SILrKIAN OF THE DEE VALLEY. 283 



8. The Denhiglishire Grits and Flags. 



There are three distinct subdivisions of this series in the Dee- 

 valley area, (i.) The lowest of these consists of the banded slates of 

 Penj'glog quariy &c. These have been identified with the Brathay 

 Plags of the Lake district, which are quite similar ; they contain 

 also the same fossils. Prom the beds of this series at Penyglog I 

 obtained Acroculia haJiotis, which has not yet been found below the 

 upper Coldwell beds of the Lake district. 



The same beds occur in the course of a stream about -1- mile south 

 of Maeshir, where they are faulted against the Tarannon Shales : 

 they here have somewhat the same lithological character as the 

 Graptolitic Mudstones, but contain numerous well-preserved speci- 

 mens of Graptolites, among which were Monograptus jpriodon and 

 J/, vomerinus in great abundance ; Retiolites Geinitzianus and Or- 

 thoceras also occurred. 



These beds in the Dee valley are succeeded by (ii.) Gritty beds, 

 e. g. those above Penyglog quarry, which are probably on the 

 horizon of the lower, and perhaps also of the middle Coldwell beds, 

 than which, however, they are much coarser, indicating a shallower 

 sea at this period than was the case in the Lake district. 



(iii.) The highest Silurian beds of the Dee valley are well 

 exposed on the hill near Llangollen upon which Dinas-Bran Castle 

 stands. They have been correlated with the Llansannan Shales by 

 the Geological Survey. In lithological character they resemble the 

 upper Coldwell beds of the Lakes, having the same peculiar character 

 of breaking into prismatic blocks ; they also form the marked screes 

 so typical of the upper Coldwell beds. This identification is 

 rendered more certain by the correlation of these beds by the Geo- 

 logical Survey (Mem. Geol. Surv. vol. iii.) with the Llansannan 

 Shales, which have a well-marked upper-Coldwell fauna. In the 

 case of these shales, however, as in the case of the Dinas-Bran beds, 

 man)^ of the fossils which occur are not found in the Lake district 

 lower than the Kirkby-Moor Plags and Bannisdale Slates. 



The following list of fossils from Dinas Bran is compiled from 

 the Woodwardian Museum and my own collection : — 



Cucullella coarctata, Phill. 



Ctenodonta. 



Theca Forbesii, Sharpe. 



Holopella gracilior, M'Coy. 



Orthoceras tenuicinctum, Fortl. 



laqueatum, Hall. 



Cjcloceras ibex, 8gw. 



Spongarium Edwardsii, Murch. 



Serpulites dispar, Salt. 



Pavosites fibrosus. 



Actinocrinus pulcher. 



Ceratiocaris. 



Ehyiichonella navicula, Sow. 



nucula, Sow. 



Ambonychia acuticostata, M'Coy. 



Of these fossils, Spongarium and Ehynchonella occur in the Ban- 

 nisdale Slates and Kirkby-Moor Flags of the Lakes ; but most of the 

 other fossils are found in the upper Coldwell beds. The identifi- 

 cation of these beds with the upper Coldwell beds depends on the 

 following facts : — (i.) the lithological and petrological character ; 

 (ii.) the stratigraphical position ; (iii.) the correlation by the Geo- 

 logical Survey with the Llansannan Shales, -which have a decided 



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