46 AQUEOUS AGENCIES. 



longations of the perpetual snow-caps are called glaciers. The exist- 

 ence of glaciers and their motion are necessitated by the great law of 

 circulation, so universal in Nature. For in those countries where 

 glaciers exist, the waste of perpetual snow by evaporation is small in 

 comparison with the supply by the fall of snow. There would be no 

 limit, therefore, to the accumulation of snow on mountain-tops, if it 

 did not run off, down the slope, by these ice-streams, and thus return 

 into the general circulation of meteoric waters. Glaciers extend not 

 only far below the snow-line, but even far below the mean line of 32°. 

 In the Alps the snow-line is about 9,000 * feet above the sea-level, 

 while some of the glaciers extend down to within 3,225 feet (Prest- 

 wich) of the same level, i. e., more than 5,000 feet below the snow-line. 



Necessary Conditions, — The conditions necessary to the formation 

 of glaciers are : 1. The mountain must rise into the region of perpetual 

 snow, for the snow-cap is the fountain of glaciers. 2. There must be 

 considerable changes of temperature, and therefore alternate thawings 

 and freezings. This condition seems necessary to the gradual compact- 

 ing of snow into-glacier-ice. The want of this condition is apparently 

 the cause of the non-existence, or small development, of glaciers in 

 tropical regions. 3. A moist atmosphere is favorable to their produc- 

 tion, for the moister the climate the greater is the snow-fall, and the 

 smaller is the waste by evaporation, and therefore the greater the 

 excess which must run off by glaciers. This is an additional reason 

 why glaciers are not formed under the equator ; for the great capacity 

 for moisture of the air in this zone increases the waste while it decreases 

 the fall of snow. This is also the reason of the scanty formation of 

 glaciers in the Sierra Mountains, and their abundance and magnitude 

 in the Alps. 



Ramifications of Glaciers. — We have said glaciers are valley-prolon- 

 gations of the ice-cap. Now, mountain-valleys are of two kinds, viz., 

 1. The deeper and larger longitudinal valleys, between parallel ranges ; 

 and, 2. The transverse or radiating valleys, transverse in case of ridges, 

 and radiating in case of peaks. The longitudinal valleys may be formed 

 either by erosion or by igneous agencies folding the crust of the earth ; 

 but the transverse or radiating valleys are always formed by erosion. 

 It is these valleys of erosion which are occupied by glaciers. In coun- 

 tries where there are no glaciers they are occupied, of course, by 

 streams. We have already shown (p. 9) how these valleys commence 

 near the top of the mountain as furrows, which, uniting, form gullies, 

 and these, in their turn, forming ravines and gorges, thus becoming less 

 and less numerous, but larger as we approach the base of the mountain. 

 In the same manner, therefore, as streams ramify, so also do glaciers. 



* Dana's Manual cf Geology. 



