ALLUVIUM. 61 



to the sun, and forms a crust including the subja- 

 cent sand. In this state the water comes in quietly, 

 detaches successive portions of this crust, in large 

 or smaller pieces, which are borne away by the 

 retreating tide. May not this silent and hitherto 

 unnoticed transportation counteract, to a certain 

 extent, the operation of other known agents 1 It 

 is not philosophical, I admit, to impute important 

 effects to slight and apparently inadequate causes ; 

 but it is equally unphilosophical to neglect trifling 

 phenomena until the nature and extent of their 

 agency has been thoroughly investigated." 



Boussingault says : " The final result of the dis- 

 integration of rocks, and of the decomposition of 

 those minerals which enter into their constitution, 

 is the formation of those alluvia which occupy 

 the slopes of mountains that are not too steep, the 

 bottoms of valleys, and the most extensive plains. 

 These deposits, however formed, — whether of stones, 

 pebbles, gravel, sand, or clay, — may become the basis 

 of a vegetable soil, if they are only sufficiently loose 

 and moist. Vegetation of any kind succeeds upon 

 them at first with difficulty. Plants, which by their 

 nature live in a great measure at the expense of the 

 atmosphere, and which ask from the earth little or 

 nothing more than a support, fix themselves there 

 when the climate permits. Cactuses and fleshy 

 plants take root in sands ; mimosas, the brocua, the 

 furze, and others, show themselves upon gravels. 

 6 



