Page:Early Man in Britain and His Place in the Tertiary Period.djvu/103

] deserted by the expiring energy; and the volcanoes of the Western Islands, inert and cold, became a prey to the ravages of the elements which have reduced them to their present condition. How vast this destruction was has already been placed before the reader in treating of the Meiocene mountains, p. 44.

The Pleiocene rivers in Britain were in their present positions, though they flowed at a higher level, and those on the west opened upon the Atlantic coast-line, then far away from their present mouths, as is shown in the map (Fig. 10).

The researches of Professor Prestwich and Mr. Searles V. Wood into the mollusca of the British Pleiocene strata show that the temperature of the sea gradually became lowered in passing from the period of the Coralline Crag towards the upper deposits. This lowering of the temperature caused the area of the North Sea to be invaded by northern forms, and compelled the retreat of the southern species now found in the warm waters of the Mediterranean, as may be seen from the following table, constructed by Professor Prestwich, from which all the species common to the crags and the British seas of the present time are omitted.