THE SOIL OF Ti:i: FARM. 391 



fluences brought to bear upon them, have been converted into sanl and 



clay. 



2. FORMATION OF SOILS. 



Bearing in mind what has been already advanced with reference to the 

 general origin of soils, we have next to inquire somewhat more minutely 

 into the processes which have at length resulted in their formation. The 

 first destructive or disintegrating element which operates upon a rock is 

 THE ATMOSPHERE, which is brought into closer contact with the rocky 

 particles through the agency of moisture, in the form of snow, rain, dew 

 or mist. Through one or all of these forms of water, carbon dioxide is dis- 

 solved from the air which contains it, in the proportion of 4 in 10,000 

 parts. Thus the rock is brought into constant contact with a dilute solu- 

 tion of carbon dioxide in water. The action of this solution is slow, but 

 irresistible when allowed to proceed through long periods of time; ami 

 under its action the hardest rocks, such as granite for example, yield up 

 their alkalies and some silica, the felspar loses its cohesion and falls into 

 a soft powder (clay), while the quartzose constituents are left in the form 

 of sand. 



< 'IIAXCKS OF TEMPERATURE. The foregoing effect is further expedited 

 by changes of temperature, and especially by frost. Water expands as it 

 freeze*, and when water lodges in the interstices or pores of a porous rock, 

 it suddenly expands as it congeals, and the consequence is a disruption of 

 particles that speedily shows itself in the crumbling character of the rocky 

 Surface. 



TATION. As these forces proceed, a superficial layer of variable 



thicv produced, and at an early stage vegetation exerts its sway. 



Lichens are seen Crowing upon walls and the faces of quarries, and mosses 



; i'l follow at a later stage, and occupy the most unpromising situ - 



ct is to increase moisture and to accumulate vegetable 



matt, i- <>r Imiiius, which again gives rise to carbonic dioxide, and renders 



rain still moi - In ita diswlving and disintegrating actioa The 



Iso, no doubt, themselves exert a dissolving effect Thus 



gradually a soil is formed, and it will be noticed that, in this case, it is 



the rock from which it was derived. 



ill to notice differ from those already enumerated in their 



rting effects, whieh, when added to their disintegrating action, 



only accountable for the origin, but the present position 



