692 



DYNAMICAL GEOLOGY. 



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

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

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

 or neve. At a lower limit, the snow becomes compacted into ice, by 

 pressure, 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 pro- 

 cess 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 gla- 

 cier. In high latitudes, the ice increases in thickness through the 

 frozen mists of the region and owing to this it often has a stratified 

 structure. This feature was observed, and thus explained, by Wilkes, 

 who found it characterizing the great ice-barrier of the Antarctic seas. 



The following circumstances are essential to, or influence, the forma- 

 tion of glaciers. 



(1.) The region must extend above the line of perpetual congela- 

 tion. 



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

 one side of a chain of mountains may have glaciers, while the opposite 

 is bare. Abundant precipitation in winter especially favors their 

 formation. 



(3.) A difference of temperature and moisture 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 the snow-line, or that below which the snow annually 

 precipitated melts away during the year, and the distance to which 

 glaciers descend, depend mainly on the mean temperature and moisture 

 of the region, and especially the mean temperature of summer as con- 

 trasted with that of winter. The height of the snow-line on the north 

 side of the Alps is about 8,000 feet, and on the southern side about 

 8,800 feet. Below this limit, the glaciers descend 4,500 to 5,300 feet. 



The snow-line in the Pyrenees is 8,950 feet above tide level; in the Caucasus, 10,000 

 to 11,000 feet; on the south side of the Himalayas, 12,980 feet, and on the north, 

 16,620 feet; at the equator, in the Andes, 15,980 feet; in Bolivia, 18,520 feet in the 

 western Cordillera, and 15,920 in the eastern; in Mexico, 14,760 feet; in Chili, near 

 Santiago, 12,780 feet; in Norway, 5,000 feet in its middle portion, and 2,300 feet at its 

 northern extremity; in Kamchatka, 5,200 feet; in Alaska, 5,500 feet. 



The lower limit of a glacier sometimes varies several miles, in the 

 course of a series of years. A succession of moist years increases 

 the thickness of the glacier, and thereby its tendency downward ; 

 while dry years have the reverse effect. If the moist years have also 

 long, hot summers, the descent and lengthening of the glacier will be 



