the Geological Structure of North Wales. 251 



north. They alternate here and there with porphyry, contain many 

 fossils, and in their highest portion near Llansaintfraid Glyn Ceiriog, 

 contain two bands of limestone. At this place they seem to form a 

 regular ascending section, conducting without a break to the over- 

 lying Upper Silurian flagstone. 



The author then notices the undulating country east of the Ber- 

 wyns, extendingfrom the Severn, near Pool, through the ramifications 

 of the Vyrnwy and the Tanat. Calcareous slates, with many fossils 

 nearly resembling those of the Bala limestone, are repeated again 

 and again by rapid undulations : the facts are illustrated by sections. 



The beds have a prevailing strike about N.E. ; but it is frequently 

 interrupted, and they are twisted out of their course so as in some 

 tracts to strike east and west ; and in other places the strikes and dips 

 are entirely anomalous. The whole system in some places seems to 

 dip under the older rocks of the Berwyns, in others it is placed side 

 by side with them, the junction planes being vertical ; and again the 

 same system is seen to be thrown off with an eastern dip from the 

 flank of the older chain. Everything indicates great derangement, of 

 a later date than those which gave the impress to the Carnarvon chain, 

 and probably contemporaneous with the movements which placed 

 the beds of the north end of the Berwyns in the anomalous position 

 above described. Part of the system here noticed has been described 

 by Mr. Murchison, and is classed in the Caradoc sandstone. 



Lastly, the author notices a comparatively low country near the 

 line of the Holyhead road, extending westward to the neighbourhood 

 of Bettws y Coed, in which the strike of the Carnarvon chain(N.N.E.) 

 is but feebly impressed. The beds undulate, and are sometimes 

 almost horizontal ; but here and there they are thrown into ridgesjwith 

 the N.N.E. strike ; and in all these different positions they are over- 

 laid by the Upper Silurian ridges. These beds are at the northern limit 

 of the Merioneth and Carnarvon ridges, are high in the ascending sec- 

 tions, and near Penmachno, Bettws y Coed, &c, contain many fossils. 



Upper division of the slate rocks. — Denbigh flagstone, &c. 

 The author traces in detail the line of demarcation between the 

 rocks of this and of the preceding division. From Conway to a 

 point a few miles south of Llanrwst, this demarcation is represented 

 by a great fault ; afterwards by an irregular line (traced on a map), 

 partly south and partly north of the great Holyhead road. A few 

 miles below Corwen it crosses the valley of the Dee, passes over the 

 crest of the hills, and strikes down the valley of the Ceiriog, in the 

 lower part of which it is cut off by the mountain limestone. The 

 strike of this upper group is affected by great breaks and undulations, 

 but on the whole is about west by north, and east by south ; and 

 its prevailing dip is towards the north. Its structure is explained in 

 detail, and illustrated by three sections : the first (commencing with 

 the slates, porphyries and calcareous slate of the older division, south 

 of Llansaintfraid Glyn Ceiriog) passes through a peculiar mass of 

 dark roofing-slate, and is thence continued through Llangollen and 

 Dinas Bran to the terrace of mountain limestone. The second, 

 commencing a few miles to the west of the former, crosses the upper 



