243 



avalanche, followed and pressed on by the latter in hasty flight and 

 exerting an enormous pressure, often over great distances, sometimes 

 2 or 3 miles beyond the actual course of the avalanche. 



The destructioD, which is due to the snow-masses of such a slide, are 

 insignificant in comparison to those of the compressed air current, since 

 the snow is light, aud by the fall dispersed and scattered and may pass 

 through a thinly stocked forest without doing much damage. One of 

 such air currents preceding an avalanche is reported to have carried a 

 full-grown larch tree bodily over the tower of a prison, lodging it 300 

 yards beyoud, and to have laid low timber drift 200 to 300 feet beyond the 

 avalanche. Such slides are called dust, or poivder avalanches. They 

 usuallj^ occur during a heavj^ snow-fall, and are but rarely occasioned 

 by winds afterwards, if perchance the snow has been able to preserve 

 its loose and dr^y condition and could keep in position long enough. ' It 

 seems that most of the snow-slides of the Eocky Mountains are of this 

 nature. 



If the snow falls when the temperature is not low, it is wet, heavy 

 and compact, and hangs to the soil closely. If not much snow fell, it 

 remains lying undisturbed, provided the soil is not wet or slippery and 

 the ground not very steep. The larger the mass of snow and the 

 warmer the temperature, the greater is the danger of a movement of 

 the mass in itself and a consequent slide. In such a slide the snow re- 

 mains compact and does not disperse unless falling over precipices. It 

 does not therefore exert much pressure upon the air, flows more like a 

 snow-stream, now sliding and now rolling over and balling together. 

 Its velocity in spite of its great weight, on account of its friction against 

 all obstacles on its road, is much less than that of a dust-slide and its 

 effects reach over a smaller space. These may be called ground slides, 

 or true slides. 



Lastly, a third kind of avalanche is mentioned, with which we have 

 nothing to do in this country, except perhaps in Alaska and Nevada. 

 These are the glacier avalanches, which are formed when large masses 

 of the glacier disconnect themselves from the main flow, and falling 

 over a precipice break up into something like a dust avalanche. 



MECHANICAL AND ATMOSPHERIC CONDITIONS. 



Apart from the nature of the snow the following conditions are of 

 moment in the formation of avalanches : 



First, the geological formation of the mountains : Compact rock-for- 

 mations are less favorable to formation of avalanches, especially ground- 

 slides, than stratified rocks. The latter offer more opportunity on the 

 side of the dip than on the side of the outcropping (head). Granite 

 and granitic gneiss mountains are therefore less liable to avalanches 

 than slates, limestone, Fiysch* — only often the angle of inclination 



* A formation composed of alternating layers of black slate and sandstone. 



