Page:Early Man in Britain and His Place in the Tertiary Period.djvu/104

76 From this table it is clear that the southern shells were being driven away from their habitation by the depression of the temperature of the water in which they had lived, and that they were being slowly replaced by those of a northern habit; the increase in the number of the latter, as Sir Charles Lyell acutely points out, being from 5⋅0 per cent in the Coralline Crag to 10⋅7 per cent in the Red, and 14⋅6 in the Norwich Crags. This was due to a more intimate connection with the Arctic Ocean, and to the consequent invasion of the British area by currents of cold water. But we have other evidence that this was the case. Professor Prestwich calls attention to a large block of porphyry in the Coralline Crag at Sutton, which is undoubtedly ice-borne, and Sir Charles Lyell mentions unworn and angular chalk flints in the Red Crag which have been transported by the same agency. From these facts we may infer the presence of floating ice in the North Sea in the Pleiocene age, and it is very probable that this was brought about, not merely by a general lowering of the temperature in the northern regions, such as Scandinavia, but also by the submergence of the tract of land uniting Iceland with the continent of Meiocene Europe, by which currents of cold water from the Polar regions obtained free access to the North Sea of the Pleiocene age, from which they had before been shut out by a barrier of land.