20 CHYMISTRY APPLIED TO AGRICULTURE. 



lime, magnesia, and oxide of iron. Those from moun- 

 tains of the quartzeous formation are composed, almost 

 entirely, of siliceous earth, and give rise to soils of an 

 analogous character ; and so on of the rest. 



It would, however, be erroneous to suppose that the 

 lands formed by the waste of mountains are throughout 

 of the same nature, or contain the same principles, in the 

 same proportions, as the rocks from which they have been 

 produced: Upon this supposition it would be necessary 

 that all the substances, originally contained in any one 

 kind of rock, should be of equal specific gravity, and pos- 

 sess an equal affinity for water ; and this is not the case. 

 Those, the particles of which are held in the closest 

 union, are deposited first, whilst the others are carried on 

 by the current ; silica, and the oxides of iron, predominate 

 in those which are first deposited ; then lime, alumina, and 

 magnesia. 



It is very interesting to trace the changes which take 

 place in alluvial soils, according to their distance from the 

 rivers which brought them ; whether we consider, in these 

 changes, the division and mixture of the constituent prin- 

 ciples, or the varieties which they present at different dis- 

 tances from the sources of their origin. 



Independently of the various degrees of specific gravity 

 and hardness which exist amongst the earthy principles, 

 there are other causes which contribute powerfully to affect 

 the nature of alluvial lands. Rivers receive, in their 

 courses, many tributary streams, which, mingling the frag- 

 ments that they carry with the spoils of the others, 

 modify to an illimitable extent the soils which they pro- 

 duce. It frequently happens, that this mixture of the mud 

 of two rivers, produces a soil more fertile, than would 

 have been formed by either of them singly ; the qualities 

 of one serving to correct the deficiencies of the other. 

 Thus the washings from moutnains of the quartzeous form- 

 ation, combined with the argillaceous and calcareous por- 

 tions of the wrecks of other mountains, constitute a more 

 productive soil than would be furnished by either sepa- 

 rately. 



The greatest part of those lands now appropriated to 

 the richest culture, are but the ruins of those imposing 

 mountains, the sides of which, rent away and carried off 

 by torrents, are in their passage reduced to dust, and de- 

 posited in the valleys to form the basis for agriculture. 



