DESERT SAND-DUNES 45 



pushing force and, swinging round from either 

 side in a right-handed and left-handed spiral, curls 

 upwards on the lee of the central peak. Thus in 

 addition to the eddy with horizontal axis formed 

 on the lee of a long ridge there is here an eddy, 

 or a pair of eddies, with inclined or vertical axis 

 on the lee of a peak. Sometimes the effect of 

 these eddies is to build up against the lee side 

 of the peak a mound or bank tapering to lee- 

 ward, with its axis parallel to the wind. This is 

 deposited where the spiral whirls, coming in from 

 either side and each heavily loaded with sand, meet 

 and ascend. 



In other places on the sandbanks I found a 

 series of waves of no great lateral extension, each 

 ridge being a single broad mound tapering from 

 the centre until it merged at either end in the 

 general level of a plain of loose sand. From the 

 highest and central part of each ridge there pro- 

 jected at right angles a tongue of sand tapering 

 to leeward until it merged in the general level of 

 the plain. This tongue of sand had an absolutely 

 sharp ridge, and was concave on either side, being 

 apparently modelled by whirls of air. 



There was, however, yet a third variety of the 

 action of secondary eddies on the lee side of peaks, 

 for there were places where depletion instead of 



