236 CONSOLIDATION OF UNSTABLE SLOPES. 



swollen torrents, but more or less perennial rills gush down the 

 mountain side and the larger watercourses never run dry. The 

 water that sinks into the ground is partly absorbed by the roots 

 and finds its way through the leaves back into the atmosphere 

 which it keeps moist, is partly retained by them by the force of 

 capillarity and their own suction power, while a third part filters 

 downwards and re-appears below to feed or form springs. At no 

 time have the upper layers of the soil to bear more than a fraction 

 of the weight of water within them and the almost imperceptible 

 underground flow is never a danger to its stability. If snow fulls, 

 the trees catch and retain a large portion of it, and what reaches 

 the ground is held in place, until it melts, by the trunks of the trees 

 and the low vegetation below, whereby heavy drifts and, as long as 

 the trees stand, avalanches and the slow downward glacier-like 

 movement of the snow are prevented. When spring comes, the 

 snow thaws gradually, first on the tree tops, then on the ground 

 below, latest of all in the deep, tree-sheltered ravines. Like the 

 rain water, a part of the melted snow sinks into the soil, while 

 another is absorbed by the dead vegetable covering over it, through 

 which covering the remaining portion trickles down slowly to the 

 drainage channels. The crowns of the trees, as long as they are in 

 leaf, shelter the soil and retard evaporation from it, and if the 

 crowns become leafless, the covering of dead leaves and grass and 

 other low vegetation and the spongy network of roots are still 

 there to prevent it from losing its moisture except very gradually. 

 Under the crowns of the trees and the ground-covering of vege- 

 table detritus the soil can seldom become baked and cracked by a 

 torrid sun and is, even at the surface, to a great extent protected 

 from frost. Lastly, the mass of trunks, branches and leaves breaks 

 the force of wind and hail. 



ARTICLE 2. 



PROTECTIVE WORKS. 



Protective works undertaken to render treeless, unstable slopes 

 fit to be sown or planted up with forest species need only be limited 

 to (A) holding together crumbling rock or soil, (B) checking the 

 excessive velocity of the flood waters at certain points in the streams 

 and gullies, and (C) providing a free outlet for rain and snow water, 

 so that none of it may sink into the ground sufficient to soak and 

 convert into liquid mud, or to saturate, break up and undermine, 

 the rock below. 



A. To accomplish the first of there three objects, the danger- 



