Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/562

476 If it be remembered that along one line of outcrop alone, from Appletreeworth Beck to Shap Wells, the Graptolitic Mudstones can be traced almost continuously succeeding the limestone for a distance of about twenty-four miles, it will be seen to be almost impossible that any want of conformity, however slight, could exist without there being, at the same time, a transgression of the Mudstones over the Limestones. It is true that, owing to the circumstance that the calcareous matter of the Coniston Limestone series is disposed in the form of irregular lenticular masses or concretionary layers, the lithological character of the bed immediately below the Mudstones is not invariably the same, being at one time a limestone and at another a calcareous shale. This, however, is due to an irregularity of deposition which obtains throughout the entire limestone series; and we have failed to find any evidence that the Graptolitic Mudstones ever rest upon any of the lower beds of the Coniston Limestone. As the latter group is of small thickness, and as its main line of outcrop is a very long one, such an overlap must occur, supposing unconformity to exist; and in all probability we should even find the Mudstones passing across the limestones and resting upon the older Borrowdale series.

The absence therefore of any unobserved overlap is, under the circumstances, the strongest possible proof that the Mudstones are entirely conformable to the Coniston Limestone.

That the Graptolitic Mudstones constitute a geological horizon of a definite character, and of much more than mere local importance, is shown by the fact that they can be recognized in Ireland in circumstances similar to those under which they occur in the northwest of England (see Appendix). They have also been recognized in Sweden, in Carinthia, and in Bohemia, while future researches will doubtless bring corresponding strata to light in other Lower-Silurian regions.

In Sweden Dr. Linnarsson has shown (Geol. Mag. June, 1876) that the so-called "Upper Graptolitic Schists" are the equivalents in that country of the Graptolitic Mudstones of the north of England. These Upper Graptolitic Schists, as seen in Westrogothia and Ostrogothia, are the highest Silurian rocks exposed to view, so that they add nothing to the evidence as to the age of the Coniston Mudstones.

In Scania, however, they are overlain by undoubted Upper Silurian beds; and in Dalecarlia they are surmounted by a locally developed limestone (the "Leptsena Limestone" of Tornquist), which appears to form either the summit of the Lower Silurian or the base of the Upper Silurian, being in turn covered by the undoubted Upper Silurian "Encrinurus-beds." Upon the whole, therefore, the evidence to be derived from the Swedish area entirely corroborates the view that the Graptolitic Mudstones are of Lower-Silurian age.

In Carinthia, beds corresponding precisely with the Graptolitic Mudstones have been described by Dr. Guido Stache (Die Graptolithen-Schiefer am Osternig-Berge in Kärnten); and the parallelism of the two deposits has been fully noticed by this distinguished