446 WIND-SWEPT RIDGES. 



clear of snow by the action of the winds. So also, 

 snow slides, descending a steep incline during heavy 

 storms, may bring down large masses of snow, some 

 of which may remain unmelted throughout the year, 

 at a point much below the usual limits of permanent 

 snow it is therefore evident that the question of snow- 

 lines is one which is beset by many uncertainties, and 

 is liable to vary according to the interpretation which 

 is put upon the facts. 



As regards the snow itself, its condition is also 

 subject to considerable variations, according to the 

 latitude and climate of the region where the moun- 

 tains are situated, and also according to the state of 

 the weather at the time of the snowfall. In northern 

 latitudes, for instance, when snow has fallen during 

 perfectly still weather, accompanied by an intense cold, 

 it is often, when examined under a magnifying glass, 

 found to consist of regular crystalline forms of ex- 

 quisite beauty. The different types of these snow 

 crystals, as they occur in the arctic regions, have 

 been minutely described at greal length, and by means 

 of illustrated diagrams, by Captain W. Scoresby, the 

 arctic navigator. * The same thing has been noticed 

 as regards the snow falling upon the high Alps, when 

 the air is calm. But " when the atmosphere is dis- 

 turbed the snow assumes a new condition, which is 

 that of small frozen pellets, a little larger than a pin's 

 head."f 



It is this which forms the blinding dust well known 



* See Scoresby's Arctic Regions. Vol. i., pp. 426 to 432 and plates 

 viii to xi inclusive, of 96 different forms of snow crystals. 



f See The Alpine Guide to the Central Alps, by John Ball, F.R.S., 

 p. Ixi of Introduction. 



