G. L. Elks — Graptolite Zones in Arenig Rocks. 203- 



corner beyond the ferry house and bridge, are much contorted and 

 disturbed, and of a nature totally different from any of those just 

 described. The Arenig Series seems to be terminated by the 

 Bangor-Carnarvon fault, which crosses the river close to the- 

 Castle and brings on the contorted beds. 



(b) BigJit Bank. 



A similar but less complete succession may be found on the right 

 bank of the river below the beds previously described by Marr 

 (loc. cit.). Flags which seem to belong to the zone of B. hirundo 

 are exposed in the railway cutting at two points, and below the 

 Castle the calcareous flags, which form the lowest visible members 

 of the Arenig Series in this district, are seen again. 



The zone of D. bifidus is also seen in a quarry by the mill near 

 the Brickworks, where the river makes a big bend, doubling back 

 on itself for a short distance and then continuing in a direction 

 almost at right angles to its previous course. The beds are here 

 bent into a slight fold, the centre of which is occupied by greatly 

 slickensided calcareous flags ; graptolites are found in the associated 

 thinly bedded shales, which are much iron-stained ; in general the 

 fauna resembles that of the beds at Pont Seiont, but the Didymograpti 

 are rarer and the Diplograptidas relatively more abundant. The 

 beds have yielded — 



Didi/mograptus b'lfidus (Hall), rare. Climacograptiis confertus, Lapw. 



Biplograpius dentatus, Brong. 



Beds on approximately the same horizon are also exposed at 

 Peblig Mill, where greasy blue-grey calcareous mudstones are seen 

 in the quarry below about 20 feet of drift. They are underlain at 

 the north end by a series of lighter-coloured ochreous flags with 

 darker bands, which give place downward to blue-grey mudstones. 



The only fossils obtained from this locality were Bidymograptus^ 

 hifidiis and Biplograptus dentatus, both found only after much searching. 



Menai Straits. 



Closely connected with the lowest beds seen in the Seiont Eiver, 

 though shifted slightly to the north-west by the Bangor-Carnarvon 

 fault, are the beds which come out on the Menai Straits beneath 

 the Carboniferous rocks, and to which attention was first directed by 

 Greenly (Geol. Mag., 1898, p. 560), The beds are well exposed 

 at low tide between the Suspension and Tubular Bridges. Working 

 westwards from the Suspension Bridge, the nearly horizontal 

 Carboniferous beds are at first all that is visible, but shortly some 

 fissile red-coloured slaty beds appear beneath them with a general 

 E.S.E. dip. They are at first only observable on the foreshore, but 

 soon rise into a low cliff, then disappear beneath the Carboniferous,. 

 to reappear before reaching the rocky point opposite the rocks 

 in the Strait known as the Swillies. They form the rocky point 

 itself, and an examination of them shows that the red colouring 

 characteristic of the higher beds is merely due to staining ; the 



