FORMATION OF ARABLE LANDS. 21 



It is not possible to refer to any other causes than those 

 I have just pointed out, the formation of the arable lands 

 of the valleys ; those which are found upon the vast table 

 lands, which crown the tops of mountains or extend along 

 their sides, must have had some other origin. The con- 

 stant action of air and water, alone, might have produced 

 the plains, but so gradually, that their effects would only 

 be perceptible after a lapse of many ages, if other agents 

 did not conspire with them to hasten the decomposition of 

 the rocks, and to convert them into arable land. 



The decomposition of such rocks, as are by their want 

 of density permeable by water, must be much more rapid 

 than that of those, in which the particles are more closely 

 united ; and rocks, of which the constituent principles pos- 

 sess some affinity for air and water, will yield much more 

 readily to their action, than those in which no such affinity 

 exists. 



In order to account for the action of air and water, upon 

 rocks, it is necessary to consider, that many among them 

 contain lime, very imperfectly saturated, and usually some 

 oxide of iron, at its lowest state of oxidation ; the lime is 

 constantly disposed to imbibe from the atmosphere its car- 

 bonic acid, whilst the oxide of iron combines with its oxy- ^ 

 gen ; th«se combinations will be very rapid, if neither the 

 lime nor the oxide of iron is united to any other sub- 

 stances, which, not possessing the same affinities for the 

 constituents of the atmosphere, oppose its action upon 

 them. 



Rocks are frequently moistened by water for a consid- 

 erable length of time, without being much affected by it ; 

 but when it has at length insinuated itself into their pores, 

 and become there converted into ice by the cold, it de- 

 stroys by its expansion the cohesion of their particles, pro- 

 ducing rents and fissures, and thus giving access to the 

 air, which combines with the lime and oxide of iron, and 

 produces' an immediate change in all the surfaces exposed 

 to its action ; from this moment the process of decompo- 

 sition goes on more rapidly than before. The lichens and 

 mosses, which fasten themselves upon the surfaces of rocks, 

 continue and increase the change ; their delicate roots are 

 constantly enlarging the crevices caused by the water, by 

 the effort they make to insinuate themselves into them ; 

 and by their decay they afford light successive layers of 

 pulverized vegetable matter. 



