GLACIERS. 671 



of the grandest in the Alps — is another ice-cataract. As the glacier 

 commences its steep descent, it becomes broken across, and thus 

 great sections of it plunge on in succession, separated partly by- 

 profound transverse chasms. Fig. 951 gives the outline of the lower 

 part of the glacier, am being the cataract, mb its terminal portion 

 or foot, from the extremity of which the river Ehone issues, and 

 c, c, c, transverse crevasses of the cascade. The same is shown in 

 profile in fig. 952, in which c, c, etc. are the transverse crevasses. 



Other glaciers in some of the higher valleys of the Alps reach 

 the edge of a precipice to descend, perhaps thousands of feet, in a 

 crashing avalanche, in which the ice is broken to fragments. 



5. Formation of Glaciers. — The uppermost portion of a glacier con- 

 sists of snow and frozen mist, deposited in successive portions, and 

 usually more or less distinctly stratified. This part is called the 

 neve. At a lower limit, the snow becomes compacted by pressure 

 into ice, owing to the depth of the accumulations ; and here the 

 true glacier portion begins. Below the limit of perpetual frost 

 there is occasional melting in summer, with alternate freezing ; and 

 this process aids in changing the mass, as well as the surface-snow, 

 to ice. The stratification of the neve is not generally distinct in 

 the icy glacier. 



The following circumstances are essential to, or influence, the formation 

 of glaciers. 



(1.) There must be an elevation, or range of heights, above the 

 line of perpetual congelation. 



(2.) Abundant moisture is as important as for rivers ; and hence 

 one side of a chain of mountains may have glaciers, and not the 

 opposite. 



(3.) A difference of temperature between summer and winter is 

 requisite ; for otherwise the snows will be melted to the same line 

 throughout the year, and will not descend much below the line of 

 perpetual congelation. 



The level of perpetual congelation, and the distance to which 

 glaciers descend, depend on the mean temperature and moisture of 

 the region, and especially the mean temperature of summer as 

 contrasted with that of winter. The height of the snow-line, or 

 that of perpetual congelation, is that in which 32° F. is the summer 

 temperature. Below this runs the year-line of 32° F., along which 

 32° is the mean annual temperature. Still below this lies the glacier- 

 limit,— that is, the lowest limit of the glacier. At Mont Blanc, the 

 snow-line is 2000 feet above the 32° year-line, and the glacier-limit is 

 4500 to 5300 feet below it, or 9000 feet above the sea. In the Pyrenees, 

 the snow-line is also 2000 feet above that of 32° ; in the Caucasus, 



