"340 DR. E. GREENLY ON THE SUCCESSION AND [vol. lxxix, 



•coast-line. Suppose this infold to be cut by ruptures due to the 

 tendency to subsidence on pitch, and these again by thrusts 

 whereon the narrow core of Fydlyn Beds was driven at an angle 

 of about 30°, on to the Church Bay Tuffs of the outer part of the 

 southern limb of the infold. There would then eventuate the 

 Telations which are seen in the cliff, and we can thus also account 

 for the infold contracting, instead of expanding, in the direction 

 •of the pitch. 



IV. The Penmtnydd Zone of Metahorphism. 



(1) The passage from the Gwna Melange to the 

 Penmynydd Zone. — Another section across this passage has 

 lately been discovered on the Aberffraw coast. In the nook of the 

 bay north-east of Bryn-llwyd, about 30 to 50 yards east of the 

 passage on the buttress already described (' G. of A.' foot of p. 124), 

 the Gwna melange contains augen of a reddish grit and of a brown 

 limestone. Yet, only 4 feet from them across the strike, Pen- 

 mynydd-Zone mica-schist appears, with well- developed micas, 

 the quartz and the limestone becoming saccharoidal, while rapid 

 minor folding also sets in. More still : the fissile matrix of the 

 adjacent grit-augen is found on examination to have already 

 ^become a true mica-schist. We have, therefore, here a real inlier 

 of the Penmynydd Zone, although only about 10 feet thick at its 

 outcrop, and no better exposure of the passage to the Penmynydd 

 .Zone is known in the Mona Complex. Re-examination of a slide 

 (E 6139, from the western cliffs of the bay) gives further confirm- 

 ation. The greater part of the slide is a calcareous Gwna Green - 

 Schist, yet at one side of it is a band brilliant with white mica, 

 ;some crystals of which are 1 mm. long and - 25 mm. broad. The 

 Pemnynydd-Zone anamorphism is therefore selective, developing in 

 seams already rich in the elements of mica, while closely adjacent 

 seams of different composition are still at the stage of Gwna 

 Green Schist. The remarkable rapidity of the passage, in both 

 ■oases, confirms the suggestion ('G. of A.' p. 127) that dynamic 

 metamorphism was facilitated by change of material, as well as 

 ■of conditions. 



(2) The hornblende-schists. — At the 178-foot level east- 

 morth-east of Plas-berw, and at several other places, these rocks 

 • contain (' G. of A.' pp. 114-15) many quartz-albite augen, which 

 by waning of albite pass gradually into ordinary quartz-augen. 

 That they belong to the metamorphic process is certain, for some 

 of them are foliated internally, while others are cased in a film of 

 paragonite. The stage at which they developed was a late one, 

 for some of them truncate the foliation, though slightly. Yet it 

 was not the final anamorphic stage, for these schists are cut and 

 shifted by foliated quartz- veins containing needles of hornblende, 

 which are dynamically connected with the transverse folding. 

 That these augen are segregations is abundantly evident from 

 .their general relations. It is, therefore, of much interest to find 



