282 TEANSPOETATION OF EOCK DEBEIS 



standpoint. It is stated^ that during the prevalence of this 

 wind, on the 6th and 7th of the month mentioned, the air be- 

 came filled with flying particles caught up from the ploughed 

 fields, fire-blackened prairies, public roads, and sandy plains. 

 The particles formed dense clouds and rendered it as impos- 

 sible to withstand the blast as it is to resist the blizzard 

 which carries snow in winter over the same region. The soil 

 to a depth of 4 or 5 inches in some places was torn up and 

 scattered in all directions. Drifts of sand were formed in 

 favorable places, several feet deep, packed precisely as snow- 

 drifts are packed by a blizzard. It seemed as if there were 

 great sheets of dust and dirt blown recklessly in mid air, and 

 when the wind died down for a few moments, the dirt, fine 

 and white, appeared to lie in layers in the atmosphere, clouding 

 the sun and hiding it entirely from sight for an hour or more 

 at a time. (See also on p. 163.) 



Over the wide, dry, and bare flat-topped terraces of the upper 

 Madison valley the wind sweeps in a strong steady current 

 for days together, or during the heated portion of the year, 

 when the sun pours from a cloudless sky its hottest rays upon 

 the parched soil, starts up spasmodically here and there in the 

 form of small whirlwinds made visible by the dust they carry, 

 and which wander spectre-like across the plain to noiselessly 

 disappear in the distant mid air. 



Dust columns of this nature are common in all arid regions, 

 and doubtless have been observed by the many who have 

 crossed the Humboldt desert in Nevada. Seated comfortably 

 in a Pullman car, one may at times see at a single view not less 

 than a half dozen of these geological spectres, each in the distance 

 doing its apportioned task and silently disappearing, laying down 

 its load of sand as its strength gives out and leaving it for its 

 successor.^ 



Under proper conditions such of these wind-blown sands as 

 are too heavy to be carried into the air as dust accumulate 

 upon the surface in the form of drifts, or dunes, all lying with 

 their longer axes approximately at right angles with the pre- 

 vailing currents. Excepting during periods of calm, such are 



^ American Geologist, June, 1889, p. 398. 



^ Professor J. A. XJdrlen estimates that the dust in a cubic mile of lower 

 air during a dry storm weighs not less than 225 tons, while in severe storms 

 it may reach 126,000 tons (Popular Science Monthly, September, 1886). 



