245 
was necessary against reckless advisers to keep it intact, until now the 
beneficial effect is fully recognized, and reforestations have been begun 
everywhere under the protecting walls or other safe ty-works wherever 
the danger of avalanches exists. 
Yet even timber-forest: is not always absolute protection against 
avalanches; since the place of incipient formation of the slide may lie 
above timber-line. Such avalanches, formed above the forest line, if 
small, first break a wedge into the forest below them, but by repeated 
* action the wedge is enlarged and gradually a road broken through the 
forest. Large avalanches break, even in their first attempt, through the 
best rooted full-grown forest of entire mountain sides, hurling earth, 
~ rock, and timber into the valley below. 
| Water, the great mover of the earth, is an active agent in the forma- 
tion of avalanches. Not only the spring and seepage water, but also rain 
| and melting snow-water exert their influence. Ground-slides (in the 
Alps) oceur mostly in warm weather in the spring, when the snow melts. 
Then the snow settles and becomes more compact, has a greater specifie 
weight, and therefore more tendency to slide; the snow water pene- 
trates through to the soil, and if the soil is not frozen, saturates it and 
then seeks to flow oif between soil and snow, by which the hold of the 
snow on the soil is loosened, the latter hens slippery, and the sliding 
facilitated. How soon this influence of water becomes active depends 
on how soon the soil is filled up with water, and this again on the kind 
of soil and subsoil. 
Clay soils soon fill up in their upper strata and the snow-water sooner 
begins a superficial flow; a penetrable soil on the contrary with pene- 
trable subsoil does not attain saturation at all and the dangeris avoided. 
If the soil is frozen, the water can not penetrate at all and the sliding 
takes place the sooner. Rgin of courseadds to the water which loosens 
the snow. 
Configuration, as bas been said, may prevent formation of avalanches 
by presenting a number of points of support. Yet where there are 
sink-holes or troughs in which the snow accumulates, the melting snow: 
water collects below the snow and loosens the masses, which may thunder 
into the valley, following the course of the ravine. 
Lastly, even an exterior pressure or disturbance may loosen the masses. 
A stone or an icicle falling, or snow dropping from the branches of 
a tree, when accompanied by strong wind, is lable to start the snow. 
So can one avalanche, by the concussion of the air which it produces, 
Start others in its neighborhood. It has also been frequently observed 
that a sound, as of a gun, of church bells, of an explosion from mines, 
etc., may start the snow. This has given rise to the proposal to start 
the slides by shotguns before they are likely to become dangerous. 
_ On the other hand, sometimes in the stillest weather the danger may 
: be greatest, as the snow falling during such weather accumulates to 
Jarge amount before it breaks loose; while in stormy weather smaller 


























