268 PEOF. A. H. COX AND ME. A. K. WELLS ON THE [vol. lxxvi, 



The band is then shifted north-eastwards by the Dolgelley Fault. 

 East of the fault the beds are well seen on the south side of the 

 diabase intrusion that runs immediately south of Dolgelley town. 

 Throughout this part of their course the more shaly layers are 

 often burnt and spotted by the neighbouring intrusions. After 

 crossing the Aran stream just above Dolgelley, the outcrop is once 

 more interrupted b}^ faulting — the Ceunant Repeating Fault — 

 causing the beds to reappear a mile farther up the Aran Valley. 



The fact that the beds have not been recognized by previous 

 observers within the district now mapped is probably accounted for 

 by reason of the mode of occurrence of the strata : they are either 

 intimately involved among diabase intrusions, or else they are 

 covered by screes from the rhyolitic rocks above, while over long 

 stretches they are entirely cut out by faulting, as, for example, 

 along the northern slopes of Myn3 r dd-y-Gader. 



No fossils, other than worm-tracks and some quite indeterminable 

 graptolites, have been obtained from this arenaceous series. But 

 there can be no doubt that we are here dealing with the local base 

 of the Arenig : the stratigraphical position of the series and its 

 characteristic lithology furnish evidence enough. It will be noticed 

 that the lithology corresponds almost exactly with that of the 

 Basal Grit and JExtensus Flags of Arenig, 1 and with the Tudwal 

 Sandstones of the Lleyn Peninsula. 2 



(2) Arenig (and Lower Llanvirn) : the Lower Acid, or 

 Mynydd-y-Gader Rhyolitic Series. 



The arenaceous strata of the Basement Series are immediately 

 overlain hy a series of rhyolitic rocks. The series takes its name 

 from Mynydd-y-Grader, a hill south-east of Llyn Gwernan, where 

 the rhyolitic nature of the rocks' was first determined by Mr. Lake 

 & Prof. Reynolds. 3 These authors found that both rhyolitic lavas 

 and rhyolitic ashes are represented, and that the lavas tended 

 to thin out and disappear when traced from east to west. The 

 westward thinning of the volcanic rocks continues, and becomes 

 still more pronounced in the area at present under consideration, 

 where the series is represented almost entirely by rhyolitic tuffs, 

 lavas being only doubtfully present at one or two points. 



West of the Gwynant, moreover, definite slate-bands appear 

 within the rhyolitic series, so that the sequence there seems at first 

 sight rather different from that on Gelli-llwyd and other localities 

 east of the Gwynant and nearer to Myirydd-y-Gader. It is evident 

 that considerable changes take place as the beds are followed along 

 the strike ; but, between the Gwynant Valley and Gelli-llwyd, the 

 outcrop of the rhyolitic rocks is interrupted by a granophyre sill, 

 and accordingly the exact gradations of the lateral changes are not 

 easy to follow. For this reason, it will be most convenient to 



1 Q. J. G. S. vol. lxi (1905) p. 618. 



2 Ibid. vol. bad (1915) p. 108. 



3 Ibid. vol. lii (1896) p. 516. 



