322 Dust Whirls. 



case, wliich has been described as a saltatory action, fine sand.. 

 The fine dust tends to rise high into the air, and it may be soinje- 

 time before it settles down to the ground again. During this- 

 period it may travel far. and there can be no doubt that a con- 

 siderable portion of it is carried .beyond, or "exported" from 

 the sub-arid areas. 



In this way the general surface of these areas as a M'hole tends-- 

 to be lowered. 17 The dust which falls in other portions of these sub- 

 arid areas, tends to increase the thickness of soils locally, and also 

 to make these soils of a more heterogeneous character. ^^ Such 

 soils are of course subject to removal by dust whirl and other 

 aeolian action, as well as by rain action, but certain areas may 

 receive more wind-blown material than they lose by the same- 

 agency. 



The fine sand removed by saltatory action immediately settles' 

 again when the dust whirl dies out, but during its journey it would" 

 tend to collect and to remain in hollows. There is thus probably 

 a general drift from the higher to the lower country (with some- 

 exce^^tions), which aids in keeping the general surface level, and 

 thus in the formation of a vast high level plain — the " new 

 plateau " of the Avi-iter.^^ 



An interesting account of a dust-stonn in soutli-western North 

 America, and an estimate of the amount of dust precipitated have- 

 been recently given. 20 It has been concluded that owing to strong 

 convectional air currents, " an enormous quantity of dust must 

 have been eroded from these arid regions — (New Mexico and 

 Arizona) — lifted into the upper atmosphere, and carried with 

 the storm a thousand miles or more to the north-east, where it was 

 brought doAvn by the snow and sleet, which had formed at a great 

 altitude in the air." It was calculated that not less than a million 

 tons of organic and inorganic material fell, and probably many 

 times that amount. I'he dust whirls discussed in this paper cannot 

 of course be compared, from an erosional point of view, with such- 

 a storm, but nevertheless, the difference is but one of degree. 



17. This idea of "exportation" has of course been brought forward by 

 earlier writers, such as von Richthofen and Davis. 



18. The mixed character of soils owing to the action of the wind gene- 

 rally has been fully described by Free. Op. cit.»p. 109. 



19. Bull. 61, Geol. Surv. W. Austral.. Perth, 1914, p. 525. 



20. See "Geographical Review," December, 1918, pp. 514 and 515. 



