ON THE ALLUVIAL DEPOSITS. ,377 



which will probably be as durable. But more infor- 

 mation than the knowledge of soils can give, has too 

 often been expected from it: since the accessory cir- 

 cumstances of drainage, retentiveness, climate, &,c. too 

 commonly defeat the prognostics of the Analyst. 



The alluvia of rivers form the soils of those plains 

 and deltas so noted for their fertility. It is the slow 

 motion of water which has rendered Egypt, Bengal, 

 Lombardy, and a thousand similar tracts, the seats of 

 inexhaustible fertility. Thus also are stored up, in 

 lands now far remote from their rivers, those soils 

 which these deposited under former positions ; and it 

 is here chiefly that we may witness the important con- 

 sequences arising from the destruction of the solid 

 earth. In nature's hands, the barren and useless sum- 

 mit of the mountain generates an empire: the eternal 

 harvests of Italy are produced from the naked Alps. 

 But though the basis of all soils consists in the com- 

 mon earths, the most important ingredient is the pro- 

 duce of vegetable decomposition : and thus, through the 

 marvellous circle of nature's chemistry, the elements 

 of organized bodies, dissipated by the winds or dis- 

 persed through the earth, return to their original forms. 



Alluvial Rocks. 

 m 

 I must still notice a class or rocks which could not 



well have found a place before 1 , and which, from their 

 daily production, are best ranked here. If their geo- 

 logical importance is not very great, they oiler some 

 valuable evidence, to which I have resorted more than 

 once. 



I have shown that the consolidation of sand and 

 fragments can be effected by carbonat of lime, by iron,, 

 and by solutions of silica: but it is doubtful if the 

 last cause operates to any extent at present. I know 



