30 THE PRINCIPLES OF AGRICULTURE. 



Many of the smaller hills have been formed by bodies 

 of moving water and ice, which have ground out valleys 

 between them. 



The formation of hills, and other changes upon the 

 face of nature, have been brought about gradually. Fall- 

 ing rains and running streams are slowly reducing the 

 size of the hills by wearing them away and carrying them 

 to the valleys below. In some cases, on the other hand, 

 swift running streams and rivers are wearing out and 

 increasing the depth of the valleys between the hills. 



The Soil If we examine some soil with a microscope, 

 we shall find that, while it contains some other sub- 

 stances of a different nature, a large part of it is com- 

 posed of finely divided particles of rock. These are 

 generally of a similar nature to the larger rocks that 

 are scattered through the soil, and have been produced 

 from large rocks by the grinding and crumbling forces 

 of nature. 



The greater part of our present soil, however, was not 

 formed directly out of the original rocks of the first 

 crust of the earth. The soil formed by the first crumb- 

 ling of the original rocks generally solidified, or petri- 

 fied, into rock again, and this process of crumbling and 

 solidifying continued through several alternations until 

 our present soil was formed. 



Nearly all the present rocks were at some time soil. 

 Conglomerate stones, sometimes called "pudding-stones," 

 are examples of an ancient soil, containing stones of 

 different kinds, which has been transformed into solid 

 rock. Sandstone was once a bed of sand. 



Specimens of rocks from the original crust are now to 

 be found only in a few scattered localities in those sec- 

 tions which first appeared above water, where, by virtue 



