•338 DB. E. GREENLY ON THE SUCCESSION AND [vol. lxxix, 



Bull Bay. — The white schists of the western cliffs of this bay 

 {'G. of A.' pp. 312-15) must also be regarded as Fydlyn Beds. 

 They are often as white as at Fydlyn, and weather in the same 

 manner, but are more schistose. Under the microscope, we find 

 the characteristic microfelsitic matrix, now highly micacized. 

 •Some are pyroclastic, but one of them retains (although its matrix 

 is now a micaceous schist) some of the characters of a porphyritic 

 rhyolite, with subquadrate phenocrysts of quartz and many of 

 unquestionable albite in tolerably good preservation. Granules of 

 leucoxene are also present. 1 



Stratigraphical considerations. — The identification of 

 Fydlyn Beds at Mynachdy, Wylfa, and Bull Bay is of the greatest 

 importance. For, as they adjoin the Gwna Beds and pass gradually 

 into them, the hypothesis that they are the lowest-known member 

 of the Bedded Succession is confirmed (see also p. 342). Further, 

 in view of the magnitude of the Carmel-Head thrust-plane, on 

 both sides of which they are now known, it is evident that they 

 are no mere local development, but an horizon of widespread 

 volcanic activity of rhyolitic type. Moreover, this makes their 

 identity with the great mass of the Penmynydd-Zone mica-schist 

 more probable than ever. 



Regional tectonics. — Their relationships also confirm the 

 view (' G. of A.' pp. 177, 215-16) that the succession in the 

 Northern Region is inverted. For, if we compare their condition, 

 at all three places, with that of the rocks which occur on the other 

 side of the Gwna Beds, it is evident that anamorphism is waning 

 as we pass from younger to older members of the succession. 



Local tectonics. (1) Mynachdy. — It will be seen (fig. 2, 

 p. 336) that there are two tracts of Gwna Beds above the Carmel- 

 Head thrust-plane. One is a narrow fringe along the coast ; the 

 other is inland, Church Bay Tuffs (which have lately been separated 

 out on the maps) rising to the south of it. The Fydlyn Beds lie 

 between these two tracts, and, as the Gwna Beds of the inland 

 area are unusually felspathic along their northern margin, it is 

 evident that the Fydlyn Beds can on\j be a few yards away, and 

 must be bending round south-eastwards. Now, if, as has been 

 urged, the succession be inverted, the Fydlyn Beds must be taken 

 in on a major isoclinal infold (' G. of A.' p. 217). In and west 

 of Porth Gron, the southern limb of this infold is cut out by 

 coming on to a bend in the Carmel-Head thrust-plane ; while, east 

 and south-east of Porth-padrig, its northern limb is cut out by the 

 Padrig slide. But it must have an eastward pitch, for in that 

 direction the Mynachdy gneiss appears, and must occupy the core 

 of the infold. Where Gwna Beds reappear beyond the gneisses 

 east of Mynachdy, however, no Fydlyn Beds have been detected, 

 and the northern limb of the infold must be cut out once more ; 



1 The slides of these Fydlyn rocks are E 12362-66, 12368-71. 



