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 destruction, which is due to the snow-masses of such a slide, are 
insignificant in comparison tothose of the compressed air current, since 
the snow is light, and by the fall dispersed and scattered and may pass 
through a thinly stocked forest without doing much damage. One of 
such airecurrents preceding an avalanche is reported to have carried a 
full-grown larch tree bodily over the tower of a prison, lodging it 300 
yards beyond, and to have laid low timber drift 200 te 500 feet beyond the 
avalanche. Such slides are called dust, or powder avalanches. They 
usually occur during a heavy snow-fall, and are but rarely occasioned 
by winds afterwards, if perchance the snow has been able to preserve 
its loose and dry condition and could keep in position long enough. It 
seems that most of the snow-slides of the Rocky 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 itselfand 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, Flysch*—only often the angle of inclination 

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

