Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/277

 be found, however, in the Caradoc strata of Ireland, and in the Hudson-River group of North America.

F. Saxon and Bohemian Areas. — I have now endeavoured to show that two great migrations of Graptolites took place from the area of the south of Scotland at the close of the Upper Llandeilo period — one westward through Ireland to America, the other southward into the north of England. I shall now try to show that a third great migration took place at the same time from the same area, in a south- east direction, into the Silurian seas of Saxony and Bohemia.

The Graptolitiferous rocks of the " Grauwackenformation " of Saxony are believed by Geinitz to form the summit of the Lower Silurian series ; so that they would correspond pretty closely with the upper portion of the Caradoc series. That the Graptolites of this series were derived from the British area is shown almost conclusively by the subjoined list of Saxon species identical with British forms : —

Saxon Graptolites derived from Britain.

Species derived from the Upper Llandeilo of the south of Scotland.

Diplograpsus pristis, His.

— — palmeus, Barr.

— — folium, His.

Climacograpsus teretiusculus, His.

Graptolites sagittarius, His.

— — Nilssoni, Barr.

— — tenuis, Portl.

— — lobiferus, McCoy.

— — Sedgwickii, Portl.

Rastrites peregrinus, Barr.

Species derived from the Coniston Mudstones.

Graptolites priodon, Bronn.

— — colonus, Barr.

All these species occur in the mudstones of the Coniston series ; but it is tolerably clear that they were not derived from this source, but from an area like the Upper Llandeilo of the south of Scotland, in the seas of which the genus Didymograpsus was represented. Two Didymograpsi, namely, occur in the Silurians of Saxony ; and one of them, at any rate, seems to be identical with a Scotch species. It is also clear that the channel of communication between the British and Saxon areas remained open for a long period, since the Silurian rocks of Saxony contain Graptolites priodon and G. colonus, both of which are wanting in the Upper Llandeilos of Scotland, but appear in the later deposits of the Gala group and in the mudstones of the Coniston series. It is probable, therefore, that more than one migration of Graptolites took place at different periods from the British to the Saxon area.

Almost all that has just been said about Saxony would apply to the case of the Bohemian area ; but I must not omit to notice that M. Barrande has already recognized the British derivation of the Bohemian Graptolites (Defense des Colonies, 1870). We may therefore regard the stocking of the Saxon area with Graptolites as being merely an early stage in a steady easterly migration of these organisms from British seas towards the close of the Lower Silurian period.

M. Barrande recognizes two chief Graptolitic zones in the Silurian