248 ACTION OP FORESTS ON THE PLOW OP RIVERS. 



and from that time vegetation could begin to take hold and complete 

 the extinction. 



" This remark is important in this way, that if the disappearance 

 of a forest always gives birth to torrential disturbances, it does not 

 always hold true that one can put a stop to them by the planting of 

 a forest alone, 



" Much as an unstable ground is protected by being wooded — 

 though it maintains itself and behaves, in a hydrological point of 

 view, as do the most solid lands, if the wood come to disappear, if 

 the ground be deeply ravined, if the bottom of the ravine continues 

 to be easily undermined and washed away — it becomes extremely 

 difficult to establish vegetation on the mountains, which continually 

 crumble away, and which with this instability no longer retain any 

 trace of vegetable soil. 



" In the Alps there are numerous cases of old mountains which 

 crumble away when the foot of the slope is undermined by the water. 

 And one is thus left, if he desire to effect a radical and prompt ex- 

 tinction of a torrent, to give, artificially, to the bottom of the ravine 

 a power of resistance to undermining and washing away, by appro- 

 priate works of consolidation. 



" But be this as it may, the potent action of forests is beyond all 

 question. Whatever be the character of the woods — timber forests, 

 coppice-wood, or simple shrubbery — all contribute to give firmness to 

 the soil, to retard and to regulate the flow of the water drained off. 



" In comparing the different kinds of woods, it may be said that 

 lofty timber forests, with their vast apparatus of foliage at a great 

 elevation above the soil, are of most use with a view to meteorological 

 and hydrological effects ; and that young trees serve perhaps better 

 to insure the consolidation of the soil on steep declivities. But as 

 generally, on poor land, the soil of timber forests covers itself with 

 branches, &c., it follows that a mixture of the two kinds of woods 

 accomplishes best the end which it is sought to effect." 



He speaks with similar expHcitness in regard to the effect of 

 gazonnement ; and in reference to artificial structures other than 

 those which he advocated, he remarks : " MM. Scipion Gras et 

 Phillipe Breton have also loudly proclaimed, in a way the most 

 explicit, that the boisement of the valley appeared to them the most 

 efficacious measure which could be adopted against torrents, and that 

 it was only in default of proceedings with a view to extinction behig 

 adopted — the application of which, when they wrote, was still 

 surrounded with obscurity and uucertaiiity — that they proposed the 



