CAUSES OF DENUDATION. 235 



preciably the surface of the hillside ; while the fallen rain, increas- 

 ing its mass and density by means of the suspended and dissolved 

 matters it takes up, and moreover gathering velocity and momen- 

 tum as it rushes down the hillside and tears up gravel, stones and 

 larger fragments, carries almost everything away before it. 



(xi) Wind, especially when it drives rain or hail. 



(xii) Lying snoic its weight, and, when it has frozen and 

 formed a single solid mass with the surface of the hillside, its 



O 



breaking away in avalanches and its constant slow glacier-like 

 movement downwards. 



(xiii) Absence of forest growth. A covering of trees protects a 

 hill side by attenuating or completely nullifying the destructive 

 action of all the preceding causes except earthquakes. The dense, 

 strong network of roots, anchored fast in the solid rock below, 

 holds together, even on very steep slopes, loose, crumbling rocks 

 or soil, which would otherwise slip or be washed down into 

 the valley. When rain falls, the crowns, and, to a smaller extent, 

 the trunks of the trees, sustain the first shock of the drops. A 

 portion of the water descends on the soil below in spray, another 

 trickles down the trunks of the trees, a third drips slowly, but 

 with very much less velocity and, therefore, momentum than the 

 original rain-drops, from the leaves and branches, while a fourth 

 is evaporated back into the atmosphere, and a fifth adheres by the 

 force of attraction to the trees or is absorbed by them superfici- 

 ally. Thus of these five portions, only three reach the ground, 

 comprising, according to Ebermayer, only about 75 per cent, of 

 the total precipitation ; and then it must be remembered that a, 

 by no means, inconsiderable portion of this quantity does not 

 reach the ground at once but goes on dripping from the tree- 

 tops for some time after the shower itself has ceased. Of this 

 75 per cent that ultimately reaches the ground, one portion de- 

 scends at once by way of the roots into, and is held by, the entire 

 depth of soil penetrated by those roots, a second is absorbed by tha 

 covering of dead leaves and low vegetation (vegetable mould absorbs 

 nearly twice its weight of water, moss up to six times), while a 

 third, if any remains, ultimately filters down slowly through this 

 loose impeding mass into the drainage channels, taking as many 

 days to reach the valley below as the shower took minutes to fall. 

 Thus the water never rushes down the hill side, and can do no 

 damage to the soil, which is moreover protected by its dead and 

 living vegetable covering and the close network of roots; the streams 

 and, at other seasons, dry gullies and ravines rarely become 



