THE EARTH. 51 



eight inches thick, which has been formed by the 

 leaves, branches, and bark, which fall and rot 

 upon the ground. I have frequently observed, 

 on a Roman way which crosses Burgundy for a 

 long extent, that there is a bed of black earth, of 

 more than a foot thick, gathered over the stony 

 pavement, on which several trees, of a very con- 

 siderable size, are supported. This I have found 

 to be nothing else than an earth formed by de- 

 cayed leaves and branches, which have been con- 

 verted by time into a black soil. Now, as vege- 

 tables draw much more of their nourishment from 

 the air and water than they do from the earth, it 

 must follow, that, in rotting upon the ground, 

 they must give more to the soil than they have 

 taken from it. Hence, therefore, in woods kept 

 a long time without cutting, the soil below in- 

 creases to a considerable depth ; and such we 

 actually find the soil in those American wilds 

 where the forests have been undisturbed for ages. 

 But it is otherwise where men and animals have 

 long subsisted ; for as they make a considerable 

 consumption of wood and plants, both for firing 

 and other uses, they take more from the earth 

 than they return to it : It follows, therefore, that 

 the bed of vegetable earth, in an inhabited coun- 

 try, must be always diminishing; and must, at 

 length, resemble the soil of Arabia Petrea, and 

 other provinces of the East, which having been 

 long inhabited, are now become plains of salt and 

 sand ; the fixed salt always remaining, while the 

 other volatile parts have flown away." 



