FORMATION OF SOILS. 



In addition to these mechanical causes of waste, we have to 

 consider the influence exerted by chemical forces in effecting the 

 disintegration of rocks, such as the action of the oxygen and 

 carbonic acid of the air, as well as that of water, upon their 

 constituent parts. Whilst we apply the term waste to the 

 effects produced by mechanical agencies, we shall confine the 

 term disintegration to the effects produced by chemical forces. 

 The latter causes may be very gradual in their operation, not 

 being limited in regard to time. Hence we cannot refuse to 

 acknowledge the existence of their action, even though the effect 

 produced may not be sensible during the life of an individual. 



Many years are necessary before the polished surface of an 

 exposed fragment of granite loses its polish ; but in process 

 of time this is effected, and the large fragment falls to pieces 

 under the influence exerted upon its constituents by the chemi- 

 cal forces. 



The action of water is so much connected with that of oxygen 

 and of carbonic acid, that it is scarcely possible to consider their 

 effects apart. 



Many kinds of rocks, such as basalt and clay-slate, contain as 

 an ingredient protoxide of iron. This oxide has a great tendency 

 to absorb oxygen from the air, becoming the higher oxide known 

 as peroxide of iron. This property is especially apparent in our 

 rich ferruginous soils. The surface of such soils to a certain 

 depth is of a red or brownish-red color, an indication that it con- 

 tains peroxide of iron ; whilst the black or brownish-black color 

 of the subsoil indicates the presence of the protoxide of the same 

 metal. It often happens that the subsoil is thrown upon the sur- 

 face in the course of subsoil-ploughing, and the consequence on 

 such soils is, that their fertility is destroyed for a certain number 

 of years. The injury thus received continues until all the sur- 

 face-soil again becomes red, that is, until all the protoxide of 

 iron is converted into the peroxide. 



It is known that a crystallized salt of iron loses its coherence 

 on exposure to air, and crumbles into a powder by the absorption 

 of oxygen. In a similar manner the disintegration of most 

 minerals is effected, for their ingredients are susceptible of en- 

 tering into union with oxygen. In consequence of the formation 



