TORRENTS OP THE HIGH ALPS. 49 



absorbs a portion of the waters which are imbibed in the spongy humus, and 

 so far it diminishes the sum of the washing away forces. ' 



("It follows from this that a forest, in establishing itself on a mountain, 

 actually modifies the surface of the ground,,)which alone is in contact with 

 atmospheric agents, and all the conditions find themselves then modified as 

 they would be if a primitive formation had been substituted for a formation 

 totally different. Whence it is not more astonishing to see the same soil 

 alternately cut up or free from torrents, according as it is despoiled or 

 clothed with forests, than it is astonishing to see torrents cease when we 

 come to primitive formations, or reappear suddenly on friable limestone. 



" In accordance with this we find — ^first, the development of forests 

 brings about the extinction of torrents ; second, the destruction of forests 

 redoubles the violence of torrents, and may even cause them to reappear. 

 And nothing is more easy than to explain these new actions. It will be 

 remembered what are the causes which call forth and maintain the violence 

 of torrents : it is, on one hand, the friability of the soil ; and, on the other, 

 the sudden concentration of a great mass of water. Now, we know already 

 that the forests render the soil less liable to be washed away ; we know also 

 that they absorb and retain a portion of the rainfall, and prevent instan- 

 taneous concentration of the portion whieh they do not absorb. Conse- 

 quently they destroy both the one and the other cause. They prolong the 

 duration of the flow, and they render the floods at once more prolonged, less 

 sudden, and less destructive. 



" It may be understood from this how forests, in invading the hassins de 

 reception, may have contributed powerfully to stifle certain torrents. Whilst 

 the waters were creating for themselves the most convenient slopes, the 

 forests were retaining the soil which was ready to go, was rendering it more 

 solid, was consequently diminishing the mass of earth washed away, and 

 above aU was opposing itself to the concentration of currents. They were 

 augmenting all the resisting, all the existing, obstacles, and were diminish- 

 ing all the motive powers ; and they were coming thus to hasten by a double 

 efficacy that epoch of stability in which the force of the waters would find 

 itself in equilibrium with the resistance of the soil. There is one circum- 

 stance which ought to render their triumph still more speedy, — it is, that 

 the torrent, in proportion as it is enfeebled, abandons to them a soil more 

 and more stable and favourable to vegetation, in such a way that this 

 augments every day their forces in proportion as the torrent loses force. 

 In fact, if the expression may be allowed, it is reinforced by the effect. 



" By this I do not mean to say that the torrents can never become extinct 

 of themselves. That would be in contradiction to what I have said, and at 

 the same time to facts observed, for there are examples of torrents being 

 extinguished without the presence of forests, and solely through the erosion 

 of the mountains — as, for^instance, the torrent of Saint Joseph, near Mones- 

 tier. But I say that the forests expedite the accomplishment of this efi'ect, 

 and that they can produce it where the other circumstances are not yet 

 producing it. 



" Thus nature, in summoning forests to the mountains, places the remedy 

 side by side with the evil. She combats the active forces of the waters ; to 

 the invasions of the torrents she opposes the aggressive conquests of vegeta- 

 tion. On those mobile revers she spreads a solid layer which protects them 

 against external attack, somewhat in the manner that a facing of stone 

 protects an earthen embankment. It is worthy of remark, that the little 



D 



