I shall confine attention in the first place to the alpine lands, for it is in the low grounds at the base of those mountains that the relation of the loss to the glacial and fluvio-glacial deposits can be most clearly made out.
It has now been ascertained that glaciers have on three successive occasions filled the great mountain valleys of the Alps and descended to the low grounds. The earliest advance I have already described—this constitutes the first glacial epoch of Swiss geologists. It was followed by a long spell of genial conditions when the great glaciers melted away, and retired to the inner recesses of the mountains. Many relics of the flora of this genial epoch have been preserved. Thus in the valley of the Inn, near Innsbruck, certain deposits have yielded an assemblage of plants similar to that which we now meet with in the valleys of the mountain regions south of the Black Sea—most of the plants being existing species. The mean annual temperature of the regions in which that flora now flourishes is 57° to 65° F., while that of Innsbruck at present is only 47°. But in the genial epoch of which I speak, the flora in question flourished on the mountain slopes over-looking Innsbruck at elevations of 3,600 to 3,900 feet, where the mean annual temperature in our day does not exceed 40°. This is enough to show us that the climatic conditions of the alpine valleys must formerly have been considerably more genial than at present. From this and similar evidence in other alpine valleys we may safely infer that the retreat of the glaciers was the result of a great change of climate, and that during the first interglacial epoch the snow fields and glaciers must have retired to the highest ridges of the mountains.
The plant beds just referred to are not only underlaid, but overlaid by bottom or ground moraines, the overlying moraines belonging to the second glacial epoch. It was during this epoch that the glaciers of the Alps attained their greatest development—the snow line becoming depressed to 4,700 feet below its present level. The glaciers now pushed their way into the low grounds considerably beyond the limits reached by their predecessors in the first glacial epoch. That the second, like the first glacial epoch, was of long duration is shown by the amount of erosion effected by the ice flows and the enormous extent of their bottom and terminal moraines.
Overlying the ground moraines of that epoch we again come upon alluvial deposits in many places, which are crowded with the remains of a temperate flora—a flora resembling that of the low grounds of Switzerland and north Italy in our own days. It is obvious, therefore, that when such a flora flourished in the great valleys of the Alps the climate could not have been less genial than the present; the snow line must have again retreated to a higher level, and the névés and glaciers were probably not more extensive than they are now. This constitutes the second interglacial epoch of Swiss geologists. Ere long it was followed by a third general advance of the glaciers, which once
