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round each stone get washed away, and the stones, becoming loose, follow ; the muddy/ 

 water collects into streamlets charged with materials which by their friction help to gutter 

 the ground ; it then forms surging streams full of mud and stones, which plough up the 

 bed, undermine and eat away the banks, and end in flooding the rivers below. In 

 time, the mountain side appears scored with innumerable ravines and becomes trans- 

 formed into a barren waste devoid of vegetation or water. The more the ground becomes 

 cut up, the greater the rate of destruction. This is due to the continual spread of the 

 bared surface subject to rapid erosive action, and to the fact that a larger amount of solid 

 material borne along by the water increases its destructive power in proportion. The evil 

 may thus attain in a small number of years as I will show it has been the case in some 

 parts of Southern Europe a magnitude which might have at first b >en thought pre- 

 posterous. 



ACTION OF FORESTS ON THE SOIL. A very different effect may be observed 

 when rain falls over a forest (a) the rain itself, though more frequent, is less intense ;: 

 (7>) a large proportion .of the water, averaging fully one quarter of the total amount*, is 

 retained by the tree-tops with their vast expanse of foliage, and restored to the atmos- 

 phere by evaporation ; the trunks and the undergrowth help to retain some of the water 

 (c) When the rain at length reaches the ground, the saturated vegetation serves still to 

 break the shock of heavy showers, and to keep the water dispersed over the surface, 

 where it is imbibed slowly by the spungy humus which is capable of absorbing an 

 immense quantity. If we could see a section of a wooded slope, it would sqow a layer one 

 or two feet thick of decayed leaves and humus, in which the fibrous rootlets form a 

 close matting ; this layer is capable of holding twice its weight of w^ater.f The roots 

 also break up the sub-soil, increasing its porosity and causing the storage of moisture, 

 out of the reach of direct evaporation. 



In this manner, the forests store immediately by far the greater part of the rainfall, 

 and yield it little by little, trickling in water of perfect fluidity, instead of forming swollen 

 streams charged with mud and gravel. Where they have not been destroyed, strips of 

 forest are usually found distributed along the banks of streams, and between the moun- 

 tains and the valleys, in such a way that nearly all the water that falls over a tract of 

 mountain country must trickle through some portion of the barrier of forest soil, acting 

 both as a reservoir and a filter, before it can reach the nearest watercourse. The cause of 

 this remarkably useful distribution of forests is simple enough. Trees thrive best where 

 they can get most moisture, if the drainage remains sufficient. Thus, in plantations the 



*Bavarian observations show that the tops of high trees retain 25 to 30 per cent, of the rainfall ; 

 while Marshal Vaillant found that in the forest of Fontainebleau the proportion amounted to 30 per cent, in 

 summer, and 50 per cent in winter, or 40 per cent, throughout the year. 



f According to De Gasparin, the humus of forests, with a density of 1-225 can retain 1-99 of its weight 

 of water. It follows that each foot of humus could store a sheet of water equal to 2t) inches of rainfall. 



