Current Literature. 245 



(5) Top Avalanches. 



If snow commences to slide over a previously formed snow 

 crust it is called a top avalanche. If such slides break through 

 this crust they may develop into one of the first two mentioned 

 forms. Such slides originate on south slopes where snow melts 

 and freezes frequently. 



{4) Glacier Avalanches. 



They consist, as the name suggests, of bodies of ice that break 

 loose from glaciers. They occur mostly during the month of 

 September when warm weather lasting for a period has thawed 

 the ground, and the pulling force becomes greater than the ad- 

 hesion. At the terminal the accumulated ice remains frequently 

 several seasons. These avalanches are always attended by a 

 thunder-like noise. This noise may be the origin of many names 

 given to rivers, as Thunder Creek, in the Cascade Mountains 

 in the State of Washington. Hurricanes which precede these 

 avalanches are of the worst kind and damage the forests by 

 breaking and uprooting trees, splitting the bark on the upper 

 hill sides, stripping the limbs and branches from the tree. 



Small bodies of snow or avalanches with a short course are 

 called simply snow slides. 



Regarding the period of occurrence, whether occurring once 

 or several times in a year, periodically, or rarely, depends on 

 many factors, such as topography, prevailing winds, snow drifts, 

 amount and quality of snow, the forms of snow fields, new burns, 

 or wind breaks, etc. In some places in the Alps, avalanches oc- 

 cur only in many decades, so that only the oldest people can 

 remember them, whereas in other places new ones occur fre- 

 quently and in places where no one expected them, especially 

 during stormy weather. To the subheading "Season" we may 

 add that according to statistics of 1878-88, out of 1,200 ava- 

 lanches, 45% occurred during the month of February, and 24% 

 during the month of March. The breaking loose of avalanches 

 depends mainly on the wind and temperature and may occur 

 during any time of the day. Cold and stormy days cause dust 

 avalanches especially during the winter, or on cold spring days 

 after a heavy snowfall, whereas ground avalanches mostly oc- 



