FORMATION OF ARABLE LANDS. 



19 



be classed according to their nature, that the degree of 

 fertility of each, and the kind of cultivation to which it is 

 best suited, may be known. 



Not one of these earths is by itself well adapted to cul- 

 tivation, but by their mixture they correct the qualities, or 

 supply the deficiencies of each other ; the best soil is that 

 which unites the greatest number of the properties most 

 suited to vegetation. 



There are few soils that do not contain, in addition to 

 the abovementioned earthy and saline principles, some 

 portion of substances, resulting from the decomposition of 

 animal and vegetable matter, by which, other circum- 

 stances being the same, their fertility is very considerably 

 influenced. 



ARTICLE III. 



Of the Formation of Arable Lands. 



Arable lands are almost entirely produced by the de- 

 composition, from various causes, of the rocks which form 

 the basis of our globe. The water, which flows in tor- 

 rents from the tops of the mountains, abrades their sides, 

 and detaches from them large portions of rock, which 

 being afterward swept by the force of the current, and 

 constantly dashed and rubbed together, have at length 

 their corners and edges broken off, their forms rounded, 

 their surfaces smoothed, and their size diminished, till 

 they form, successively, pebbles, gravel, sand, and mineral 

 slime. 



The number and magnitude of the stones found in the 

 beds thus deposited, depend upon their distance from the 

 mountains whence they have been brought, upon the 

 harder or softer character of the rock whence they have 

 been broken, and upon the force of the currents by which 

 they have been acted upon. 



Nearly all the lands of our rich valleys owe their origin 

 to the decomposition of rocks, and we can judge of the 

 nature of the principles which compose them, by knowing 

 those of the mountains whence they have been brought. 

 The deposits from granitic mountains, consisting of quartz, 

 feldspar, and mica, form soils mixed with silica, alumina, 



