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tions of the Vymwy 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 Ocher places the strikes and dips 

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

 dip under the older rocks of the Berw^ms, 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 sand- 

 stone. 



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 ridges 

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

 overlaid by the Upper Silurian ridges. These beds are at the northern 

 limit of the Merioneth and Carnarvon ridges, are high in the ascend- 

 ing sections, 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 upjDcr 

 groups of flagstone, which are contorted, and in some places nearly 

 vertical; and it is prolonged to the tabular hills of the Denbigh 

 flagstone, south-west of Ruthin. The third (commencing with ridges 



