part 4] UPPER OKDOVICIAX OP THE BEKWYN HILLS. 491 



A point to be noted is that the bed of volcanic ash which is 

 well developed in the south-west, in the lower" part of the sequence, 

 rapidly dies out to the north-east. This ash may be seen in the 

 road-cutting leading up to the Vyrnwy Dam. The rock is a fine, 

 somewhat impure, pumiceous ash, with a considerable admixture of 

 sedimentary matter. On tracing the bed to the north-east along 

 the vertical limb of the anticlinal fold, we can follow it with ease 

 to within a point 1000 yards north-west of Gwreiddiau, where it 

 forms bold crags, on the Avestern side of the valley which cuts 

 through the ridge to connect the Vyrnwy valley with that of the 

 Tanat at Pen-y-bont-fawr. The ash which forms these crags ex- 

 hibits several peculiarities. It consists of lumps of ashy material, 

 up to several inches in diameter, embedded in normal blue mud- 

 stone ; besides the ash, however, there are nodules of calcareous 

 matter the calcite of which is in large plates enclosing numerous 

 dark specks. The bed appears to have been formed from volcanic 

 materia] under strong current-action. North-east of the vallej'' no 

 beds of ash have been observed at this horizon. This absence 

 cannot be accounted for by faulting, as the Caradoc-Ashgill 

 boundary runs without a break. 



The ash may be traced from Llanwddyn for some distance in a 

 northerly direction along the western flank of the Berwyn dome ; 

 but I have not traced the ash-bed (as shown on the 1-inch Geological 

 Survey map) into the ashes mapped on the northern side of the 

 dome. There can be no doubt, however, that this bed belongs to 

 a centre of eruption which lay on the west or north-west, rather 

 than to the vents that give rise to the Cwm Clwyd ash, which 

 came from the necks in the neighbourhood of the Carboniferous 

 Escarpment. 



Orthis-actonice calcareous group. — The sandstones with 

 O. alternata pass up without abrupt lithological change into the 

 overlying subdivision. This succeeding group is of a more 

 calcareous nature, and in places, particularly near the summit, 

 passes into fairly pure lenticular limestones. Interstratified with 

 these are beds of impure volcanic ash of no great thickness, and 

 thin beds and wisps of angular fragments of felspar. As a rule, 

 the purer limestones are not highly fossiliferous, and at Pen-y- 

 Garnedd they appear to have been considerably dolomitized ; but 

 the associated calcareous shales are in places crowded with fossils 

 which are particularly conspicuous in the weathered, buff-brown, 

 soft, decalcified mudstones. 



The most characteristic fossils of this group are : — 



Orthis CNicolella) actonlm Sowerby. 

 Platystrophia biforata (Schlotheim) ; 



typical large form. 

 Phacops (Pterygometopiis) jukesi 



(Salter). 



Asciphus powisi (Murchison). 

 Lichas laxatus M'Coy ; large form. 

 Gybele verrucosa (Dalman). 

 Motiticulipora lycopevdon (Say). 



A. small quarry about 550 yards north-east of the ruin of the 



