336 



DE. E. GREENLY ON THE SUCCESSION AND [vol. lxxix, 



MnO, P 2 Sl CO* and C not found. 



I-IV. Quartzite of Graig-wen in Gynfor, near Portli-wen Bay on the northern 

 coast. [See slides E 10953-54, also 10951, 10955.] C G. of A.' pp. 79, 311. Anal. 

 Dr. J. W. Mellor. 



V. Quartzite of Llangefni Old Windmill (Graig-fawr Windmill of the 6-inch 

 map), an inlier among Carboniferous rocks. [E 9954, 10696.] ' G. of A.' p. 350. 

 Name of analyst not stated. 



VI Average of Analyses I-V. If, also, we include the two silica-percentages 

 (96'40 and 99'60) mentioned in ' G. of A.' p. 79, then the average percentage of 

 silica is 96'47. 



The apparently capricious alkali-percentages are clue to planes, 

 which cany films of a wkite mica. 



III. The Fydlyn Beds. 



Mynackdy. — When the Survey Memoir and 1-inch map were 

 published, Fydlyn Beds had not been recognized with certainty in 

 the northern region. In November, 1921, however, while I was. 



Fig. 2. — Tlie Fydlyn Beds at llynachdy (from tlie 

 6-inch maps). 



b=Ordovician. 

 MS = Church Bay Tuffs. 

 MG=GwnaBeds. 

 MF=Fydlyn Beds. 



M= Gneisses. 



C.H.T. = Carmel-Head thrust-plane. 

 P.y .E . = Porth-yr-Ebol. 



P.G. = Porth Gron. 



P.P.=Porth-padrig\ 



re-examining the perplexing coast-sections between Porth-yr-ebol 

 and Porth-padrig, Mynachdy, some of the ' confused types ' 

 (' G-. of A.' p. 295) struck me as so like the Fydlyn rocks that I 

 wondered that I had not suspected their identity before. The 

 result of comparison with Fydlyn and of subsequent re-examination 

 was decisive. The rocks, though much decomposed, and rather 



