122 LITEEATUBE ON TORRENTS. 



still, qapable pf producing are no longer gnawed to the root, there is a real 

 awakeniilg of the forces of nature. All the buried seeds spring to life. 

 From the very first year the ground changes its aspect, it begins to show 

 a green hue. In autumn the plants shed their seeds. From year to year 

 the vegetation spreads and takes possession of the place more and more 

 extensively. 



" In proportion as this change is produced in the state of the surface, 

 the water arrives less rapidly and in less abundance in the ravines and in 

 the hollows of the soil. Its power of dkrrying off material is weakened ; 

 from the first it has no longer the strength to carry off the larger stones, 

 which roll to the bottom of the ravines and stop there. 



" To the former work of erosion, and of carrying off material, succeeds 

 the opposite action of colmatage — depositing mud, &c., in its course — and of 

 levelling. This action, at first slow, increases with a rapidly advancing 

 progression. The tendency to effect a general levelling extends throughout 

 the whole section of the ground over which the water flows ; and a retarda- 

 tion of swiftness succeeds to acceleration. The vegetation promoted by the 

 fertilizing colmatage takes on an energetic development ; it invades more 

 and more the bottom of the ravines. It is there a characteristic sign of 

 victory being assured to the vegetation. 



" When the upper slope is surmounted by crests formed of crumbling 

 rocks, these summits, more exposed to the destructive action of the elements, 

 continue to produce masses of minute debris, which sustain the action of 

 colmatage on the lower slopes. Eavines, and all depressions of the soil, 

 tend more and more, to efface themselves. The soil goes on rising in these 

 unceasingly. In this new permeable and minutely subdivided layer the 

 turf developes itself with more and yet more vigour, and it finishes by 

 reaching a considerable thickness — it is often 30 centimetres (or 12 inches) 

 thick. One may estimate from this the influence produced by the thick 

 layer of turf or herbage. 



" In regard to the consolidation of the soil, the protection is complete. 



" In regard to hydrology, the abs6rption of the water by this permeable 

 layer is so much the greater as — be it in consequence of a greater levelling 

 of the surface, or be it through the effect of the long herbage — the flow of 

 it is subjected to a very great retardation. 



" The levelling, in extending indefinitely the section, reduces the mass of 

 water to an extremely thin sheet ; and then each shoot of herbage easily 

 breaks this sheet, so that the water which can only acquire velocity by a 

 certain concentration is broken up to such a degree that all flow is impossible, 

 excepting in some extraordinary case as when a water-spout breaks and 

 pours out itself on a single point. 



" Woods also induce colmatage on the higher slopes which they occupy 

 when the crests are denuded and formed of crumbling rocks, throwing off not 

 only fine debris, but also very often stones and large pieces of rock. 



" In certain forests all the trees are severely grazed on the upper side by 

 the shock of stones which roll from the upper part of the mountain. When 

 these projectiles are launched with very great velocity they roll to the foot 

 of the mountain; but most frequently they stop on the lower slope, and 

 form, by their accumulation, a layer of variable thickness. 



" When the colmatage acts slowly and regularly it is extremely favourable 

 to the vegetation. It is then one of the causes of the beauty of the woods 

 on the upper slopes. 



