264 [Nov. 29, 



antly) Graptolites ludensis, a fossil which occurs throughout the 

 whole of the upper series. He was then in doubt whether to refer 

 this slate to the protozoic or Upper Silurian rocks ; but he now 

 regards it as the lowest of the Upper Silurian rocks in the Llan- 

 gollen district. On the southern flank of Cyrn-y-Brain, that is, 

 on the northern edge of the Llangollen trough, this slate re- 

 appears. 



To the north of the Llansaintfraid valley, on the present line of 

 section, is a great thickness of Denbighshire flagstone, which the 

 author, now as before, separates into three subdivisions.* The 

 lower of these consists of flagstone, passing into bands which are 

 hard and quartzose, or into earthy semi-indurated shale. The 

 middle subdivision resembles the lower, but is more indurated. 

 The upper flags consist of softer beds, which are more or less slaty, 

 and contain only a few fossils ; but these are surrounded by other 

 beds, harder and more quartzose, which are very fossiliferous. An 

 example of these fossiliferous beds occurs at the summit of Castell 

 Dinas Bran. 



All the above subdivisions of the Denbigh flags contain impres- 

 sions of Orthocerata. The lower and middle subdivisions afibrd 

 numerous compressed traces of the fossil, determined by Professor 

 E. Forbes to be a Creseis ; and associated with the Creseis are 

 found Graptolites ludensis, and sometimes LeptcBua lata. The 

 middle and the upper subdivisions, of which the latter has been 

 considered by Mr. Bowman (Trans. Manchester Geol. Soc. vol. i. 

 p. 203.) as non-fossiliferous, yield Cardiola interrupta and Tere- 

 bratula Wilsoni, although rarely. In the upper subdivision, at 

 Castell Dinas Bran, Terebratula navieula and T. semisulcata are 

 very abundant. (For the other fossils of Castell Dinas Bran, see 

 Proc. Geol. Soc, vol. iv. p. 221.). From the abundance of the Creseis 

 which, taken as a whole, the Denbigh flags contain, the name of 

 " Creseis flagstone " might serve as a good local name to give to 

 this group. 



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-westei'n base of Cefn Fedw, the upper Silurian rocks 

 {b') again appear, and are succeeded by the palseozoic 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. 



* Mr. Bowman has separated this part of the Upper Silurian rocks of North 

 Wales into subdivisions, which he has compared with those of the entire Ludlow 

 series of Mr. Murchison : to these views of Mr. Bowman the author signifies 

 his dissent. 



