ch. m FIXATION OF UNSTABLE SOILS 287 



With inland-dunes and smaller masses of shifting 

 sands, the question of how to deal with them is a more 

 complex one. It has already been seen in the first part 

 of this volume (Chap. III., " Locality ") that it is mostly in 

 regions of extremely small rainfall that these sand-drifts 

 are the most difficult to stop, and that where there is a 

 sufficient precipitation of moisture the vegetation soon 

 springs up on them and fixes them, unless interfered 

 with by man or domestic animals. The example given 

 is of the sand-drifts in Kordofan in the "Western Sudan, 

 which have been covered by open forests of Acacia 

 Verek and with herbaceous growth, the most prominent 

 of which is the Cenchrus catharticits, a grass with 

 hooked seed-capsules forming most irritating burrs, 

 which help its distribution. 



But the work of Nature, unaided, is often slow, and 

 it may be that for some cause the onward march of these 

 sands must be arrested. The first step will be to exclude 

 all animals from the area, so that the surface may not be 

 constantly broken up and young plants destroyed. If 

 the rainfall is sufficient, the measures for fixing the 

 sands may generally be of a fairly simple nature. In 

 Europe open wattle-fences of sufficiently loose texture 

 to offer not too strong a resistance to wind and sand 

 are erected, and made stronger by means of struts, 

 and the area is loosely covered by sods of turf, which are 

 either scattered irregularly over the surface or arranged 

 in a chess-board pattern of hollow squares, in the middle 

 of which seeds of trees, shrubs, or sand-binding grass 

 are dibbled in. In the Tropics, instead of wattle-fences, 

 thorn-fences or " zaribas " may be used. These consist 

 of piled-up masses of thorny branches which resist a con- 

 siderable pressure from the wind, as the sand soon trickles 

 through the interstices of the branchlets and fixes them 

 in the ground. If the wind is particularly strong their 

 butt-ends can be turned towards the wind and stuck 

 into the ground, the leafy ends being to leeward. 

 These fences soon become crates full of sand, and they 

 must be added to as they grow. As the fences are 



