Remains of birds are among the rarest of fossils, and few have been described save from the more recent formations. With the exception of Archœopteryx from the Jurassic, and a single species from the Cretaceous, no birds are known in the old world below the Tertiary. In this country, numerous remains of birds have been found in the Cretaceous, but there is no satisfactory evidence of their existence in any older formation, the three-toed footprints of the Triassic being probably all made by Dinosaurian reptiles.

In the preceding chapters we have endeavoured to throw light upon some of the more salient characters of volcanos—the features of their architecture, the nature of their products, and the peculiarities of their attendant phenomena. In doing so, however, we have preferred to select our illustrations from some of the less familiar, though, as we hope to have shown, by no means less interesting volcanos of our continent; and have of set purpose avoided the oft described examples of Vesuvius, Etna, the Auvergne, and the Eifel.

B. Mountain Limestone Proper.—Both the Hecla Hook and Liefde Bay strata, and the strata included in the foregoing division, consist of fresh-water formations, devoid of any trace of marine fossils; which proves that Spitzbergen, during the immense period of time in which these strata were deposited, formed a considerable continent. Later, however, these circumstances were altered, inasmuch as that part of the globe was again covered by sea, from which, with only an inconsiderable, perhaps local, break (the strata of the Coal-formation in Bell Sound), during a long sequence of geological periods, strata of great extent, containing marine fossils, were deposited. The oldest and most extensive of these strata belong to the Mountain Limestone formation, which in this part of the polar regions is specially developed and rich in fossils.

Following up the notes which recently appeared on Cretaceous Aporrhaïdæ, I now propose to continue from time to time notes on other Cretaceous Gasteropoda. At present I have selected for illustration “Scalariform Shells.” The most recent papers which refer to this subject are witnesses how much revision of the genera and species is needed to which our British Cretaceous representatives of this type have been assigned. I first take the family of Scalidæ, and will, as briefly as possible, endeavour to recapitulate what is at present known of this family.

A Good deal of attention has been lately bestowed on the Raised Sea-beaches around the British Islands and the evidence afforded by them as to alterations in the level of the land. Some geologists seem to entertain the idea that such beaches were originally accumulated on horizontal lines; and they have drawn the conclusion, that when one portion is found to be higher than another, it necessarily indicates an uneven rise in the surface of the earth. This idea, however, is erroneous; and the object of this paper is to prove, from the data supplied by the Ordnance Survey and the Admiralty Charts, that the littoral deposits at present accumulating around Ireland are being formed at varying levels.

Most of these islands appear to be of volcanic origin, their centres rising to high-peaked, cone-shaped, mountains, of from 500 to 6000 feet in height. The weather, east and south-east, sides are clothed with thick forests from base to summit; but the north-west and west sides in many of them have but a thin sprinkling of trees, the slopes being covered with a thick, wiry grass.