t)iSCUSSION OF DATA 731 



currents continue for much longer periods than currents produced by 

 wind waves, the work produced by tidal currents evidently somewhat 

 more resembles drifting than does washing effected by waves raised by 

 winds in water near shores. It is also evident that, owing to various com- 

 pensations and interactions of tidal and other currents, there must exist 

 in the sea all gradations between alternating currents, interrupted cur- 

 rents, and continuous currents. 



In the atmosphere no process takes place that is quite comparable 

 with the washing effected by shore waves. The atmosphere has no upper 

 surface which intersects the slopes of the continents. But there are 

 some winds which in their duration and alternation with each other 

 somewhat resemble tidal currents in the sea. These are the diurnal winds 

 on seacoasts and the cyclonic winds. The former of these are believed to 

 be of minor significance for atmospheric sedimentation, and the latter 

 are subject to so many irregularities and to such effective systematic 

 influences from the planetary winds that the sum total of their work is 

 more like drifting. In particular cases, as in dune regions on the coast, 

 the wind may work in a manner somewhat like washing performed by 

 wave currents in water. We might then call their sedimentary work 

 winnowing. But this winnowing will in its effects probably be more 

 like work performed by tidal waves than by shore waves. 



Silting is sedimentation in water that results in the deposition of 

 somewhat fine material, which is suspended in the entire body of water, 

 or at least in some considerable part of a main body of water. The 

 water is sufficiently agitated to keep in suspension a part, but not all, of 

 the load it carries. A part is deposited. Compared with drifting and 

 washing, silting is a slow process and takes place in quiet waters. To 

 some extent, silting goes on, however, almost everywhere, even in more 

 rapidly moving currents. It is the principal mode of sedimentation in 

 lakes and seas away from shores. It is also a very important part of 

 sedimentation effected by rivers during floods. 



In the atmosphere a similar process of sedimentation is one of the 

 most common occurrences. This is generally known not as silting, but 

 as dusting. Dusting consists in the settling of fine material from a 

 relatively deep and quiet body of the atmosphere. The material must 

 be fine enough to have become dispersed in the atmosphere during strong 

 winds, but too coarse to be held in effective suspension at times and in 

 places of lesser movement of the air. The process is universal in its 

 geographical distribution. In most places the resulting sediment is so 

 small in amount that it never can accumulate on land of moderate re- 

 lief where erosion is at all effective. But over wide and extensive plains 



