Disintegration of Rocks. 5 



and earthy masses. Some of its most obviously power- 

 ful effects are seen in the regions of glaciers and drift 

 ice. In warm latitudes glaciers are found only at those 

 great elevations on mountain ranges that rise above the 

 limits of perpetual snow. On the Himalaya, the loftiest 

 peaks of which are about 31,000 feet high, the greater 

 glaciers descend to the level of about 14,000 feet; in 

 the Alps, in the lower glacier of Grrindelwald, to about 

 3,300; and in the Grlacier du Bois to 3,350 feet above 

 the sea. In the north of Norway, Greenland, and the 

 southern part of South America, and in the Antarctic 

 continent of Victoria Land, the large glaciers descend 

 to the sea-level. In the two last-named regions, towards 

 the poles, surfaces of vast extent are covered by ice in 

 the form of universally diffused glaciers. 



A glacier in temperate regions is chiefly supplied by 

 the drainage of the snow that falls on those parts of 

 the mountains which rise above the limits of perpetual 

 snow ; and its size is commensurate to the height of the 

 mountains and the extent of area drained. Pressure of 

 the yearly accumulating snow, and in less degree the 

 summers heat and the winter's cold, or, indeed, the 

 summer day's thaw and the nightly frost, gradually 

 change snow into ice, which experience proves, acts 

 as a whole, like a plastic body, and glaciers progress 

 down valleys at slow rates, proportionate to the steep- 

 ness of their inclination, the volume of ice, and the 

 season of the year moving faster in summer and au- 

 tumn, and slower in winter. The effect of this motion 

 in these icy masses is to grind, polish, scratch, and 

 groove the rocky valleys over which the glaciers pass, 

 removing asperities, and giving portions of the rocky 

 floor rounded and mammillated forms, termed roches 

 moutonnees. A necessary result of this action is the 



