252 The Rev. A. Sedgwick's Outline of 



o 



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 

 of sandstone and conglomerate at Gam Brys near Pen Tre Voelas) 

 is carried over the whole group, nearly north and south, to the es- 

 carpment of the mountain limestone near Abergele. 



The first two sections give the following ascending series : — 



1. Hard quartzose slates alternating with greywacke and beds of 

 porphyry : it is fossiliferous and of great thickness. 



2. A great mass of calcareous slates with two subordinate beds of 

 limestone and with many fossils. 



3. Dark roofing-slate with a few Graptolites. 



4. A great thickness of Denbigh flagstone, &c, extending to the 

 mountain limestone. This is separated into three subdivisions : — (a.) 

 Lower flagstone series passing into hard quartzose bands and into 

 earthy semi-indurated shales. It has impressions of Orthoceratites, 

 and numerous compressed traces of fossils mistaken for Orthoceratites, 

 but considered by Mr. Forbes as a species of Pteropoda (Criseis). 

 Some of its beds exhibit many impressions of Graptolites Ludensis. 

 (b.) Beds resembling the former, but more indurated, also contain 

 here and there many fossils, among which Cardiola interrupta and 

 Terebratula Wilsoni are enumerated, (e.) Softer beds, more or less 

 slaty, with few fossils, surrounded by harder and more quartzose 

 bands with very numerous fossils (e. g. the summit of Dinas Bran). 



In the preceding section Nos.l and 2 belong to the older division ; 

 No. 3 is considered doubtful ; but the whole of No. 4 is unequivo- 

 cally Upper Silurian. 



The third section gives the following ascending series : — 



1. Ridges of old rock with Caradoc sandstone fossils. 



2. Great masses and beds of conglomerate and coarse sandstone 

 unconformable to the preceding. The conglomerates disappear in 

 the ascending section, and the coarse sandstones pass into a finer 

 structure, and alternate with bands of dark coarse slate, here and 

 there with a true cleavage. Among these beds are a few Upper Si- 

 lurian fossils. 



3. A great thickness of Denbigh flagstone, generally agreeing 

 with No. 4 of the preceding sections. 



4. Great ridges of roofing-slate alternating with thick beds of 

 coarse greywacke. The group is contorted and traversed with a few 

 mineral veins, which are worked near Bronhaulog. These beds 

 contain fossils described in a memoir by Mr. Bowman (see the Pro- 

 ceedings of the Geological Society, vol. ii. p. 667*). 



5. A thick mass containing beds like those of the lower groups, 

 but often passing into a rotten slate or mudstone. 



6. Mountain limestone. 



This last group (No. 5) is overlaid further to the west by red 

 conglomerates described by Mr. Bowman. R.ed sandstones and 

 conglomerates also appear under the mountain limestone near Ru- 

 thin, which the author refers to the old red sandstone. 



He concludes that the groups of this upper division cannot be 

 * Or Phil. Mag. S. 3. vol. xiii. p. 225.— Edit. 



