Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/561

Rh corals which have been found are not such as to give any guide to the age of the strata, all that can be said being that they belong to a type not hitherto recognized in the underlying Coniston Limestone.

The clear evidence borne by the Crustacea, Brachiopoda, and Cephalopoda as to the Lower-Silurian age of the Graptolitic Mudstones, is still further substantiated when we consider the characters of the Graptolites themselves, by far the most abundant fossils in this series. Without entering into any minute analysis of the Graptolites found in these rocks, it may at once be stated that they constitute an assemblage of forms of an unequivocally Lower-Silurian aspect. This is proved by the presence, in abundance, of representatives of the Diprionidian genera Diplograptus and Climacograptus, by the remarkable variety of the species of Monograptus, and by the presence of the genus Rastrites. At the same time, that the Graptolitic Mudstones are not low down in the Lower Silurian series is equally clearly shown by the total absence of the genera Didymograptus and Dicranograptus.

As regards the species of Graptolites, Climacograptus teretiusculus, Diplograptus pristis, D. palmeus, D. tamariscus, Monograptus Sedgwickii, M. triangulattis, M. spinigerus, M. intermedius, M. gregarius, M. Sagittarius, M. fimbriatus, M. lobiferus, M. Nilssoni, Rastrites peregrinus, R. distans are all found in the Moffat shales of the south of Scotland, and are more especially characteristic of that division of the Moffat shales to which Mr. Lapworth has given the name of the "Birkhill Group." Thus nearly three fourths, or seventy-five per cent., of the total number of Graptolites known in the Mudstones, including all the common and characteristic species of the group, can be specifically identified with forms which serve to mark the Lower Silurian rocks of the southern uplands of Scotland, the position of which has never been questioned.

Taking all the various fossils now known from the Graptolitic Mudstones together, it is impossible to doubt that the balance of the palæontological evidence is overwhelmingly in favour of the view that this formation is of Lower-Silurian age.

Such being the case, the Graptolitic Mudstones must correspond in position with the highest beds of the Bala series or with the lower portion of the Llandovery group; and this is the direction in which we believe all the evidence tends. As to the precise physical relations between the Graptolitic Mudstones and the subjacent Coniston Limestone, we are of opinion that the two groups are strictly conformable to one another. Not only are the Mudstones invariably found in their proper position, resting upon the limestone, as seen when there are sections of these groups, but no discordance can be detected, as regards the dip and strike, between the two series where they cannot be seen in actual contact.

Moreover (and this appears to us to be an argument of the greatest weight) it cannot be shown that there is any overlap of the Graptolitic Mudstones upon the Coniston Limestone, the former always resting, so far as we have seen, upon the highest bed of the latter.