270 DENUDATION OF THE LAND Chap. VI. 



earth, weighing above 23 pounds, will 

 annually reach tlie bottom. Here a thick 

 bed of alluvium will accumulate, ready to be 

 washed away in the course of centuries, as 

 the stream in the middle meanders from side 

 to side. 



If it could be shown that worms generally 

 excavate their burrows at right angles to 

 an inclined surface, and this would be 

 their shortest course for bringing up earth 

 from beneath, then as the old burrows col- 

 lapsed from the weight of the superincum- 

 bent soil, the collapsing would inevitably 

 cause the whole bed of vegetable mould to 

 sink or slide slowly down the inclined sur- 

 face. But to ascertain the direction of many 

 burrows was found too difficult and trouble- 

 some. A straight piece of wire was, how- 

 ever, pushed into twenty-five burrows on 

 several sloping fields, and in eight cases the 

 burrows were nearly at right angles to the 

 slope ; whilst in the remaining cases they were 

 indifferently directed at various angles, either 

 upwards or downwards with respect to the 

 slope. 



In countries where the rain is very heavy, 



