Correspondence — Mr. C. Lapivorth. 479 



containing the chief Mudstone Graptolites, is covered up by a second 

 Graptolitic schist, with transitional forms, and that is followed in 

 turn by a limestone containing a preponderance of Llandovery fossils. 



(e.) The Colonies and the Band E. e. 1 (asserted by Salter to be of 

 Llandovery age) of Barrande's Silurian Basin of Bohemia contain an 

 admixture of the Mudstone Graptolites with transitional and Upper 

 Silurian forms ; the first-named making up, perhaps, about one- 

 half of the total Graptolitic fauna. 



We see, therefore, that the beds containing the typical Coniston 

 Mudstone fauna are followed in Ireland, Scotland, Germany and 

 Sweden, by rocks containing many new forms, and having a general 

 fauna principally composed of an admixture of Lower and Upper 

 Silurian species ; and that the transitional and supposed Upper 

 Silurian Graptolite-bearing zone of Bohemia, hitherto looked upon as 

 the equivalent of the Mudstones, and sometimes instanced as proof 

 positive of the possibility of their Upper Silurian age, is, on the 

 contrary, of far more recent date. 



Thirdly — (a.) None of the Graptolites of the Mudstones survive 

 into the true Upper Silurians of the succeeding Coniston Flags.^ 



(6.) All the corresponding Birkhill fossils have died out before we 

 reach the Upper Silurians of South Scotland. 



(c.) Not one of the Graptolites yielded by the Kiesel Schiefer 

 proper of Thuringia occurs in the unequivocal Upper Silurian of the 

 neighbouring district of Bohemia (Band E. e. 2 of Barrande). 



(cl) None of the Mudstone Graptolites are met with in the 

 Wenlock or Ludlow beds of Shropshire and Hereford. 



The Upper Silurian beds here cited contain Graptolites in great 

 abundance, so that the absence of the Mudstone forms cannot be 

 explained away by reference to non- suitability of habitat or the like ; 

 while it must be recollected that the Graptolitic faunas of these widely 

 separated Upper Silurians are practically identical. Hence on any 

 view it is evident that the period of time which intervened between 

 the close of the typical Mudstone epoch and the commencement of 

 that of the true Upper Silurian was sufficiently lengthy to admit of 

 the gradual %ing out of all the Mudstone Graptolites and the con- 

 temporaneous evolution of their Upper Silurian successors. If this be 

 true, the chief break in the Lake District is not between the Lime- 

 stones and the Mudstones, but between the latter and the Coniston 

 Flags. In Westmorland we may form some little idea of its mag- 

 nitude by the marvellous change it wrought upon the Graptolites ; 

 and in Scotland and Germany by the thick and far-spreading masses 

 of those Middle Silurian rocks, of which we have little or no trace in 

 the Lake-district itself. 



That the Coniston Mudstones are not of Bala age is tolerably 

 clear from their position with reference to the Coniston Limestones ; 

 and if Mr. Aveline is right in his opinion that there is a slight 

 physical break between the two groups, this may be regarded as 

 demonstrated. Again, that they are not of Upper Silurian age is 

 placed beyond doubt by the complete absence within them of Qpper 

 1 "With one exception, M. Sisingeri, Carr. 



