328 THE JOURNAL OF GEOLOGY. 



at least i,000 times (under ordinary circumstances perhaps nearer 

 100,000 times) less per unit of bulk of the carrier than in most 

 waters where sedimentation occurs, it is likely true that floccula- 

 tion in aerial sediments is not as great as that which takes place 

 in aqueous sediments. Thus the finest materials carried by the 

 air are not deposited in so great a proportion with the coarse 

 material, as they would be if the atmosphere carried a greater 

 load. The finest sediments, say particles below .002 mm. in 

 diameter, settle only during extreme calms, if not first caused to 

 gather in flocculi. This extremely fine material is retained by 

 the atmosphere and must be carried everywhere over the entire 

 surface of the globe, and must also be deposited everywhere, but 

 in such small quantities as not to be noticeable. No small part 

 of it, it may be surmised, is carried from the land and precipi- 

 tated into the sea. But the coarser sediments, say particles 

 between .002 and .1 mm. in diameter, are less easily retained in 

 the air and therefore occasionally deposited in favorable localities 

 in such quantities as to become an object of geological signifi- 

 cance. It is maintained that in these deposits from the atmos- 

 phere there should be a scarcity of the finest materials. 



It should be remembered, however, that there are great 

 differences in the prevailing wind velocities and that this circum- 

 stance will naturally bring together materials ranging through 

 great differences in coarseness. It has lately been shown ^ that 

 such differences are great, even within the limits of a minute of 

 time. As a result there will be a chance for a considerable range 

 in size of particles composing the bulk of any aerial sediment, a 

 range which it is believed might be expressed for the diameters 

 of such particles by the numbers i and 100. Of course the 

 range of the extremes will be much greater. 



Deposition of dust will take place where wind is caused to slackest 

 its speed. 



This is so self-evident that it appears superfluous to mention 

 it. It may be presumed that such a slackening will take place 

 over continental basins, where the general direction of the wind's 



' S. P. Langley : Internal Work of the Wind. 



