66 GENERAL CONCLUSIONS. 



Very fine sand (diam. ^ — jijj- mm.) vV few miles. 



Coarse dust (^ — gV min) 200 miles. 



Medium dust {{-j—^^ mm.) 1,000 miles. 



Fine dust (^V """• ^^d less) Aroimd the globe. 



It is evident that the place of greatest deposition is 

 never far from the place of greatest erosion, when the 

 eroded terrane consists of coarse as well as fine materials. 

 It is generally marked by the accumulation of dune sand. 

 From this point deposition decreases, owing to the trans- 

 versely horizontal and the vertical dispersion of the load 

 by spreading winds and owing to the previous settling of 

 the coarser particles. A limit is sooner or later reached, 

 where aqueous erosion is more rapid, than the accumula- 

 tion of atmospheric sediments. Beyond this limit the 

 latter will of course not appear. 



It is also evident that the different grades of materials 

 are so far separated from each other in the direction ot 

 the wind movement, that even with considerable changes 

 in velocity, the principal area of the deposition of sedi- 

 ments of one grade will not far encroach upon that of the 

 deposition of materials much coarser or much finer. 

 Gravel or coarse sand, for instance, will never be carried 

 to the region of the main dust deposit, nor will the fine 

 sand. For any particular locality a wind sediment will 

 hence be quite uniform in composition in a single triangle. 



In nature we must, however, expect to find a mul- 

 tiplicity of these triangular areas of wind action, wher- 

 ever the conditions are such that erosion by the atmos- 

 phere may take place. They must be found overlapping 

 and inclosing each other. The sediment in any particular 

 place may hence be found to contain grains of varied 



