Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/398

Rh conditions, and similar rolled and broken fossils with those so characteristic of most of the glacial beds around the Irish Sea.

The upheaval of the bed of the glacial sea, was not sudden but gradual. The phenomenon, so well described by Professor Forchhammer in his essays on the Danish drift, indicating a conversion of a muddy sea of some depth, into one choked up with sand banks, though not universal, are equally evident in the British Isles, especially in Ireland and the Isle of Man. In the latter locality, the marl beds containing bivalves of the second or third region in depth, are capped by a great thickness of sands and gravels, occasionally containing littoral shells, but rolled. On these sands the larger boulders usually lie. In Ireland Captain James has found the littoral shells in abundance in the sandy beds, especially in such localities as were evidently close upon the ancient coast line. The period of the extinction, as a race, of the reversed variety of Fusus antiquus may be referred to that of the upheaval of the Irish sandy beds.

The close of the glacial epoch, ending in this upheaval of the bed of the glacial sea, marks the commencement of a new era in our fauna, and, as we have already seen, of our flora. As a great part of the area, previously occupied by water, now became land, the banishment of a number of species necessarily took place, many of which, in consequence of the change of conditions arising from the causes of their expulsion, retired for ever. The remodelling of our area, which afterwards took place, the formation of the Irish Sea, of the German Ocean, and of a new line of coasts, events calling new influences into play, introduced the existing population of our seas. Part of our glacial testacea had been extinguished, part retired to more congenial arctic seas, and a few disappeared from the coasts of Europe, while they continued inhabitants of the shores of America. A considerable number, however, returned to the seas of their ancestors, where they became and remain the associates of numerous forms, some newly called into being to people the new-formed seas, some coming with favouring currents from the warmer seas of the south. Among the latter were a number of forms which had not always been strangers to the British Seas. During the genial times preceding the glacial epoch, more than fifty species of testacea, inhabitants at present of our seas, lived in them whilst the crag beds were in process of formation but disappeared under the chilly influences of the sub-arctic epoch which succeeded. As either a re-creation (a notion inconsistent with the line of argument I have adopted in this essay) of these forms must have taken place, or else a re-migration of them from some distant point where they had lived on, amid favouring conditions, when banished from the British Seas,—an inquiry