142 



PHYSICAL GEOLOGY 



intermediate regions the height increases toward the equator. In the 

 Alps the snow line is 8500 feet above the sea; in the western United 

 States and in British Columbia the higher mountains are covered with 

 perpetual snow; in Massachusetts it has been shown by kites that 

 glaciers would exist at an altitude of 11,470 feet; and it is estimated 

 that glaciers would develop in the Scottish Highlands if the average 

 temperature were lowered three degrees. 



The position of the sun with reference to a mountain range in- 

 fluences the height of the snow line. In the northern hemisphere, for 

 example, other things being equal, the snow line will be lower on the 

 north side of a mountain than on the south side, since the former 

 receives heat from the sun fewer hours each day. Certain forms of 

 topography also favor the retention of snow. For instance, snow 

 gathers to greater depths in deep ravines than on a level surface, as it 

 is blown in by the wind and protected from the sun's heat so that it 

 may remain from one winter to the next. A moist climate also favors 

 a low snow line on account of the greater snowfall, since more time 

 is required to melt, or evaporate, a thick layer of snow than a thin one. 

 On the Himalayas the snow line is 3000 to 4000 feet lower on the south 

 than on the shaded north side, because of the greater amount of snow 

 precipitated there by the moist, south winds from the Indian Ocean. 

 The few inches of snow which fall on the north slope may be melted 

 in a few warm summer days, while the several feet of snow on the 

 south side may not disappear, even when subjected to a longer period 

 of warmth. In dry climates the snow may disappear entirely by 

 direct evaporation. As far as temperature is concerned portions of 

 Siberia are under glacial conditions, but the climate is so arid and the 

 snowfall so scanty that the snow which falls is soon evaporated. 



Formation of Ice in Snow Fields. — Snow differs from ice in being 

 composed of fine crystals, loosely consolidated and separated from 

 one another by air, whereas ice consists of crystals in contact. In a 

 snow field there is every gradation from fluffy snow to granular snow 

 or neve and finally to solid ice. The change from one state to the 

 other is well shown in snowdrifts of the temperate zone, which become 

 granular if they exist for a few months, the granules being about the 

 size of small hailstones. If they exist still longer, the drifts are rep- 

 resented by small mounds or ridges of solid ice. The transforma- 

 tion from snow to neve and then to ice is very gradual and is accom- 

 plished (1) by the pressure of the overlying snow which forces the air 

 from between the snow crystals and thus tends to compact them; 



