OF THE S0CB:S OF THE SOUTH OF IRELAND. 269 



Pembrokeshire there is an unconformity between the two forma- 

 tions ; this is the direction in which lie the tracts of the south of 

 Ireland on the one hand, and Devonshire on the other ; and owing 

 to the overlap of the Old Eed Sandstone near Caermarthen onto the 

 Lower Silurian rocks, the passage-beds are hidden from view * ; we 

 have therefore no opportunity for comparing the Lynton and Pore- 

 land beds with their presumed representatives in time north of the 

 Bristol Channel until we reach the district of Usk, a considerable 

 distance therefrom. Nevertheless there seems no objection to the 

 supposition that the purple and reddish sandstones, shales, and con- 

 glomerates of the Eidge of the Trichragt, underlying the so-called 

 " Old Eed Sandstone " near Llandovery, are the representatives of 

 the Foreland beds on the one hand, and of the Glengariff beds on 

 the other. 



The Pickwell-down Sandstone and the Old Eed Conglomerate of 

 the south of Ireland are, it may be assumed, represented by the 

 " White Sandstone and Conglomerate " underlying the " Limestone 

 Shale," this latter being the representative in a greatly reduced 

 form of the Carboniferous Slate of the south of Ireland, and the 

 Barnstaple, Pilton, and Marwood beds of Devonshire t. Between 

 these two horizons lie the red, grey, and greenish sandstones and 

 marls, with earthy limestones of " the Cornstone group," which 

 must therefore represent the Middle and Lower Devonian beds de- 

 posited under conditions differing from those under which the 

 Devonshire strata were formed. Mr. Godwin-Austen and Prof. 

 Eamsay are of opinion that these beds are lacustrine § ; I would 

 venture to suggest, as more probable, that they were of estuarine 

 origin, and connected with the open sea which spread over the 

 Devonian region to the south. Mr. Salter discovered in 1862 a 

 bed of Serpula (S. advenci) in the marls of the " Cornstone series " 

 at Caldy Island in South Pembrokshire, showing the introduction 

 of marine conditions at least in this district. Prof. Dew al que has 

 pointed out that similar red sandstones and marls occur amongst 

 the Devonian rocks of Belgium, separating the Calcaire de Prasne 

 from the Calcaire de Givet, both being highly fossiliferous marine 

 limestones. I have myself seen these beds, which are laid open to 

 view in several fine sections along the valley of the Ourthe, near 

 Liege. It must be admitted, however, that although " the Corn- 



* De la Beche, ibid. p. 60. 



t Hor. Sect. Geol. Survey, sheet No. 3, by Sir H. T. De la Beche aud Prof. 



I In these shales, as they occur in Pembrokeshire, the late Mr. Salter recog- 

 nized numerous forms characteristic of the Pilton and Marwood beds, such as 

 Avicula damnoniensis ( a very abundant and typical foi*m), Cucullceo. trapezium, 

 Rhynchonella laticosta, Bellcrophon bisulcatus, and two other species, besides 

 other unnamed forms oi Pletirotomana, Nucula, Sangii.inolites, Modiola, A.rinus, 

 and Discina. That the Limestone Shale is the equivalent of the Barnstaple, 

 Pilton, and IMarwood (or Cucullaca-) beds, seems, therefore, to be well supported 

 by fossil evidence. (Quart. Journ. Geol. Soc. vol. xix. p. 478.) 



§ Phys. Geol. Great Brit. p. 105. The absence (so far as known) of the Old 

 Red Sandstone fishes from the marine Devonian beds is remarkable, and is ex- 

 plained by the authors above referred to. 



