SOIL. 203 



it always contains. The resistance of the greywackes, and of the 

 sandstones depends in a great measure on tiie nature and cohesion 

 of the cement which unites their particles; their power of resisting, 

 however, is generally inconsiderable, and these rocks fall down 

 pretty rapidly into sandy soils. 



The modifications experienced hy the constituent minerals of 

 rocky masses, do not happen solely from changes in the molecular 

 state of their elements ; their chemical nature is further deeply 

 changed, and some of their original principles disappear. The fel- 

 spars, for example, into the constitution of which potash and soda 

 enter, abandon almost the whole of these alkalies, in passing into 

 the state of kaoline. This is made manifest by a comparison of ihe 

 analyses of the mineral in its two states. Besides the alkali which 

 is lost, we also perceive that in kaoline, the proportion of alumen 

 relatively to that of silica, is much greater than in the undecomposed 

 felspar, a fact which, according to M. Berthier, demonstrates that 

 the alkali is removed in the state of silicate. 



The final result of the disintegration of rocks, and of the decom- 

 position of the minerals which enter into their constitution, is the 

 formation of those alluviums which occupy the slopes of mountains 

 that are not too steep, the bottoms of valleys, and the most extensive 

 plains. These deposites, however formed, whether of stones, peb- 

 bles, gravel, sand, or clay, may become tlie 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 

 whick 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 broom, the furze, &c., show 

 themselves upon gravels. These plants grow, and after their death, 

 either in part or wholly, leave a debris which becomes profitable to 

 succeeding generations of vegetables. Organic matter accumulates 

 in the course of ages, even in the most ungrateful soils in this way, 

 and by these repeated additions they become less and less sterile. 

 It is probable that the virgin forests of the new world have thus 

 supplied the wonderful quantity of vegetable mould, in which the 

 present generation of trees is rooted. At Lavega de Supia, in South 

 America, the slipping of a porphyritic mountain covered completely 

 with its debris, to the extent of nearly half a league, the rich plan- 

 tations of sugar-cane which were there established. Ten years af- 

 terwards I saw the blocks of porphyry shadowed by thick groves of 

 mimosas ; and the time perchance is not very remote when this new 

 forest will be cleared away, and the stony soil, enriched with its 

 spoils, will be restored to the husbandman. 



The chemical composition of the earth, adapted for vegetation, 

 must of course participate in the nature of the rocks and substrata 

 from which it is derived ; and the elements which enter into the 

 constitution of mineral species ought to be found in the soils, which, 

 »y the elFect of time or human industry, may serve for the repro- 

 duction of vegetables. It is on this account that it becomes inter- 



