108 PHYSICAL GEOGRAPHY. tudes there is comparatively little moisture in the air. The fall is heaviest in the cool temperate regions. 281. Snow Line.—Regions of Perpetual Snow. —The snow which falls on mountains is slowly pressed down the slopes by the weight of the snow above. The distance it will move down the mountain before melting depends on a number of circumstances, The lower limit of the line is -ealled the snow line, above which are the regions of perpetual snow, in which the ground is covered with snow throughout the year. The height of the snow line depends— (1.) On the amount of the snowfall. The greater the fall, the farther down the mountain the snow will move before melting. (2.) On the temperature of the valley. The warmer the valley the higher the snow line. The snow line is, therefore, highest in the trop- ical regions, and lowest near the poles. (3.) On the inclination of the mountain slope. The steeper the slope, the more rapidly the snow will move, and the farther it will go before melt- ing, therefore, the lower the snow line. According to Guyot, the snow line, subject to variations, is about three miles above the sea in the tropics; rather less than two miles in the temperate latitudes; and less than a mile near the northern extremities of the conti- nents; while still farther north, on the polar islands, the snow line is but a few hundred feet above the sea. Over the polar oceans, the winter snows are but partially melted, and help to produce the huge ice-floes of these regions, SNOW LINE. Europe.—Norway, lat. 70° N....ceec.sceeeeees 3,400 feet. “« «“ SCE GQOaN err eeterces 5,500“ 3 Alps, lat. 46° N. (south side) 9,200 “ es Ee S “(north side). 8,800 “ Asia.—Altai Mountains, lat. 50° N..... 7,000 ‘ “Himalayas, lat. 31° N..........ccceeeees 17,000 “ Africa.—Kilimandjaro, lat. 3° S..ceseeseee 16,000 “ North America.— Rocky Mountains, lat. ABOM NE Gen coos ea eee TS 12,467“ South America.—Andes, Ecuador, lat. 1° 8. 15,800 “ « ‘i See Tapa bactS he ces 3,700“ The snow line is generally lower in a moist atmosphere than in dry air, because of the greater fall of snow in the former case than in the latter. As a rule, that slope of a range which is exposed to the prevalent wind has a lower snow line than the opposite slope. The position the slope occupies in relation to the vertical rays of the sun, also exerts an influence on the height of the snow line. 282 Glaciers are immense masses of ice and snow, which move almost imperceptibly down the higher mountain valleys or slopes. Their upper parts are formed of soft snow; their lower por- tions of clear, hard ice. Their origin is as fol- lows: The weight of the huge snow fields, which form above the snow line, presses the mass slowly down the slopes. The pressure, due to the weight of the layers, but especially the pressure which is produced when the mass is forced through a con- traction in the valley, squeezes out the confined air, to which snow, in great part, owes its white color, and the lower part of the glacier thus be- comes changed into a compact mass of pure ice. The alternate thawing and freezing to which the mass is subjected below the snow line, also con- tribute to the change from snow to ice. The change is most thorough in the lower parts of the glacier, where the ice is marvellously clear. Its color, when seen in great depths, is of a deep azure blue; in the middle portions of the glacier the ice is coarse and white. The higher region of but partially changed snow is called the névé region. Here the snow occurs in coarse white grains. The process of formation is a continuous one. The névé region is supplied by fresh falls of snow, which replace those pressed down the slopes. \, 288. Drainage of Snow and Ice.—Glaciers closely resemble rivers, since they receive the drainage of their basins through the solid mate- rial which flows into them; their motion, how- ever, is much slower. Like rivers, they have their tributaries, and their peculiarities of flow and velocity. Several glaciers often unite and flow on as one mass; but their solid condition prevents the in- termingling which occurs in rivers, and the sepa- rate streams can generally be distinctly traced throughout the remainder of their course. Like rivers, the top and middle portions move more rapidly than the sides or bottom, owing to the diminished friction. \ 284. Peculiarities of Glaciers—The surface of the glacier is often comparatively smooth; but when irregularities occur, either in the direction of the valley, or in the slope of its bed, the glacier is broken into deep fissures, called crevasses. These are most numerous on the sides, from which they extend either obliquely up the stream, or directly across, in deep transverse fissures. The former are generally due to a bend in the valley, one side being compressed and the other extended; the latter, to steep and abrupt descents in the bed. Crevasses are, therefore, rapids in the ice stream. Crevasses vary in breadth from mere crevices, that a knife-blade can scarcely penetrate, to yawning chasms over 100 feet in width. The depth of the wider crevasses is generally profound. Their vertical walls afford a con- venient opportunity for studying many peculiarities of formation. Looking down the walls of the crevasses, the ice appears of a deep azure blue. The surface ice is a dirty white.