22 GEOLOGY. 



of dust, and winds of moderate velocity to shift sand. Exceptionally 

 strong winds sometimes move small pebbles, but winds of sufficient 

 force to move larger pieces of rock are rare. It follows, therefore, 

 that the impact of the wind has little direct effect except on surfaces 

 covered with dust and dry sand. 



The transportation of material by the wind is limited by the size 

 of the particles to which it has access. Dust particles expose more 

 surface to the wind relative to their mass than sand grains. Winds 

 which are unable to carry sand may still carry dust, and winds which 

 are able to shift sand no more than a trivial distance may blow dust 

 great distances. 



The common conception of wind as a horizontal movement of some 

 part of the atmosphere is not altogether accurate. Every obstacle 

 against which wind blows causes deflections of its currents, and some 

 of these deflections are upward. Furthermore, there are exceptional 

 winds, in which the vertical element predominates. Particles of dust 

 are often involved in these upward currents, and by them carried to 

 great heights, and in the upper air are transported great distances. 



Transportation and deposition of dust/ — The universality of the 

 transportation of dust by the wind is well known. No house, no room, 

 and scarcely a drawer can be so tightly closed but that dust enters 

 it, and the movements of dust in the open must be much more consider- 

 able. The visible dustiness of the atmosphere in dry regions during 

 wind-storms is adequate and familiar proof of the efficiency of the wind 

 as a transporter of dust. 



Under special circumstances, opportunity is afforded for rough 

 determinations of the distance and height to which wind-blown dust is 

 transported. Snow taken from snow-fields in high mountain regions 

 is found to contain a small amount of earthy matter. Its particles are 

 often found to be in part volcanic, even when the place whence the snow 

 was taken is scores or even hundreds of miles from the nearest volcano. 

 There is probably no snow-field so high, or so far from volcanoes, but 

 that volcanic dust reaches it. If this be true of all snow-fields, it is 

 probably true of all land surfaces. In the great Krakatoa ^ eruptions 

 of 1883 large quantities of volcanic ash (pulverized lava) were pro- 

 jected to great heights into the atmosphere. The coarser particles 



* For an excellent study of the erosion, transportation, and sedimentation per- 

 formed by the atmosphere, see Udden, Jour, of Geol, Vol. II, pp. 318-331. See also 

 Pop. Sci. Mo., September, 1896. 



' The Eruption of Krakatoa. Committee of the Royal Society, 1888. 



