158 HEAT OF HIGHER LAYERS OF AIR. 



common snow-level, yet become in summer quite 

 free from snow, because they lie at a distance from 

 the main mass of lofty mountains. "Where, on 

 the other hand, a great number of snowy moun- 

 tains stand close together, the everlasting snow 

 often comes down to a level at which, on single 

 mountains, it would be melted away every 

 summer. 



During the winter months, the snow-line over 

 the whole of Europe falls to the level of the sea, 

 and finds its limits only between 30° and 35° N. L. ; 

 that is to say, south of this latitude we find no 

 instance in which the snow remains lying on the 

 plains. 



On single shaded spots, of the mountains, and 

 in gullies, into which the sun's rays can seldom or 

 never penetrate, ice and snow often remain 

 through the whole year at a considerable distance 

 below the snow-line. In many gorges and deep 

 valleys, which run up to the heights of everlasting 

 snow, masses of ice are pushed down towards the 

 lower grounds, and are carried forward by the 

 mere continual thrust of the ice above them to 

 regions where the general surface of snow can no 

 longer remain unmelted. These valleys filled with 

 ice form the Glaciers. The eternal ice of the 

 glacier stands in a most remarkable contrast to the 

 mountain-sides which bound them, and which in 

 summer are perfectly snowless ; many of these we 



