Vol.51.] SERIES OF SOUTH DENBIGHSHIRE. . 21 



and correspond with the Pen-y-glog Grit and Moel Ferna Slates 

 rather than with the Pen-y-glog Slates themselves. These mud- 

 stones are followed by thin muddy shales with rare flaggy ribs, 

 which contain Monograptus colonus, M. Nilssoni, and Gardiola 

 interrupta, and which therefore represent the Nantglyn Flags. They 

 are succeeded by hard, thick flags with shales, in which Monograptus 

 leintwardinensis, M. Salwet/i, and M . Roemeri occur ; and these must 

 be the equivalents of the Leintwardinensis-beds of Pen-y-vivod. The 

 upper beds of the Silurian in the Long Mountain are almost barren, 

 so that we can say no more than that they are the representatives 

 of the beds above the Leintwardinensis-zone. 



Welsh Borders (Central and Southern). 



In the typical Silurian area the Wenlock Shales contain Mono- 

 graptus priodon and other graptolites of the Pen-y-glog Slates. 

 The Lower Ludlow Shales contain M. colonus and Cardiola inter- 

 rupta like the Nantglyn Flags ; and hence we may probably place 

 the Pen-y-glog Grit and Moel Ferna Slates between these two, at 

 about the horizon of the Wenlock Limestone. 



The Leintwardine Flags with Monograptus leintwardinensis clearly 

 correspond with the Lzintwardinensis-heds, of the Llangollen basin, 

 and the Dinas Bran Beds must therefore be of Upper Ludlow age. 



Lake District. 1 



In the Lake District, as has already been pointed out by 

 Prof. Hughes and Mr. Marr, the succession is remarkably like that 

 in Denbighshire. The Brathay Flags with Monograptus priodon, etc. 

 correspond exactly with the Pen-y-glog Slates. They are followed 

 by the Lower Coldwell Beds, which are coarse, grey, unfossiliferous 

 grits, and no doubt represent the Pen-y-glog Grit. These are 

 succeeded by the Middle Coldwell Beds, which are calcareous, flaggy 

 grits, with a tolerably abundant fauna. No precise representative 

 of them is to be found in Denbighshire ; but from their position 

 they probably correspond, in part at least, with the Pen-y-glog Grit 

 and Moel Ferna Slates. The next subdivision, the Upper Coldwell 

 Beds, consists of gritty flags of great thickness, in which Monograptus 

 colonus, Actinocrinus pidcher, Orthoceras primcevum, etc. occur : 

 both lithologically and palaeontologically they therefore resemble 

 the Nantglyn Flags. 



The Upper Coldwell Beds are succeeded by the Coniston Grits, 

 which are generally unfossiliferous. Prof. Hughes describes a band 

 which contains Aoidaspis Hughesi in abundance ; and he finds a 

 similar band in the grits above the Nantglyn Flags of the Vale of 

 Clwyd. This species, however, in the Lake District at least, is not 

 confined to one horizon. In the Llangollen basin, although the 



1 See Marr, Quart. Journ. Geol. Soc. vol. zsxxiv. (1878) p. 871 ; Geol. Mag 

 1892, p. 534. 



