MECHANICAL EFFECT OF FORESTS. 43 



fertile lands. The rate at which this process proceeds 

 depends on the geological origin and the formation of the 

 surface ; the less binding the soil and the looser the formation, 

 the greater the damage will be. If, on the other hand, such 

 a- slope is covered with a well preserved forest, the roots of the 

 trees and the layers of humus keep together and protect the 

 soil against the action of water ; besides, the crowns intercept 

 and retain, at any rate for a time, a considerable portion of 

 the water. On the whole, a series of obstacles are opposed to 

 the movement of the water, which reduce its velocity and 

 force, or at any rate divide it into numerous small channels. 

 The beneficial effect of tree vegetation in this respect can be 

 observed in most mountain ranges, and especially in the Alps 

 from France to Austria. Wherever, in those parts, extensive 

 deforestation has taken place, the consequence has been the 

 gradual formation of a series of torrents in all places where 

 the surface did not consist of hard rock ; the debris brought 

 down has covered more and more fertile land at the base of 

 the torrents, and this evil has grown to such an extent that 

 not only in France, but also in the other Alpine countries, 

 great efforts have been made to re-afforest the denuded areas 

 at a great outlay. When once the evil has been created, 

 immediate afforestation is not possil)le ; it must be preceded 

 by the construction of dams, dykes, walls, etc., to steady the 

 soil, until the young forest growth has had time to establish 

 itself and once more lay hold of the surface soil. 



Tlie importance of maintaining a complete cover of vegeta- 

 tion in all such cases was recognised many years ago, so that 

 already in the Middle-Ages so-called "Protection Forests" 

 existed which the then existing laws protected against devas- 

 tation. Although the effect here described is perhaps most 

 complete in the case of a well-stocked forest, similar effects 

 are produced if the soil is covered in other ways, as, for 

 instance, by a dense growth of heather, by turf, etc. 



Forests protect the soil not only in the hills, but also in low 

 lands, wherever it consists of so-called moving or shifting 



