l)l!IFTI.\(i SA.NI). 



2:5 



Table XIV. General Average of 

 the Composition of Dune Sand 

 (based on the averages for each 

 locality where samples have been 

 taken). 



Length or 



diameter in 



mm. 



Average. 



tures arrange tliemBelves in two series decrensing on cillicr 

 side of the maximum. The coarse admixtures form a less 

 rapidly decreasing- series than the fine, the former extend- 

 ing over five grades in the general average and the latter 

 over only three grades. The extension of either is di- 

 minished by prolonged wind action, which results in more 

 perfect elimination of grains near either extreme. 



The occurrence of the maxi- 

 mum at the same or nearly the 

 same point in nearly all the dune 

 sand taken at many different lo- 

 calities, challenges our special 

 notice. The size of a particle ca- 

 pable of being transported by 

 the current of a fluid varies as 

 the sixth power of the velocity of 

 the current. The diameter of the 

 particles, therefore, varies as the 

 square of the velocity. If the 

 velocity is doubled, the diameter 

 of particles transported may be 

 increased four times. The range 

 of velocities of dune-making 

 winds, as usually measured, cer- 

 tainly exceed a doubling of their 



speed, and it might be expected that the bulk of the sand, 

 in some place at least, should consist of grains many times 

 as large as in others. It may be that sometimes there is 

 a scarcity of such sand for the wind to work on, but this 

 will not alone account for the uniform composition of the 

 dune sand. Wind velocities ai-e usuallv measured some 



16—8 

 8-4 

 4—2 

 2-1 

 1- 



T2"' 



1 _ 



or 



1 _ 



■ 1 



