478 Correspondence — Mr. C. Lapworth, 



which yields a very distinct assemblage of Graptolites (about, 25 

 species), a few of which are confined to its lower beds, and a few 

 occnr only in its highest zones. 



(h.) In the neighbouring Silurian district of South Scotland is 

 found a bed of precisely similar lithological character (the Birkhill 

 Shales), black below, and grey, purple and black above. In these 

 shales perhaps every fossil of the Mudstones is represented ; and, 

 strange to say, they have a similar vertical distribution ; the forms 

 peculiar to the top beds of the Mudstones being also strictly con- 

 fined to the highest bands of the Scotch deposit. Now these Birk- 

 hill Shales are said by the ofi&cers of the Scottish Survey to be 

 imbedded in strata of Upper Llandeilo age. 



(c.) The same beds, with nearly all the same fossils arranged in 

 the same way, are continued into Ireland, where they emerge from 

 below a great thickness of almost barren grits, regarded by the 

 Irish Survey as of Bala age ; while the black band itself is said to 

 belong to the Llandeilo. 



(rf.) The characteristic Graptolites of the Mudstones can be col- 

 lected in abundance in the highly prolific beds of Pomeroy, which 

 yielded to Portlock the magnificent series of Irish fossils figured in 

 his Eeport, and which have subsequently been looked upon as 

 undoubtedly of Bala age. 



(e.) The same group of Graptolites occurs in the Kiesel Schiefer 

 of Thuringia — a band of black and grey shales which plunges below 

 the great mass of the Middle Silurian rocks of that district. 



(/.) Many of the most characteristic species (unmixed with 

 earlier or later forms) swarm in the very lowest band only of 

 Linnarsson's Upper Graptolite Schist of Sweden, which reposes 

 immediately upon calcareous beds with Bala fossils. 



Thus, on the assumption that the authoritative correlations made 

 by the Scotch and Irish Surveys are correct, this especial group of 

 Graptolites has hitherto been met with only in strata that (i.) 

 underlie the chief mass of the Middle Silurian, (ii.) lie immediately 

 above the Bala limestones, (iii,) are imbedded in the main Bala 

 series, or (iv.) form a portion of the Upper Llandeilo. 



Secondly — (a.) The Birkhill Shales aforementioned are, separated 

 from the true Upper Silurian of Scotland (with Wenlock fossils) by 

 a massive series of rocks (the Gala group), containing many Grapto- 

 lites, some of which are Coniston Mudstone forms, others peculiar, 

 and others common to the Upper Silurian. 



(&.) Similar rocks floor the counties of Down, Louth, and Oavan, 

 and repose on the Mudstone equivalents already noticed. They 

 contain also a similar mixture of Birkhill, Gala, and Upper Silurian 

 Graptolites, but nowhere show any clear trace of overlying Upper 

 Silurian strata. 



(c.) Eocks of the same nature, and of great collective thickness, 

 surmount the Thuringian Kiesel Schiefer, and contain some of the 

 same transitional Graptolites in association with a mixed group of 

 Lower and Upper Silurian Brachiopoda. 



{d.) The lowest bed of the Upper Graptolitic Schist of Sweden, 



