CHAPTER I. 



ATMOSPHERIC AGENCIES. 



Origin of Soil. If we dig into the earth anywhere, 

 at a certain depth, greater in some places than in others, 

 we find rock. How was the earthy soil formed ? Per- 

 haps some imagine that it is an original clothing intended 

 to cover the rocky nakedness of the new-born earth. But 

 the very first lesson to be learned by the study of geology 

 is that all things that we see, even the most enduring 

 such as hills, mountains, rocks, etc. have become what 

 they are, usually by a slow process. 



Now, soils are no exceptions. All soil is formed by a, 

 disintegration or rotting down of rocks.- Sometimes the 

 soils remain resting on the rocks from which they were 

 formed ; sometimes they are removed to another place, as, 

 e. g., from hillsides to bottom-lands ; sometimes they are 

 carried by streams to great distances, and deposited as 

 sediments, and again raised as land ; but in all cases they 

 are formed in the same way viz., by the rotting down of 

 rocks under the slow action of the atmosphere. 



The active ingredients of the air in this process are 

 oxygen, carbonic acid (carbon dioxide), and water, as 

 vapor or as moisture. Now, rain-water contains in solu- 

 tion both oxygen and carbon dioxide. Therefore, rain- 

 water, wetting the surface and penetrating the cracks of 

 rocks, is the great agent in the formation of soil. 



Proofs of this Origin of Soils. The proofs of this 

 mode of formation are clearest in those cases in which 



