THE TUNDRAS AND STEPPES OF PREHISTORIC EUROPE. 341 



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 05° 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 elevatious of 3,600 to 3,900 feet, where the mean 

 annual temperature in our day does not exceed 40 c . 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 w T ay into the low grounds considerably beyond the limits 

 reached by their predecessors in the first glacial epoch. That the sec- 

 ond, 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 neves 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 



