233 CONSOLIDATION OF UNSTABIE HILL SIDES. 



lenough not be breached. Stones, gravel, silt, &c,, brought dowfl 

 with the water, will then gradually collect behind each dam until 

 the bed of the stream rises up on a level with it, thus ultimately 

 forraiug an even terrace having a gentle slope outwards. The 

 more rapid the fall is, the nearer together must the dams be con- 

 structed. The material of the dam will depend on the maximum 

 volume and velocity of the water coming down at any time. To 

 use paklsa solid masonry in all the dams would, by reason of the 

 expense, be impracticable, and hence only the larger and stronger 

 among them should be so built. In many cases a dry stone wall, 

 of sufficient width, will suffice, the silt collecting behind it soon 

 making it more or less tight. Another excellent style of dam is 

 ^•ep resented in Fig. 81, which explains itself. 



All dams constructed of dead material have against them this 

 ■drawback that they must be at once made of the required height, 

 and are besides very expensive. In the French Alps a tight live 

 fence of som^ strongly-rooted, easily slipped, and quick-growing 

 shrub has proved an effective substitute for masonry. The fence 

 is put up low, but gradually grows up, and it never, like other de- 

 scriptions of dams, allows the water to rush over it and fall clear of 

 it in a single mass that tears the ground away where it falls. A 

 further plan, often adopted with great success, is to erect two 

 parallel strong, tight-bound live or dead fences a few feet apart 

 from one another and to fill the interval with boulders and 

 rubble. 



When the fall is very rapid, the terraces must be completed at 

 ■once without delay and may have to be made so narrow as to 

 assume the appearance of a flight of steps. 



If the material of the mountain side is very loose, the sides of 

 the ravines may have to be wholly or partially lined with dry 

 fstone masonry and even buttressed up by cross-walls in continu- 

 ation of the Hue of the edge of each terrace. 



C. — The third system of measures is intended to attack the evil 

 ,at its very source. At the head of each drainage basin, all over 

 the mountain-side, a well-devised system of drains is cut, which 

 collect rain and snow water and give it immediate egress to the 

 larger natural driiinage channels, without allowing any consider- 

 able proportion of it to sink into the mountain-side, thereby 

 preventing its soaking and softening the latter and destroying its 

 stabihty. 



We have seen that in the drainage of swamps no regaid at all is 

 paid .to the quantity of moisture in the layer of the soil below that 



