THE ATMOSPHERE AS A GEOLOGICAL AGENT. 25 



from the underlying rock for considerable distances from the edge 

 of the cliff (Fig. 6). This may be seen at numerous points on the 

 lava plateaus of Washington. 



The presence of dust in the upper atmosphere during a rain-storm is 

 sometimes the occasion of phenomena which are often misinterpreted. 

 If there be abundant dust in the atmosphere through which rain-drops 

 or snow-flakes fall, much of it is gathered up by them, and the water 

 is thereby rendered turbid and the snow discolored. Here is to be 

 found the explanation of ^'mud-rain/' '^ blood-rain" (red dust), etc. 



Since dust is carried to a considerable extent in the upper atmos- 

 phere, its movements and its deposition are little affected by obstacles 

 on the surface of the land. A building or a hedge can only affect the 

 lodgment of that part of the atmospheric dust which comes in contact 

 with it or is swept into its lee. Since most obstacles on the surface 

 of the solid part of the earth reach up but slight distances into the 

 atmosphere, the dust of the greater part of the air settles without 

 especial reference to them, and is spread more or less uniformly over 

 the surface on which it falls. 



Fig. 7. — Diagram to illustrate the effect of an obstacle on the transportation and 

 deposition of sand. The direction of the wind is indicated by the upper arrow. 

 The lower arrows represent the direction of eddies in the air occasioned by the ob- 

 stacle. If the surface in which the obstacle was set was originally fiat (dotted 

 line), the sand would tend to be piled up on either side at a little distance from 

 the obstacle, but more to leeward. At the same time, depressions would be hol- 

 lowed out near the obstacle itself (see full line). (After Cornish.) 



Much of the dust transported by the wind is carried out over seas 

 or lakes and falls into them. By this means, sedimentation is doubt- 

 less going on at the bottom of the whole ocean, and at the bottoms 

 of all lakes. While means of determining the amount of dust blo^vn 

 into the sea are not at hand, it is safe to say that, were such determina- 

 tions possible, the result, if stated in terms of weight, would be sur- 

 prising. 



Transportation and deposition of sand. — In its transportation by 

 the wind, sand is not commonly lifted far above the surface of the 



