262 PROF. E. HULL OIT THE GEOLOGICAL RELATIONS 



markable freshwater mussel. Up to this time, however, it has not 

 been discovered in Devonshire*. 



This is not surprising when the relations of the strata in each dis- 

 trict are considered. In the South of Ireland the Old Eed Sandstone 

 deposited along the slopes of subsiding landsf, giving rise to the bed 

 of shingle now forming its conglomerate base, was favourably situated 

 for the formation of lucustrine conditions. The formation is in all 

 probability the deposit of a lake, the waters of which were inhabited 

 by the peculiar fishes of the period as well as by this large freshwater 

 mussel. It may have been otherwise, however, with the represen- 

 tative beds in the Devonian area. The Pickwell-Down Sandstone 

 is here underlain, as well as overlain, by essentially marine 

 beds, those of Ilfracombe and Mortehoe $ ; and it is possible that the 

 lacustrine conditions of the South of Ireland gave place to marine con- 

 ditions over the Devonian area during the same time. This view 

 finds support in the fact (which I will again advert to) that the re- 

 presentative beds in Belgium (Psammites du Condroz) contain marine 

 fossils. 



(3) It will now be apparent why the Pickwell-Down Sandstone, 

 though the representative of the Old Eed Conglomerate of the South 

 of Ireland, is not itself a conglomerate. The conglomerate of Ireland 

 was formed along the flanks, and over the edges, of older uncon- 

 formable beds ; but this was not so in the case of its representative. 

 Hence the difference of mode of accumulation and in composition. 

 Reduced to a tabulated form, then, it may be taken as generally agreed 

 that the following are the representative beds of the Lower Carboni- 

 ferous and Upper Devonian, or Old Red Sandstone, in each district: — 



Representative Beds. 



South of Ireland. North Devon. 



( Carboniferous Limestone. Carboniferous Limestone. 



T n T, • I Carboniferous Slate. Barnstaple beds. 



Lower Oarboni- J . p., , ^ , , 



ferous beds. } Coombola Grit and Slate I -^^^ ^^- 



(passage-beds). j 



Old Eed Sand- f Kiltorcan beds, Upcot beds ? 



stone or Upper Old Eed Sandstone and Pickwell-Down Sandstone. 



Devonian. [ Conglomerate. 



YII. Middle and Lower Devonian Beds. 



Tlie great series of marine fossiliferous strata which underlies the 



Pickwell-Down Sandstone in North Devon, and constitutes "the 



Middle and Lower Devonian " series of Sedgwick and Murchison § 



not being represented (except perhaps in its lowest part) in the 



* Anodonta has been found in Northumberland by Mr. Lebour, F.G.S., in 

 beds probably corresponding in position to the Kiltorcan beds of the South of 

 Ireland. 



t As I have shown in my former paper su^pra cit. 



X The Mortehoe slates have not yielded fossils. The absence of calcareous 

 bands and the micaceous character of the beds are indicative of the absence of 

 fossils ; but they may be regarded as the upper members of the Ilfracombe 

 series. 



§ Eep. Brit. Assoc. 1836 ; ' Siluria,' 4th edit. p. 272. * 



Baggy and Marwood beds 

 ( CucuUcsa-zone). 



