Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/532

438 favourable conditions of existence in the seas which then covered Lancashire. 3rdly and lastly. On the land emerging from the sea, the isolated glacier-areas show that the climate was severe, but yet not so severe as in the time of the continuous ice-sheet. On the Continent the traces of this lowering of the temperature are to be found in the travelled blocks and the Boulder-clay, which occupy the whole region north of the continuation of the line passing through the Thames valley eastward into Russia. To the south, however, of this line there is no evidence of a continuous ice-sheet—a fact which can only be accounted for by the climate at the time having been less severe than in the northern region. Nevertheless a mer de glace extended far over the Jura from the lofty axis of the Alps; and glaciers have left their unmistakable moraines in the valley of the Rhine, at least as far down as Suabia, as well as in Lombardy. The Alps, indeed, formed an axis, from which the ice extended far down on every side into the lower districts. M. Desor has proved that the three climatal changes which are so marked are traceable also in Switzerland. The lignite beds of Dürnten, which have furnished the remains of Elephas antiquus, rest on an ancient moraine, and are also covered by a mass of glacial detritus. It is clear, therefore, that before the accumulation of the lignite the cold was sufficiently intense to allow of glaciers occupying the horizon, that during the growth of the trees on the spot the glaciers had retreated, and that subsequently there was a reversion to the intense cold of the first Glacial period. These three changes have not been traced in any other part of the Continent.

The moraines and roches moutonnées and detritus which cover the flanks of the Pyrenees prove that they formed an axis from which the glaciers radiated into France and Spain. And similar remains detected by M. Delanoue in the valley of the Dordogne prove that the higher region of Auvergne was also covered with ice.

The mere fact of these glaciated areas being isolated shows that the Pleistocene climate was less severe in Central than in Northern Europe, although the angular condition of the superficial detritus, pointed out by Mr. Godwin-Austen, in the south of England, and the twisted and contorted river-gravels of Britain and Northern France, noted by Mr. Prestwich, that have been disturbed by the contact of ice, imply a temperature considerably lower than that which is now found in those countries.

On physical grounds, therefore, we have reason to believe that in the Pleistocene period, treated as a whole, there were two distinct climatal zones—the arctic, which extended as far down as a line passing through the valley of the Thames eastwards, and a zone with cold winter and warm summer, which extended from this line as far south as the Alps and Pyrenees. In the one the northern division of the Pleistocene mammalia found their head quarters, while the other afforded a common feeding-ground for both northern and southern animals. And the northern boundary of the latter gradually passed northwards, as the temperature became warmer, as far as the shores of the Baltic, probably in the latest stage of the Pleistocene.