260 DENUDATION OF THE LAND Chap. VI. 



water-spouts would remove all the mould 

 from a very gentle slope ; but when ex- 

 amining the steep, turf-covered slopes in 

 Glen Roy, I was struck with the fact how 

 rarely any such event could have happened 

 since the Glacial period, as was plain from the 

 well-preserved state of the three successive 

 " roads " or lake-margins. But the difficulty 

 in believing that earth in any appreciable 

 quantity can be removed from a gently in- 

 clined surface, covered with vegetation and 

 matted with roots, is removed through the 

 agency of worms. For the many castings 

 which are thrown up during rain, and those 

 thrown up some little time before heavy rain, 

 flow for a short distance down an inclined 

 surface. Moreover much of the finest levi- 

 gated earth is washed completely away from 

 the castings. During dry weather castings 

 often disintegrate into small rounded pellets, 

 and these from their weight often roll down 

 any slope. This is more especially apt to 

 occur when they are started by the wind, 

 and probably when started by the touch of an 

 animal, however small. "We shall also see 

 that a strong wind blows all the castings, 



