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THE JOURNAL OF GEOLOGY.

at least 1,000 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 flocculation 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 precipitated 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 significance. It is maintained that in these deposits from the atmosphere 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 circumstance will naturally bring together materials ranging through great differences in coarseness. It has lately been shown[1] 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 1 and 100. Of course the range of the extremes will be much greater.

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

  1. ↑ S. P. Langley: Internal Work of the Wind.
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