Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/564

478 high amount of cleavage; and fossils, though by no means wholly unknown in them, are almost always more or less distorted. At Knock they have yielded only two Graptolites, viz. Monograptus priodon, Bronn, and M. broughtonensis, Nich. & Lapw. At Skelgill, and on the high ground between this place and the vale of Troutbeck, they have afforded many specimens of small Brachiopods (Orthis and Discina) and minute Lamellibranchs. These, however, occur only in the form of external and internal casts; and, as yet, no conclusion has been arrived at concerning their specific forms.

The Knock beds are directly surmounted by the well-known and well-marked group the "Coniston Flags," a series of strata corresponding to the "Denbighshire Flags" of North Wales, and which have been clearly shown, more especially by the researches of Prof. Hughes, to be of Upper-Silurian age. As to the exact age of the Knock beds, it is not to be denied that sufficient evidence is yet wanting on which to found any positive or final opinion. They rest upon the Graptolitic Mudstones, which we have shown to be placed nearly or quite at the summit of the Lower Silurian; and they are overlain by the Coniston Flags, which are quite or nearly the base of the Upper Silurian. It is therefore clear, from their physical position, that the Knock beds must be either the basement series of the Upper Silurian, or the summit series of the Lower Silurian, or a group of passage-beds between these two.

The palæontologieal evidence at present obtained is not enough to justify us in adopting definitely any one of these hypotheses. So far as it goes, the evidence tends to favour the view which regards them as the base of the Upper Silurians—the only two species of Graptolites observed being forms common to the overlying Coniston Flags, whilst there appears to be a complete absence of the genera and species characteristic of the Graptolitic Mudstones.

Further researches, however, will undoubtedly add to the fauna of this group of beds, and enable its position to be determined with greater precision.

In the meanwhile it can only be said that the conclusion to which the few known fossils point is corroborated by the strong lithological resemblance between the "Knock beds" and the "Tarannon Slates" of Wales. We cannot, therefore, be far wrong in provisionally regarding the Knock beds as the base of the Upper Silurian series of the Lake-district, in which case the Graptolitic Mudstones will constitute the highest portion of the Lower Silurians of the same area.

On referring to a geological map of the British Isles it will be seen, from the strike of the Coniston Limestone in the southern portion of the Lake-district, and after it disappears under the newer