342 DR. E. GREENLY ON THE SUCCESSION AND [vol. lxxiX,. 



be expected, wherever the Gneisses and the Gwna Beds occur 

 together, to intervene between those formations. No gneisses- 

 have been found at Fydlyn ; and at Mynachdy, where they do 

 occur, it was inferred that the Fydlyn Beds must have been cut 

 out ('G. of A.' p. 216). The result set forth in this paper 

 (pp. 336-38) throws a flood of light on the whole question. In the 

 first place : as Fydlyn Beds have now been identified at Mynachdy 

 (fig. 2, p. 336), 1 in the very position where they were to be ex- 

 pected, my reading of the succession is confirmed. In the second 

 place: the contrast between their crystalline condition, which is 

 but slightly anamorphic, and that of the Gneisses with their large 

 plutonic albites and large biotites in the foliated bands, is extreme. 

 The two conditions belong to widely separated zones of the litho- 

 sphere, and cannot be genetically connected. In the third place : 

 the Gneisses, as highly crystalline as anywhere in the island, keep 

 on (fig. 2) striking at the boundary. 



Finally, a rhyolitic tuff of the Fydlyn Beds [E 12362] has 

 yielded a fragment of plutonic mosaic through which runs a 

 folium with parallel flakes of a white mica, and thus of gneissose 

 character. This specimen, which is from Fydlyn south cliff, was 

 obtained at a point about 150 feet below the base of the 

 Gwna Beds, the lowest horizon at which any derivative comj)Osite 

 fragment had hitherto been found, and therefore a long way 

 below the only admissible break in the bedded succession. 



Secondary dynam-anamorphism. — It has been stated 

 ('G. of A.' p. 168) that, at Mynachdy, the Gneisses are wholly 

 catamorphic. But a coarse albite-granite of the gneiss [E 10642], 

 from near the junction with Gwna Beds, on the drive, about a 

 quarter of a mile east of the house, which is traversed by zones of 

 catamorphic shearing, is also traversed by r another zone, wherein 

 chlorites and white micas wind about epiclastic-looking grains, 

 and this zone bears a strong resemblance to a Gwna Green Schist. 

 There is a similar zone in a gneiss of the Gader Inlier [E 10639]. 

 The pseudepiclasts, however, are cataclasts of the same albite as 

 that of the undeformed portion of the slide. Now, the white 

 micas and chlorites of this zone are undoubtedly authigenic, hence 

 catamorphism is here transcended, and anamorphism, patently 7 of 

 dynamic origin, has set in. The degree to which it has developed 

 is about the same as that of the Gwna Beds of the Llanfair- 

 ymghornwy Belt, in which this gneiss occurs. In fact, we see here 

 the dynam-anamorphism of the Bedded Succession in process of 

 being superimposed upon the Gneisses, of the structures of which 

 it is quite independent. It follows that the ciystalline characters 

 of the Gneisses are not an intensification of those of the Bedded 

 Succession, but are their own. 



Seven new pieces of evidence, therefore, go to confirm the view 



1 Fig. 94 (' G. of A.') will need to be modified, by insertion of a wedge of 

 Fydlyn Beds between the gneiss and the Gwna Eeds at the western infold. 



