90 THE SOIL. 



Northern and Middle States, the soil is made up, in a 

 considerable degree, sometimes wholly, of sands or clays, 

 drifted from the north. These are often called diluvial 

 soils, from a belief, once in vogue, that they had been 

 brought to the places where they are found by the action 

 of a deluge (diluvium.) 



303. When the native forests are cut down, and the 

 land cleared of the undergrowth, and broken up by the 

 plough, the soil is almost uniformly found to be fertile. 

 In most parts of America, this virgin soil will bear large 

 crops of grain and other valuable plants, for many years 

 in succession, without manure. This fertility is owing 

 to the fact that the surface has been occupied by forest 

 trees and other forest plants for countless centuries. By 

 the decay of the leaves, fruit, roots and trunks, the 

 ground has been covered with a coat of humus or forest 

 mould ; and by weathering, the long continued action 

 of the atmosphere, and other great agencies of nature, 

 the minerals in the soil have been brought into a state 

 suitable for the food of plants. 



304. To give some instances of this action. The oxygen 

 of the air, combining with the iron or oxide of iron in a 

 particle of granite, makes it swell and crumble, and, at 

 the same time, releases the potash or other element which 

 had been associated with the iron, and leaves it ready to 

 be taken up by the roots of a plant. Carbonic acid acts 

 in a similar way iipon lime and magnesia. 



305. But the carbonic acid docs not act alone. Car 

 bonic acid is always ready to be dissolved or absorbed by 

 water ; and water, thus charged with it, has not only the 

 power of dissolving limestone and magnesian rocks, but 

 exerts a slmv but certain inlluenee by which even granite 

 and the other hardest rocks are gradually crumbled ; very 



