going on, it could not have escaped. And it has 

 escaped. The same relations are seen 170 yards away to the west, 

 where, too, the strikes of the two gneisses diverge at an angle of 

 45°. Also, at the northern boss on the eastern side of the 

 alluvium, and at the forking of the roads west-south-west of 

 Clegir-mawr, the foliation of biotitic gneiss is truncated by un- 

 foliated or feebly foliated basic rock. The whole body of evidence 

 therefore compels me to abandon a view previously expressed 

 (' G. of A.' p. 903), and enables us to escape from a perplexity 

 created by that view (' G. of A.' p. 904). For it is now evident 

 that the basic magma was introduced after the granitoid per- 

 meation had ceased. Yet, as the basic intrusions have no chilled 

 selvages, the permeated rocks must have still maintained a high 

 temperature. 



The Gneissic Succession. — The foregoing evidence enables 

 us to revise, and also considerably to extend, our scheme of the 

 chronology of these ancient rocks, which may be summarized as 



