SHARPE ON NORTH WALES. 285 



lent flagstone, the Denbighshire flagstone of Professor Sedgwick. 

 These flagstones are largely quarried, and are worked along the 

 planes of bedding. They are entirely free from cleavage ; whereas 

 a true slaty cleavage frequently aifects the shale that alternates with 

 the flagstone. 



3. Thick beds of liver-coloured indurated shale, distinguishable 

 only by their position from the similar shales (No. 2) of the Lud- 

 low series. 



4. Grey slate, the lowest bed of the Wenlock series according to 

 Mr. Bowman. This slate is very largely quarried in the Glyn and 

 Oernant quarries, north of Llangollen, at the southern foot of Cyrn- 

 y-brain. It is Worked along the planes of cleavage ; but these being 

 strongly marked by transverse lines of bedding, the slates are liable 

 to split across. The dip of the beds at these quarries is south, from 

 4'5° to 80° ; the dip of the cleavage is north, 45°. 



5. Hard, gritty, blue shales, with lighter streaks, several hundred 

 feet thick. This is, according to the author, the lowest bed of the 

 Wenlock series in this district. Mr. Bowman made the aggregate 

 thickness of the four beds which he describes, 3100 feet; and if to 

 these this fifth bed be added, the aggregate thickness of the series 

 will be about 3500 feet. 



To the fossils previously noticed as belonging to this series by 

 Mr. Bowman and Professor Sedgwick, the author adds three new 

 species of Creseis, viz. Creseis ventricosa, C. obtusa, C, gracillima, 

 a description of which is given in the appendix to this abstract. 



The different species of Creseis are the most abundant fossils in 

 the Wenlock series of North Wales. Frequently they quite cover 

 the bedding planes of the large flagstones ; and they occur also in 

 beds of other texture and composition in the middle and lower parts 

 of the series. Hence they are of great assistance in distinguishing 

 the beds of the Wenlock period. 



The Wenlock beds cover a considerable area round Llangollen : 

 on the east they are bounded by Ludlow shales and mountain lime- 

 stone. On the north, their boundary runs from Pant-glas, at the 

 foot of the mountain limestone of Cefn-y-fedw, across Moel-y-faen 

 to Bryn-Eglys. Along this line they rest conformably on the dark 

 roofing-slate which is the uppermost of the Lower Silurian beds. 

 Such a conformity between the Upper and Lower Silurian rocks is 

 rarely observed in North Wales. 



From Bryn-Eglys a fault runs in a south-eastern direction down 

 Nant Morwynion to the Dee river, about three miles west of Cor- 

 wen, and cuts off" the Wenlock beds*. From that point on the Dee 

 another fault strikes down that river, and for three miles of its 

 course this second fault separates the Wenlock flagstones of Bron- 

 einion on the left bank of the Dee from the Lower Silurian beds on 

 the right bank, at the northern extremity of the Berwyns. At 

 Garth-newydd, about six miles east of Corwen, and south of the 

 Dee, Wenlock rocks appear ; and from this point their southern 



* The same fault may be traced for many miles to the north-east of Bi yn-Eglys. 



