CH. Ill 



LOCALITY 57 



sandy ridges formed by the driving action of the wind 

 on sand which has been cast up by the sea. But very 

 similar formations will also be formed inland where, 

 the rainfall being small, or other circumstances inter- 

 vening, no A T egetation springs up and the sand is driven 

 forward and encroaches on other land. The fixation 

 of the dunes of Gascony is now historic ; but, in tropical 

 countries with a small rainfall, the problem of stopping 

 the encroachment of these sandy hosts is much more 

 difficult to solve. In the province of Dongola, in the 

 Sudan, for example, the dry winds, coming from the 

 north-east across the desert, have brought waves of 

 sand which cover up cultivable land, palm-groves, and 

 woods. In Kordofan the dunes, locally known under 

 the name of " goz " (or hill), have advanced and covered 

 up the country until they reached a region of sufficient 

 rainfall, where they got fixed by vegetation in the 

 shape of open thorn-forest, the characteristic tree of 

 which, as we have seen, is the Acacia Verek. In 

 moister climates, especially near the sea, the dunes have 

 less chance of encroaching on the land behind them to 

 any large extent, as they soon get clothed by herbaceous 

 and shrubby growth which fixes them. Such dunes 

 may be found, for example, on the coasts of Java and 

 Ceylon, where they often do not exceed one or two 

 ridges. 



As regards aspect, which may be defined as the 

 point of the compass which any sloping land faces, it 

 is of importance chiefly as regards the direction from 

 which moisture-laden winds come. Within the Tropics, 

 the sun swingine; now to the north and now to the 

 south, at midday, thus greatly nullifies the effects that 

 northern and southern aspects have in regions more 

 distant from the equator. Also with the high angle 

 to which it swiftly ascends early in the day, even 

 westerly slopes, unless they are very steep, are soon 

 bathed in sunlight, while the easterly slopes remain so 

 until well on in the afternoon. Nevertheless, in the 

 Tropics of the northern hemisphere, the sun being to 



