Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/560

474 Discinocaris, are found in the darkest and most highly fossiliferous graptolitic zones. The Trilobites were obtained exclusively from a dark grey band lying between two graptolitiferous bands, about ten feet above the highest bed of the Coniston Limestone.

Lastly, all the Brachiopods, with the exception of a common but undetermined Orthis of small size, were procured from a single band, the position of which is shown with absolute clearness at several points in the course of Skelgill Beck.

The highest bed of the Coniston Limestone in this locality is fortunately actually a limestone, and not merely a calcareous shale; and it is seen at various points to be directly overlain by a thin band of the characteristic black Mudstone, not more than six or eight inches in thickness, containing numerous Graptolites (Climacograptus &c). The direct contact and juncture of the limestone and the above-mentioned graptolitiferous band can be observed at various spots; and the latter is at once surmounted by a dark grey shale, about eighteen inches in thickness, from which Graptolites are almost entirely absent, but in which the great majority of the Brachiopods were obtained; and this shale is immediately succeeded by a second graptoliferous band. The shale affording the Brachiopods is highly cleaved; and these shells are, unfortunately, so much distorted as to render their determination a matter of considerable difficulty. The Trilobites, on the other hand, occur in a zone several feet higher in the series, surmounted in turn by other graptolitiferous beds, and they are so well preserved as to admit of ready and complete identification.

Any doubt which might have been previously entertained as to the precise age of the Graptolitic Mudstones seems to be removed by the fossils recently obtained from these beds.

It has been already mentioned that these Mudstones were to be regarded as of Lower-Silurian age; and the materials at present in our hands appear entirely to confirm this view. Leaving the evidence afforded by the Graptolites to be considered separately, it is impossible to doubt that the fauna of the Mudstones is essentially a Lower-Silurian fauna. Amongst the Trilobites Agnostus trinodus, Phacops apiculatus, Calymene senaria, Trinucleus fimbriatus, and Cheirurus bimucronatus are characteristic Bala forms ; and all of them, with the exception of the last mentioned, are exclusively confined to rocks of Lower-Silurian age. Harpes Flanagani, the sole remaining form that we have been able to determine with certainty, is a Bala type from Tyrone and Desertecreat. The Discinocaris is closely allied to if not absolutely identical with D. Brouniana, a well-known fossil from the graptolitic Lower Silurians near Moffat.

Of the Cephalopoda the singular Endoceras proteiforme is a characteristic fossil of the Trenton Limestone (Llandeilo-Caradoc) of North America, and Orthoceras angulatum ranges from the Bala to the Ludlow group. The Brachiopoda, even when so much distorted as to be specifically indeterminable, have nevertheless a distinct Lower-Silurian facies; and Orthis vespertilio with Strophomena expansa are characteristic types of the Bala rocks. Lastly, the few