GENERAL CONCLUSIONS. 65 



and this tends to })i'event t'ui'ther erosion. The coarser 

 grades of sand , those containing grains from one to one 

 fourth of a millimeter in diameter, are dragged along a 

 greater distance, but they are unable to keep pace with 

 dune sand, which is mostly finer. When present in suffi- 

 cient quantity in the eroded terrane, the medium and the 

 fine sand, and especially the latter, are heaped up into the 

 dune drifts. These may creep over considerable distances 

 in course of time. The sand grains which measure from 

 one half to one eight of a millimeter in diameter, do not 

 seem to be lifted very far in a single leap by the strongest 

 wind, probabh- seldom as far as a few hundred yards, 

 and much more often only a, few feet. The very fine sand, 

 which is next in texture, appears to be mostly dropped 

 before it is carried many miles. Course dust remains 

 much longer in suspension. Most of it probably settles 

 before it is carried two or three hundred miles. The gen- 

 eral presence in all kinds of winds of medium dust renders 

 it likely that much of this may be carried as far as five 

 hundred or a thousand miles before having time to settle. 

 Dust finer than this is no doubt carried still farther. It 

 must be largely scattered around the globe and is perhaps 

 often kept floating, until it is brought down by rain. It 

 should be understood that these estimates are for such 

 winds as prevail over the continents. In a tabular form 

 they may be stated thus : 



Table of Approximate Maximum Distances over which Quartz Fragments of Different Dimen- 

 sions may be lifted by Moderately Strong Winds in Single Leaps. 



Gravel (diameter from 8 — 1 mm.) A few feet. 



Coarse and medium sand (diam. 1 — { mm.). .Several rods. 

 Fine sand (diam. ^ — ^- mm.) Less than a mile. 



