2 THE FERTILITY OF THE SOIL [CH. 



earth was first cool enough to allow a solid crust to 

 form. When the water began to fall some of it 

 soaked into the chinks and crannies of this crust, and 

 by its expansion and contraction with change of 

 temperature caused fragments to split off from the 

 main rock. Other agencies were also effecting the 

 same end, and in course of time a great quantity of 

 this disintegrated rock matter was formed. The 

 particles did not remain where they were, but were 

 carried by wind or water into the valleys and streams 

 and many found their way to the bottom of the sea. 

 Here they mingled with the residues of plants and 

 animals, and the whole mass became consolidated. 

 Later on, when earth movements changed the course 

 of the waters and the old sea became dry land, this 

 consolidated material appeared as new rock and went 

 again through the processes of disintegration and 

 erosion. For a long period the surface of England 

 north of the Thames, and of Canada and the northern 

 part of the United States, was covered with ice which 

 pounded up and carried away many of the particles, 

 depositing them again when it melted. When the 

 particles were lying on the dry land they were subject 

 to the constant washing of the rain, the oxidising 

 effect of the atmosphere, and the shattering effect 

 of changing temperature, processes collectively known 

 as weathering. Ever since the particles first split off 

 from the original rock they have been exposed to 



