
~ Panr IL Szcr. ii. §5.] SNOW AND GLACIERS. 403 
polar circles. Between these limits it rises gradually in level till it 
reaches its highest elevation in tropical latitudes. South of lat. 78° 
N. it begins to retire from the sea-level, so that on the coast of 
northern Scandinavia it is already nearly 3000 feet above the sea. 
None of the British mountains quite reach it. In the Alps it stands 
at 8500 feet, on the Andes at 18,000 feet, and on the northern 
slopes of the Himalayas at 19,000 feet. 
Snow exhibits two different kinds of geological behaviour, (1) 
conservative, and (2) destructive. (1) Lying stationary and unmelted 
it exercises a protective influence on the face of the land, shielding 
rocks, soils, and vegetation from the effects of frost. On low grounds 
this is doubtless its chief function. (2) When snow falls in a: 
partially melted state it is apt to accumulate on branches and leaves, 
until by its weight it breaks them off, or even bears down entire 
trees. Great destruction is thus caused in dense forests. Snow 
which falls thickly on steep mountain slopes is frequently during 
Spring and summer detached in large sheets. These rush down the 
declivities as avalanches, and sweep away trees, soil, crops, and 
houses. Another indirect effect of snow is seen in the sudden rise 
of rivers when warm weather rapidly melts the mountain snows, 
Many summer freshets are thus caused in Switzerland. It is to the 
melting of the snows, rather than to rain, that rivers descending from 
snowy mountains owe their periodical floods. Hence such rivers 
attain their greatest volume in summer. A curious destructive 
action of snow has been observed on the sides of the Rocky Mountains, 
where the. drifting of snow crystals by the wind in some of the passes 
has damaged and even killed the pine trees, wearing away the 
foliage, cutting off the bark and even sawing into the wood for 
several inches." 
Glaciers? are rivers of ice formed by the slow movement and 
compression of the snow which by gravitation creeps downward into 
valleys descending from snow-fields) The snow in the higher 
regions is loose and granular. As it moves downward it becomes 
firmer, passing into the condition of nevé or firn (p.111). Gradually 
as the separate granules are pressed together and the air is squeezed 
out, the mass assumes the character of blue compact crystalline ice. 
From a geological point of view a glacier may be regarded as the 
drainage of the snowfall above the snow-line, as a river is the 
drainage of the rainfall. A glacier, like ariver, is always in motion, 
though so slowly that it seems to be solid and stationary. The motion 
also, like that of a river, and for the same reason, is unequal in the 
different parts, the centre moving faster than the sides and bottom. 
This important fact was first ascertained through accurate measure- 
1 Clarence King, Exploration of 40th Parallel, i. p. 527. 
2 On glaciers and their geological work, see De Saussure, “‘ Voyages dans les Alpes,” 
~ § 535; Agassiz, “Etudes sur les glaciers,” 1840; Rendu, “Théorie des glaciers de ia 
Savoie,” Mem. Acad. Savoie, x., translated into English 1875; J. D. Forbes, “ Travels 
in the Alps,” 1843 ; “ Norway and its Glaciers,” 1853 ; “ Occasional Papers on Glaciers,” 
1859; Tyndall, “ Glaciers of the Alps,” 1857; Mousson, “ Gletscher der Jetztzeit,” 1854, 
2Dd2 

