Glaciers. 363 



the sea, whereas the limit of perpetual snow is 8,500 

 feet. Ever melting on its surface, in its mass, and at 

 the end, each glacier is yet ever renewed by yearly falls 

 of snow, and by direct gravity on the slopes, and by 

 pressure of accumulating snow and ice behind, and by 

 melting and regelation, it is urged down the valley and 

 maintains its average size. I will not enter into all 

 the details of the structure of the ice of glaciers, 

 because that will not help us in the special geological 

 investigation now in view; but I will describe what 

 are the effects produced by a glacier in the country 

 over which it slides, and various other glacier-pheno- 

 mena affecting the scenery of the Alps, and therefore 

 affecting the scenery of our own country in past times 

 when glaciers existed here, and still affecting it in the 

 relics they have left. 



A glacier slides more or less rapidly according to 

 the mass of ice that fills the valley, and to the greater 

 or less inclination of the slope, for in these respects it 

 behaves very like a river. If we have a vast river like 

 the Mississippi flowing down a broad valley, although 

 the slope of the valley may be gentle, still the river 

 flows with rapidity, in consequence of the greatness of 

 the body of water ; so if we have a mass of ice, which 

 represents the snow-drainage of a large tract of country, 

 covered with perpetual snow, then the glacier floivs 

 with, a rapidity proportionate to the mass of ice., and 

 that rate of progress is modified, increased, or dimin- 

 ished, in accordance with the fall and width of the 

 valley, so that when it is steep, the glacier flows com- 

 paratively fast, and when the angle at which the valley 

 slopes is small, it flows with comparative slowness. 

 Like a river also when the valley expands in width, so 

 does the glacier broaden to meet the mountain sides on 



