WALTZING PILLARS OF SAND. 365 



of rocks in these deserts, which have sometimes puzzled 

 visitors to account for their appearance, are no doubt 

 caused by sand-drift, which is here a very powerful 

 wearing-away agent. Thus, the undermining of cliffs and 

 other masses of rock may be effected by the constant 

 drift of sand driven in one particular direction by the 

 prevailing winds, and the remarkable circular cavities 

 made in positions where it would have been impossible 

 for water to get at them, is probably caused by revolv- 

 ing storms, which causes the sand drift to revolve, 

 and grind their interiors with great velocity and force. 

 All along the Nile these sand-carvings are common. 



What are known as whirlwinds are merely another 

 variety of these storms, where the effect is sometimes 

 merely local, and the wind gyrates in spiral form, 

 with more or less velocity. These curious phenomena 

 seem to be most common in the great deserts of 

 Egypt and Nubia, and the month of June appears 

 to be the period during which they are most pre- 

 valent. Sir Samuel Baker, another leading authority 

 in these matters, states that their force is often 

 " sufficient to raise dense columns of sand and dust, 

 several thousand feet high," which frequently continue 

 in existence and without falling to the ground, 

 "for many hours, usually moving in circles, resembling in 

 the distance solid pillars of sand. I have frequently" (he 

 says) "seen many such columns at the same time, in the 

 boundless desert, all travelling, or waltzing, in various direc- 

 tions, at the wilful choice of each whirlwind : this vagrancy 

 of character being an undoubted proof to the Arab mind, of 

 their independent and diabolical origin. " * 



Nor indeed is this to be wondered at, seeing that 



* The Nile Tributaries of Abyssinia, by Sir Samuel Baker, 1867, p. 23. 



