THE LEVELLING POWER OF RAIN. 377 



vening island. It was a grand sight ; each downfall created a 

 cloud of spray ; the concussion in one place causing other 

 masses to give way a long distance from it, and thus the 

 crashes continued, swaying to and fro, with little prospect of 

 termination. When we glided out of sight two hours after 

 sunrise the destruction was still going on." 



We might consider here the action of glaciers in gradually 

 grinding down the mountain slopes, the destructive action 

 of avalanches, and a number of other forms in which snow 

 and ice break down by slow degrees the upraised portions 

 of the earth. For in reality all these forms of destructive 

 action take their origin in the same process whence running 

 waters and heavy rainfalls derive their power. All these 

 destructive agencies are derived from the vapour of water 

 in the air. But it seems better to limit the reader's attention 

 in this place to the action of water in the liquid form ; and 

 therefore we proceed to consider the other ways in which 

 rain wears down the land. 



Hitherto we have considered effects which are produced 

 chiefly along the courses of rivers, or in their neighbourhood. 

 But heavy rainfall acts, and perhaps in the long run as 

 effectively (when we remember the far wider region affected) 

 over wide tracts of nearly level ground, as along the banks 

 of torrents and rivers. 



The rain which falls on plains or gently undulating 

 surfaces, although after a while it dries up, yet to some 

 degree aids in levelling the land, partly by washing down 

 particles of earth, however slowly, to lower levels, partly by 

 soaking the earth and preparing a thin stratum of its upper 

 surface to be converted into dust, and blown away by the 

 wind. But it is when very heavy storms occur that the 

 levelling action of rain over widely extending regions can 

 be most readily recognized. Of this fact observant travellers 

 cannot fail to have had occasional evidence. Sir Charles 

 Lyell mentions- one instance observed by him, which is 

 specially interesting. "During a tour in Spain," he says, 

 " I was surprised to see a district of gently undulating 



