234 DYNAMICAL GEOLOGY. 



large, the suows may be permanent far below tlie line even if the summers 

 are warm. In accordance with this principle the snow-line in ivet southern 

 Chile is 6000 feet lower than it is in corresponding latitudes in North 

 America, and 3000 feet lower than in Europe ; and in dry northern Chile, in 

 latitude 33° S., it is as high as it is 15 degrees farther north (Buchan). 



But exceptionally snowy winters followed by a succession of two or 

 more cool summers may make accumulating deposits of snow in some shaded 

 valleys of high mountains that are much below the normal limit of per- 

 petual snow, and produce temporary accumulations of ice that have incipient 

 flow — a fact observed in the White Mountains, N.H. 



The height of the line of perpetual snow is 18,500' in the western Cordillera of the 

 Bolivian Andes near the equator, and 15,920' in the less dry eastern ; 12,980' on the south 

 side and 16,680' on the di'ier north side of the Himalayas ; 12,780' in the Chilean Andes, 

 near Santiago ; 14,760' in Mexico ; about 13,000' in Teneriffe ; 8400' on the northern and 

 8800' on the sunnier southern or Italian slope of the Alps ; 5000' in Norway; 3000' in Lap- 

 land ; 5500' in Alaska ; about 2000' to 2200' in Danish Greenland, where the mean annual 

 temperature at the sea level is between 13° and 33° F., it being, according to Rink, at 

 Upernavik, in 72° 48' N., 13-3 F.; at Jakobshavn, in 69° 14' N., 22-6 F.; at Godthaab, in 

 64° 8' N., 27-8 F.; at Lichtenau, in 60° 31' N., 33-2 F.; the annual range of monthly 

 means, at Jakobshavn, being 0-3 F. to 45-3 F., and at Godthaab, 11-8 F. to 48-4 F. The 

 temperature of the soil 4 feet under ground at Godthaab varies during the year between 

 31-5 F, (in March) and 40-1 F. (September). 



Lowering the mean temperature of a place, by cooling the summers, lowers the glacier- 

 limit. Great Britain and Fuegia (Tierra del Fuego) are in nearly the same latitude ; and 

 yet, in Fuegia, the snow-line is only 3000' above the sea. If, by any means, the climate 

 of Great Britain could be reduced to that of Fuegia, it would cover the Welsh and Irish 

 mountains with glaciers that would reach the sea, the snow-line being but 1000' to 2000' 

 above it ; and the same cause would place the snow-line in the Alps at 5000' to 6000' above 

 the sea, instead of 8400'. This change of temperature involves a removal of tropical 

 sources of heat, or an increase of arctic sources of cold. 



The length of flow of a glacier before it melts away depends mainly, as 

 stated above, on the thickness of the ice-mass, and largely because ice cannot 

 melt without absorbing an amount of heat sufflcient to raise the temperature 

 of a like quantity of water 143° F., the latent heat of water. As a consequence 

 of this, an ice-mass has a thin layer of cold air about it; and if also covered by 

 earth, to shut off the winds that aid evaporation and also to protect it from 

 the sun, the permanence is greatly increased. An ice-house affords a fa- 

 miliar example ; and others are the dirt-covered masses of ice only a foot 

 or two thick that linger on the north side of houses at times from winter 

 into April in the middle latitudes of eastern North America. 



In the Alps the glaciers extend down 4500 to 5300 feet below the snow- 

 line. The snows of the glacier-source in the mountains take the half-com- 

 pacted condition of the neve for a distance above the snow-line as far as 

 there are seasonal or other alternations in temperature sufficient to produce 

 occasional meltings. Over the snow-fields, the extreme cold of winter is 

 followed by months of less stringent weather, and by meltings that send 

 water down through the mass and make it coarsely granular and more or less 



