244 Forestry Quarterly. 



a snow slide, and what is merely a snow slide when occurring in 

 a developed country may be called an avalanche. In order to 

 illustrate the main points to be considered in recording statistical 

 data in tabular form as used in Switzerland the following head- 

 ings may serve. 



The cause of the origin of the avalanche determines its charac- 

 ter. There are four kinds distinguished : Dust, Top, Ground and 

 Glacier avalanches. 



(/) Dust Avalanches. 



They originate usually during cold and stormy weather in high 

 altitudes. The loose snow begins to roll like sand from steep 

 barren slopes, constantly increasing in volume as its descends. 

 The fine snow is carried by the wind, the heavier rolls on the 

 ground, the whole mass becomes compressed and is preceded by 

 a hurricane, the effect of which is usually stronger than that 

 of the avalanche itself. Such avalanches often break loose during 

 the time it snows, or are started afterwards by wind. The ve- 

 locity of such snow slides is very great, and cases are known 

 in which they travel four miles per minute and sprinkle snow- 

 like powder over large areas. 



(2) Slope or Ground Avalanches. 



If a great amount of snow falls during warm weather it re- 

 mains more compact and may start to slide at the same time 

 over a wide area, somewhat like a mass of wet snow sliding in 

 a body off the roof of a building during a sudden thaw. The 

 speed is greatly reduced through friction by sliding over the 

 ground. These avalanches are as a rule not accompanied by a 

 hurricane. If such a slide becomes wedged into a canyon it may 

 then also be a "canyon slide." Although dust and slope avalanches 

 differ in character and origin, they vary in such a manner as to ex- 

 hibit a number of intermediate stages between the typical forms. 

 A dust avalanche may develop into a ground avalanche by 

 taking with it hard snow, debris and rocks. On the other hand, 

 a ground avalanche may end in a dust avalanche as a waterfall 

 disperses into small particles by falling over rocks, or through 

 the resistance of the air. 



