DEFLATION 



55 



basal Cambric sands of Sweden, the Rothliegende of Germany, the 

 Buntersandstein of Thuringia and elsewhere. 



Deflation. This is by far the most important work of the wind, 

 and its geological significance can scarcely be overestimated. The 

 lifting of the material from the surface is largely the work of 

 eddies and irregularities of movement of the wind, including many 

 conflicting cross-currents. (Langley-57; 58.) The most important 

 are, of course, the whirling eddies of whirlwinds and tornadoes, but 

 many minor currents, due, in part, at least to the irregularities of 

 the surface are active in lifting the dust and fine sand. The ordi- 

 nary convection currents of the atmosphere carry this finer material 

 up to great heights. The force (Free-33 : 55) which moves the par- 

 ticle is due to the direct impact of the wind plus the friction along 

 its surface. The force of the direct impact varies with the velocity 

 of the wind, and for a given velocity with the cross-section of the 

 particle in the plane perpendicular to the direction of the wind, as 

 well as with the orientation of the particle, and it is of course most 

 efificient when a smooth plane is opposed to the wind. The resistance 

 of the particle varies with its mass, i. c, its size and specific gravity, 

 and to some extent its form. In general the velocity of tho. wind 

 necessary to carry a spherical particle of given specific gravity 

 varies as the square of the radius, and conversely the radius of a 

 particle which can be supported by wind varies as the square root 

 of the velocity. Thoulet (94; 95) found that a uniform upward 

 current of air will keep suspended quartz grains, the size of which 

 varied with the velocitv as follows : 



