ATMOSPHERIC DUST. 35 



I'ents next to the ground in the heaviest wind storm, the 

 composition of this dust may be said to indicate that fine 

 sand is too heavy to be effectively kept from settling in 

 such winds, that very fine sand and coarse dust are just 

 on the limits of the size which is subject to effective sus- 

 pension, and that particles which have a diameter less 

 than one thirty-second of a millimeter will not readily 

 settle from the atmosphere in a strong wind. It may be 

 inferred also that dust of the kind taken in railway 

 coaches must be capable of being lifted u]) into the atmos- 

 phere by moderately strong winds. This is also indicated 

 by the composition of some dust gathered on a window 

 sill three feet above the ground in a building at Yuma in 

 Arizona (Tab. XXI). 



Volcanic dust forms another class of atmospheric sedi- 

 ments which are transported under unusually favorable 

 conditions. It is launched from great heights, to w^hich 

 it never could have been raised by the convection currents 

 of the lower part of the atmosphere, and it is carried by 

 the upper currents, where transportation is much more 

 swift than below. Nearest the volcanic outburst thei-e is 

 no maximum limit to the size of volcanic fragments 

 which may fall, but beyond the distance of the influence 

 of the projectile force, which seldom, perhaps, exceeds a 

 dozen miles, their size is determined by the sorting action 

 of atmosphei'ic currents and hence will be a true exponent 

 of the nature of this action. 



Seven samples of such volcanic dust have been exam- 

 ined (Tab. XXII). Five of these are from quaternary 

 deposits on the western plains, one is from the Lahontan 

 sediments in Nevada, and one is fi-om a recent shower on 



