Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/557

Rh scene of the intense volcanic activity which gave birth to the Borrowdale series.

Resting directly upon the Coniston Limestone, in every locality where the summit of the latter has been recognized, is a series of dark-coloured Mudstones and grey Shales, which, though of small vertical thickness, are of especial interest from the organic remains which they contain. These beds were originally described by us (Quart. Journ. Geol. Soc. vol. xxiv. p. 296); and to them was applied the name "Graptolitic Mudstones," as indicating their general mineral character and the predominant fossils which they yield. Subsequently, as an alternative local name, the title "Skelgill Beds" was proposed by one of us for these strata, from the farm of High Skelgill, near Ambleside, where they are typically developed (Nicholson and Lapworth, Rep. Brit. Assoc. Bristol, 1875). As before stated, immediately above the highest member of the Coniston Limestone these Graptolitic Mudstones come on. The finest sections of the series, however, are to be found on both sides of Long Sleddale, in Skelgill Beck, near Ambleside, in Appletreeworth Beck, and in Swindale Beck, near Knock.

Many other localities exhibit the same beds; but they are generally badly exposed, and, from their comparatively soft nature, they often become so far worn down as to be indicated simply by a depression immediately following the outcrop of the limestone.

In their mineral characters the Graptolitic Mudstones are so well marked that they may be recognized by this alone, apart from the detection of their characteristic fossils. They consist of dark-coloured, often nearly or quite black Mudstones, which are sometimes anthracitic, the joints of which are characteristically iron-stained. These dark Mudstones are replete with beautifully preserved Graptolites, and alternate in successive bands with lighter and darker grey or even sometimes greenish shales, both the light and dark bands being more or less cleaved. Annexed is a diagrammatic sketch (fig. 3) showing the succession of the strata at Skelgill, where the Graptolitic Mud-stones are admirably exhibited.

In our earlier researches in the Graptolitic Mudstones we devoted ourselves principally to the black bands containing Graptolites, a large number of species of these organisms being described by one of the present writers from this group (Nicholson, Quart. Journ. Geol. Soc. vol. xxiv. p. 521). More recently an examination has been made, in greater detail, of the grey bands which interstratify the dark-coloured graptolitic layers, with the gratifying result that these have been proved to contain a considerable number of fossils of higher organization than Graptolites. We append a list of the more important species of Graptolites, and of the other fossils at present known as occurring in this series.