1872.] 



NICHOLSON MlGBATIOIsS OF THE GEAnOLITES. 



225 



diately posterior to the deposition of the Coniston Limestone. The 

 source whence these were mainly derived was unquestionably the 

 south of Scotland, as shown by the followiDg Table. It is to be noted 

 also that the period of the Bala Limestone (Girvan Limestone) Avas 

 not without Graptolites in the Scotch area ; for at least one Diplo- 

 grapsus and one species of Graptolites are known in this formation. 

 The following Table shows the Graptolites of the Coniston mudstones, 

 according as they are derived from the Scotch Llandeilo area, are 

 peculiar, or pass upAvards into younger deposits : — 



Graptolites of the Coniston Mudstones. 



Climaeograpsus teretiiisculus, Hi^. \ 

 Diplograpsus angustifolius, Hall. 



vesiculosus, Kick. 



paliueus, Barr, 



folium, His. 



pristis, His. 



• tamariscus, Nich. 



Eastrites Linnwi, Barr. 



peregrinus, Barr. 



Ketiolites pei'latus, Nich. 

 Graptolites lobif'ci-us, M'Coy. 



Nilssoni, Barr. 



— ■ — • Sedgwickii, Portl. 

 ■ — — Sagittarius, His. 



tenuis, Portl. 



■ Clingani, Carr. 



Diplograpsus confertus, IsicJi. 



Hughesi, Nich. 



sinuatus, Nich, 



putillus, HaU. 



Graptolites argenteus, Nich. 



bohemicus, Barr. 



fimbriatus, Nich. 



■ cliscretus, Nich. 



exiguus, Nich. 



■ turriculatus, Barr. ! 



colonus, Barr. I 



priodon, Bronn. \ 



Eetiolites Geinitzianr.s, Barr. ) 



.Derived from the Upper Llandeilo of 



( the south of Scotland. 



Not found in the Upper Llandeilo. 

 The first eight species are peculiar 

 to the Coniston Mudstones. The 

 last five species occur also in the 

 Gala group of Scotland ; and the 

 last three of these pass on into the 

 Upper Silurian. 



It will be seen from the above Table that all the genera of the 

 Graptolites of the Coniston mudstones of the north of England, and 

 sixteen of the species, or more than fifty-five per cent., are identical 

 with those occurring in the Upper Llandeilo rocks of Scotland. 

 "When we consider the relative age of these two deposits, it seems 

 clear that the Coniston area was stocked by a migration from the 

 older Scotch area. jSTot only is this the case, but it seems certain 

 that the channel of migration between the two areas remained open 

 during the whole of the period succeeding the deposition of the Bala 

 Limestone. This is shown by the fact that, of thirteen species which 

 occur in the Coniston miidstones but do not occur in the Upper 

 Llandeilo, five species are likewise found in the Gala group of Scot- 

 land ; and amongst these is the highly characteristic form, Graptolites 

 turriculatus, Barr. This would go to prove that there was ultimately 



