290 SYLVICULTURE IN THE TROPICS 



PT. IV 



covering over slopes which are particularly liable to 

 erosiou. Only in the latter case the erosion is caused 

 by water which scours the surface of the slopes, or, after 

 sinking into a friable soil, soaks it to the point of satura- 

 tion and then bursts out in a torrent of liquid mud, thus 

 forming a landslip. It may also be caused by springs or 

 watercourses, the waters of which run down or under- 



pin. 91. Shifting sands pouring over the banks of the Nile. 

 Dougola Province, Sudan. 



mine unprotected slopes and, either gradually or in 

 .sudden bursts, carry away their banks. Railway cut- 

 tings or roads cut on a hill -side or quarries may also be 

 the cause of slips by the removal of the foundations on 

 which the surface layers rest. 



These landslips not only cause a direct loss of pro- 

 ductive area, the loss being constantly increased unless 

 protective measures are taken, but the detritus washed 

 down into the valleys or plains below spread out like 

 fans, and may cover with rubbish other fertile areas, 



