further aids disintegration. These disintegrating agencies are still 

 further aided by the root-growths of plants, by the burrowing of worms 

 and other earth-dwelling creatures, and in no small degree by the gen- 

 eration of organic acids, humic, crenic, &c., by organic decay. 



From the hardest granites, basalts, and lavas, to the softest lime- 

 stones and marls, all are undergoing this disintegration ; and the soils 

 to which they give rise will vary in depth, composition, and texture 

 according to the softness and mineral character of the rocks and the 

 length of time they have been subjected to these agencies.*! 



According to Darwin t the solid rocks disintegrate even in countries 

 where it seldom rains and where there is no frost. De Kouiuick, a 

 Belgian geologist, is of opinion that such disintegration may be attrib- 

 uted to the carbonic and nitric acids, together with the nitrates of am- 

 monia, which are dissolved in the dew. 



The rocks which weather most easily and rapidly do not always ex- 

 hibit most soil, very often the reverse. A pure limestone would exhibit 

 hardly any weathered band or soil, because the carbonic acid of the rain 

 would almost at once dissolve and remove the particles it acts upon. 

 Even in the case of igneous rocks, their composition may be such that 

 those which weather the most rapidly would, likewise, show little of a 

 weathered band, owing to the same solvent action.^ 



THE SOILS FORMED BY THE DIFFERENT GEOLOGICAL FORMATIONS. 



The rocks of which feldspar is one of the constituents are the origins 

 of the clays and potash which are met with in all arable soils; feldspar 

 is a silicate of aluminium and potassium, which on disintegration forms 

 clay, a silicate of aluminium, and a silicate of potassium. 



The primitive and igneous rocks yield soils rich in potash, and the 

 fossiliferous rocks those rich in phosphoric acid. 



THE DENUDATION OF THK SOIL. 



The same agencies which form the soils are also wasting and carry- 

 ing them away. During every rain storm transportation of soil goes 

 on, as the brooks and rivers show, after heavy long-Continued rains, by 

 the yellow muddy color of their waters that they are carrying a vast 

 quantity of sediment towards the sea. The running streams bear along 

 the transported matter, and gradually deposit it as the current dimin- 

 ishes in velocity, the very finest particles being carried as long as the 

 stream remains in motion. When a river reaches :i flat or level tract 

 and over which its waters can flow in flood with a slow motion, the sus- 

 pended matter, consisting principally of sand and mud, is deposited, 

 and constitutes the alluvium, or new land, formed by such deposits at 

 the river's mouth or along its banks.f 



* Page's Economic Geology. 



t Darwin's Vegetable Mold and Earth Worms, IS&, p. 235. 



iThe Soil of the Farm. 



