Page:Early Man in Britain and His Place in the Tertiary Period.djvu/102

74 fathoms, which imply that a land barrier was in that position for a very long period. It would make but very little difference to the Map of Pleiocene Britain if we were to take the western coast-line to be marked by the 300 instead of the 100 fathom line.

It will be observed by the comparison of the Maps, Figs. 6 and 10, that the geography of Britain in the Meiocene age was modified in the Pleiocene only in two important points—the letting in of the Arctic waters into the North Sea, and their free communication with the Atlantic by the submergence of the area between the Shetlands and Iceland. In other respects, Pleiocene Britain was as it had been before, and plants and animals could migrate over the dry land then uniting France, Spain, and England together, without being confronted by a physical barrier. A geographical continuity of this kind in ancient times was considered by Professor Edward Forbes necessary for the presence of certain Spanish plants such as Arabis ciliata and Pinguicula grandiflora in the south of Ireland.

The Pleiocene mountains were similar to our present mountains, but were undoubtedly higher, because of the enormous amount of denudation, which they have undergone in post-Pleiocene ages, as well as from their then rising from a base at least 600 feet above their present bases. The line of lofty volcanic cones, also, in the Hebrides probably had not altogether lost their subterranean fires in the early Pleiocene ages. Clusters of small lateral cones or puys sprang up on their flank like those on Mount Etna, but they, too, were gradually