Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/274

 a migration in a reverse direction from the Coniston area into the Scotch area.

It may be admitted, then, that the fundamental elements of the Graptolitic fauna of the Coniston Mudstones were derived from Scotland ; but this fauna nevertheless has its own peculiar species, whilst the absence of some of the Scotch forms is very remarkable. Thus no traces have ever been discovered in the Coniston mudstones of any species of the genera Didymograpsus, Dicranograpsus, Coenograpsus ( =Helicograpsus) and Pleurograpsus, so characteristic of the Upper Llandeilo area of the south of Scotland. On the other hand, there is a great development of the species of the genus Graptolites in the Coniston Mudstones, as compared with the Dumfriesshire Shales.

It may be as well to follow out here the further course of the Graptolites of the Coniston area. Immediately above the Coniston Mudstones comes a great series of rocks to which the name of "Coniston Flags"* was applied by Professor Sedgwick; and these in turn are surmounted by a still more extensive group termed the " Coniston Grits " by the same observer. The latter formation is certainly Upper Silurian, as has been shown by Mr. Hughes ; and the former may very possibly also belong to the Upper Silurian period. Be this as it may, both formations contain Graptolites, often in considerable abundance as far as individuals are concerned ; but there is here a wonderful diminution in the number of genera and species, as compared with the Coniston Mudstones. Thus the genera Diplograpsus, Climacograpsus, and Rastrites, abundantly represented in the Coniston Mudstones, are wanting altogether in the Coniston Flags and Grits, in which there remain only the genera Graptolites and Retiolites. One or two new forms of the genus Graptolites are present ; but the commonest forms are G. priodon, G. colonus, and Retiolites Geinitzianus, all of which existed in the Coniston Mudstones.

It is clear, therefore, that the close of the period of the Coniston Mudstones was signalized in the north of England by the setting-in of conditions unfavourable to the existence of Graptolites. Of the twenty-nine species in the Mudstones, only three pass upwards into the immediately superjacent Flags, and the remaining twenty-six become, so far as this area is concerned, extinct. We shall see, however, that a considerable number of these were not absolutely extinguished, but that they migrated, a few northwards into Scotland, and the larger number in a south-east direction towards Bohemia.

D. Gala area of the Soutn of Scotland. — Returning now to Scotland, I may endeavour to trace out the further dispersal of the Graptolites of the Upper Llandeilo area of Dumfriesshire. So far

Geol. Soc. vol. xxiv.), I included under the name of " Coniston Flags " both the Coniston Mudstones and the overlying Coniston Flags proper. The researches, however, of my friend Mr. T. McKenny Hughes, combined with my own investigations, have satisfied me that the two formations must be separated upon paltaeontological grounds, though no physical break has yet been shown to exist between them.
 * In my paper "On the Graptolites of the Coniston Flags" (Quart. Journ.