82 THE ORIGIN AND NATURE OF SOILS [chap. 



rocks to soil, covering the bare surface of the former 

 wherever it may be exposed with a slowly formed 

 coating of its own debris. If the formation of soil 

 ended at this stage every rock would be covered by its 

 own appropriate "sedentary" soil, but as soon as the 

 soil is formed it begins to move. The wind and the 

 rain are always moving soil downhill ; the wind perhaps 

 may seem a trivial agency, but in rainless or even semi- 

 arid climates there are practically no other transporting 

 agencies at work, and we find vast tracts covered with 

 fine, uniform, wind-blown soil. The work of the rain is 

 most apparent when it has been long continued enough 

 to bring the rivers out in flood ; high up the country 

 every little gutter, every ditch that bounds the arable 

 land, is charged with turbid, hurrying water, which is 

 washing unseen quantities of coarser mud and sand 

 along its course. Finally the rivers themselves are 

 charged with sediment, as we can see by filling a large 

 bottle with the water from one that is running in flood 

 and allowing it to settle ; along their beds also, gravel 

 and stones of all sizes are being pushed. Wherever the 

 velocity of the current is reduced some of the transported 

 material will be deposited, hence as the streams descend 

 from the heights into flatter country they discharge the 

 coarsest part of the material they have washed from the 

 rocks above, and so give rise to the broad expanses of 

 gravel, sand, silt, etc., which underlie the water meadows 

 bordering the lower levels of all rivers. Much of the 

 suspended mud and clay, however, washes away to sea, 

 and is thrown down when the velocity of the stream is 

 entirely checked by the open water, where also the salts 

 in the sea-water exert their precipitating action upon 

 the fine particles ; in consequence, beds of newly sorted 

 soil tend to accumulate in the shallow waters just off 

 shore, and even to block up the river mouth unless the 



