18 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The lower flags consist of flagstone, passing into hard quartzose 

 sandstone and earthy semi-indurated shale. Since it is only 

 in the lower flags that an abundance has been found of Ortho- 

 ceratites, and of the fossil found by Professor E. Forbes to be a 

 Creseis, the name of 'Creseis flagstone' might serve as a good local 

 name to give to this lower group. At nearly the base of the series, 

 together with the Creseis, Leptcena lata has been found ; but it is 

 scarce. The uppermost portion of the lower flags (which has 

 sometimes been described as non-fossiliferous) contains, though 

 rarely, Cardiola interrupta and Terebratula Wilsoni. 



The lowest upper Silurian rock exposed in this line of section is 

 dark roofing slate, containing a few of the Graptolites ludensis. 

 But this is a fossil which extends upwards, through the whole 

 series of lower, middle, and upper flags. 



All the upper Silurian rocks (b) upon this line of section, from the 

 Ceiriog river to Castle Dinas Bran inclusive, are now considered by 

 the author to belong to the lower flags. The character of the 

 Llangollen fossils, taken as a whole (see list of them, Proc. Geol. 

 Soc. vol. iv. p. 221.), and particularly the abundance they contain 

 of Terebratula ?iavicula, have led the author to come to this con- 

 clusion.* 



These upper Silurian rocks are overlaid by unconformable and 

 nearly horizontal beds of mountain limestone (d d") ; and these are 

 crowned at Cefn Fedw by a capping of millstone grit (e e'). At 

 the north-western base of Cefn Fedw, the upper Silurian rocks 

 (b r ) again appear, and are succeeded by the palaeozoic mass of 

 Cyrn-y -Brain, full of Caradoc sandstone fossils. On the north- 

 western flank of this mass the mountain limestone, crowned by 

 millstone grit, again appears, and in an inclined position. 



Section VIII. 



The Teirw River, across the valley of the Dee, to Cricor Mawr, near the 

 head of the Vale of the Clwyd, 



Mean direction of the line of section, S. 20° E. to N. 20° W. 

 Horizontal base 1 1 \ miles. 



W. 30° W. S. 30° E. N. 5° W. S. 5° E. 



5J Llansaintfraid, -ft 



Cricor Mawr. Moel y Gamelyn. Q Glyn Ceiriog. B 



This Section is nearly the counterpart of Section VII., and runs 

 nearly parallel to it : it passes the Dee about 3 miles W". of Llan- 

 gollen. At the southern extremity, on the Teirw river, we have 

 the limestones (X X'), and porphyries of the Pakeozoic series («). 

 Then follows a trough containing the dark roofing slate and the 



* In this view the author differs in opinion from the late Mr. Bowman, who 

 separated this part of the upper series into subdivisions, which he compared 

 with those of the entire Ludlow series of Mr. Murchison. 



