recovered what we may call its _normal_ temperature since the Glacial Epoch. Note on Astronomical Theory.--If the Ice Age be due to the increased eccentricity of the Earth's orbit, the theory shows that a long duration of normal temperature will be followed by a group of Glacial Periods alternating between the northern and southern hemispheres, the time elapsing between the culmination of such a period in one hemisphere and in the other being about 10,500 years. While one hemisphere is in a glacial period, the other will be enjoying a specially mild,--a "genial" period. Now, according to the record of the rocks, the "genial" periods were far from being those breaks in the Glacial which we know as Inter-glacial periods. We have the immensely long warm period of the Eocene and Oligocene, the Miocene with a still warm but reduced temperature, and then the gradual cooling during the Pliocene, till the drop in temperature culminates in the Ice Age. Moreover, the duration of each glaciation during this Ice Age is usually considered to have been much longer than the 10,000 years or so given by the Astronomical Theory. Add to this that the periods of high eccentricity of the Earth's orbit, though occurring at irregular intervals, are, on the scale of geological time, pretty frequent; so that several of such periods would have occurred during the Eocene alone. Yet the geological evidence shows unbroken sub-tropical conditions in this part of the world throughout the Eocene. [Footnote 12: The older division of the Archaean rocks--the Lewisian gneisse--consists entirely of metamorphic and igneous rocks; a later division--the Torridonian sandstones--is comparatively little altered, but still unfossiliferous.] [Footnote 13: The great equatorial mountains Kilimanjaro and Ruwenzori show signs of a former extension of glaciers.] [Footnote 14: For an account of such movements, see Prof. Gregory's _Making of the Earth_ in the Home University Library.] [Footnote 15: See The _Wanderings of Animals_. By H. Gadow, F.R.S., Cambridge Manuals.] Chapter XI THE STORY OF THE ISLAND RIVERS; AND HOW THE ISLE OF WIGHT BECAME AN ISLAND We must now consider the history of the river system of the Isle of Wight, to which our study of the gravels has brought us. For rivers have a history, sometimes a most interesting one, which carries us back far into the past. Even the little rivers of the Isle of Wight may be truly called ancient rivers. For though recent in comparison with the ages of geological time, they are of a vast antiquity compared with the historical periods of human history.