SEDGWICK ON NORTH WALES. 17 



of the upper limestone bands on the Ceiriog (vide Section IV.), but are very 

 different from the corals of the lower group of the Meifod country. There 

 are very great numbers of Turbinolopsis bina, of Favosites polymorpha and F. 

 alveolaris, of Cyathophyllum, and Stromatopora. 



On the Fossils of the calcareous shale of Pen-y- Craig. — In the very remark- 

 able list of fossils (List I. column 13.) obtained from this shale, we have 

 Terebratula marginalis, a Wenlock shell, associated with Leptccna dvplicata, 

 Atrypa undata, A. plobosa, and Orthis lata, shells which have been considered as 

 characteristic of the Llandeilo flags.] 



From a review of all the preceding facts, the author concludes 

 that the Protozoic series of North Wales is of enormous thickness ; 

 that it has no defined base, the fossils disappearing in the de- 

 scending section, not suddenly, as in Cumberland and Westmore- 

 land, but gradually ; that many species are found in every sub- 

 ordinate group from the top to the bottom ; and that some species, 

 especially certain Trilobites, characterise the lower group. 



§ 6. Upper Silurian Rocks of Llangollen, Cernioge, S?c. 



The geological structure of this part of North Wales the author 

 illustrates by three sections, which he exhibited to the Geological 

 Society on a former occasion ; but which, with the help of Mr. 

 Salter, he is now able to present in a more accurate shape. 



Section VII. 



Llansaintfraid, Glyn Ceiriog, across the valley of the Dee, to Cyrn-y-brain, 

 near the head of the vale of the Clwyd. 



Horizontal base 12 miles. 

 N. 35° W. S. 35°. E . N. 17° E. S. 17° W. 



This Section may be considered as a continuation, northwards, of 

 Section IV. To render clear the position of the upper Silurian flag- 

 stones of the Dee {b b'\ lying as they do in a trough which is 

 bounded, both to the north and to the south, by a mass of pateozoic 

 rocks (a and c c'), a- portion (a) of the older series of rocks, which 

 lie to the south, and were before represented in Section IV., is here 

 repeated. On the northern side of the trough, at Cyrn-y-brain, 

 the existence of a mass of older rocks (c c'), which was before 

 suspected by the author, has been ascertained ; with the help of 

 Mr. Salter, its extent has been laid down upon the Ordnance Map; 

 and it has been inserted in the section. 



In his paper, read before the Society in June, 1843, the author 

 described the Upper Silurian Denbighshire Flagstone series as con- 

 sisting of three subdivisions ; which may be termed the Lower, the 

 Middle, and the Upper Flags. 



vol. i. c 



