358 PREVENTION OF TORRENTS AMONG THE ALPS. 



walls, suffices to arrest the surface-wasL ; but it is better to sod them at 

 once. 



Planting and mowing. — After the dams, the drains, the opening of the 

 belts and their covering, have, for the time being, fixed the soil, a vege- 

 tation should be established sutficient to hold it permanently. But 

 what ought this vegetation to be, that shall consolidate the surfiice in 

 such a way as to prepare it for all eventualities that can possibly be 

 foreseen ? Nature itself answers this important question. 



In short, when we study in the Alps the formation of great erosions, 

 we shall at once observe that the loosening of the soil on the steep slopes 

 has immediately followed the destruction of the timber, and that it has 

 only stopped when it has reached a limit where the ligneous vegetation 

 has successfully resisted further injuries to the soil. 



We find nowhere upon slopes given up to the pasturage of sheep and 

 goats that the soil has been uniformly consolidated by herbaceous plants, 

 while we everywhere meet with such grounds, which, after long periods 

 of crumbling, have been fixed chiefly by the growth of trees. This is 

 easily explained. The animals above mentioned pull up the herbs as 

 fast as they appear, while ligneous vegetables may be browsed off, but 

 not pulled out by the roots like herbs, and it is this difference that con- 

 stitutes the superiority of tree-planting over sodding. 



In the presence of these results, which strike every observer, no one 

 can venture to assert that torrents can be restrained by sodding alone, 

 or deny that reboisement is the most effectual means for fixing the soil 

 iu eroded places. Who does not furthermore know that woodlands, by 

 retarding the melting of the snows, and by hindering the flow of water, 

 compels it to sink into the soil, and thus contribute to the supply of 

 springs, everywhere so useful ? 



An important question here arises which should be fixed before going 

 further : as to what kinds and in what proportion the ravines ought to be 

 planted, in order to fix the soil with certainty, and so as to meet the 

 local wants in as satisfactory a manner as posfsible ; for in an economical 

 point of view the mountains are made for the population, and should be 

 made to adjust their products to its wants, if we would seek to check 

 emigration, the evil consequences of which need not be specified. 



In gorges, trees of full growth, by reason of the difDculties in getting 

 them out, have but a small value. Trees of large size cover the ground 

 but imperfectly, and evidently protect it less than a young and dense 

 coppice. Trees of feeble size are not a temptation to trespassers. Placed 

 on the horizontal belts (more widely spaced apart where the slope is less 

 steep) the young trees, besides benefiting these belts by the moisture 

 that they produce, favor the growth of herbaceous plants, which after 

 some years, may be gathered for use. This is a fact of some importance, 

 since among almost all of the mountains the cattle are led in winter 

 with leaves and young shoots cut green about the beginning of Sep- 

 tember. 



On the other hand, it is sufficient for stopping these superficial ero- 

 sions if the lands are kept cultivated, and the intervals between them 

 are kept sodded. All interests are, therefore, satisfied by either of these 

 methods. 



1. By planting the eroded portions in alternate belts with kinds of 

 trees that shoot readily from their slumps, that have a rapid growth, 

 and that may be easily worked at short periods of revolution. These 

 conditions are offered in the acacias, ashes, elms, maples, and white- 

 woods,' and it is well to mix them according to locality ; but we should 



' Willows, alders, poplars, and lindens. 



