Soil' Binders 51 



worse these must have been when sun and wind had 

 full play, with nothing to check 1>hcm. 



It is clear, too, that where sand or volcanic ashes 

 have been brought by the wind, the same wind may 

 in many instances scatter them again. And where 

 mud has been brought down by a river and deposited 

 within reach of the tide, there it will be liable to be 

 washed away, unless some means be taken to fix it to 

 the spot. 



Let us even look, for instance, at a railway embank- 

 ment. It has been piled by human hands with a 

 special object, and is a far more solid mass than if it 

 had been merely blown together ; but yet it to some 

 extent wastes away. Its bare, exposed surface is 

 washed and wasted by the rain, dried and blown away 

 by the wind ; for there is nothing to protect it from 

 either, to begin with. But this state of things does not 

 last long. There is always plenty of seed floating in 

 the air, ready to sow itself on any bare space it can 

 find ; so that in two or three years' time the embank- 

 ment is overgrown with grass, whose roots are so 

 matted together that further shifting of the soil is to a 

 large extent prevented. 



Of course, where seed is sown even before the soil is 

 made, as we have seen in the case of lichens, there the 

 mould as it is formed is kept in place and protected, 

 and is able to deepen undisturbed. But where sand 

 has been heaped together by the wind, or mud de- 

 posited on the coast, there something is needed to give 

 it firmness, or else it will be dispersed again. 



The sandhills on the plains of Venezuela, for example, 

 are still constantly moving to and fro, here to-day and 

 there to-morrow, except in one district where they 



