Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/563

Rh observer. Unfortunately the succession of the Silurian strata in the Osternig is still so obscure as not to permit of any safe con- clusions being drawn as to the precise stratigraphical horizon of these beds, though Dr. Stache concludes that they stand on the borderland between the Lower and Upper Silurians.

In Bohemia we find the representatives of the Graptolitic Mudstones in the "Colonies" of Barrande's Étage D and in the lower portion of the Étage E of the same eminent palæontologist.

These beds, as is well known, are referred by M. Barrande to the base of the Upper Silurian.

Without, however, entering fully into this question here, and disregarding the strong evidence which we now hold as to the Lower-Silurian age of our Graptolitic Mudstones, we would simply point out that M. Barrande himself fully admits that the succession of Silurian life in the Bohemian area was later, stage by stage, than in the northern European and British areas. Thus he supposed that the Bohemian area was peopled with a general Lower-Silurian fauna at a time when the North European and British areas were peopled with a general Upper-Silurian fauna, and he employs this supposition to explain the phenomena of the "Colonies." Admitting therefore that the "Colonies" and the lower portion of Étage E are Upper Silurian, it would in no way follow that the corresponding Graptolitic Mudstones of the north of England are also Upper Silurian. On the contrary, by M. Barrande's own theory, the Graptolitic Mudstones ought to be Lower Silurian, being thus "homotaxeous," but not "contemporaneous" with the lower part of Étage E of Bohemia. We thus see that the evidence to be derived from Bohemia, though apparently at conflict with our views as to the age of the Graptolitic Mudstones, is, when fully analyzed, an additional argument in favour of our conclusions.

The Graptolitic Mudstones are succeeded by a series of strata for which the name of "Knock beds" has been proposed (Nicholson & Lapworth, Rep. Brit. Assoc. Bristol, 1875), on account of their excellent development in Swindale Beck, near Knock. Lithologically the Knock beds present a singular uniformity wherever they are found, enabling them to be recognized with the greatest readiness. They consist principally of pale green, fine-grained slates, extremely ashy in their appearance, exhibiting numerous dendrites, and very commonly containing crystals of cubic pyrites. Along with these greenish slates are often well-marked bands of red and purple slates of the same grain and texture as the preceding; and occasionally there is met with a thin band having a grey or even a nearly black colour, though this is exceptional. The general strike and dip of the Knock beds conform with those of the underlying Graptolitic Mudstones; and there is, at present, no clear evidence of any want of conformity between the two groups.

The Knock beds have been subjected to a varying but always a