The Soil 15 



especially in the warmer temperate and tropical regions, 

 the untold milHons of coral polyps live and construct 

 great reefs, which are gradually brought to the surface 

 and exposed to the action of the waves. The Httle build- 

 ers die, but the waves carry the particles of their building 

 landward, and the lagoon behind the reef gradually fills 

 up. The Mangrove trees send their great roots into the 

 water along the lagoon, the shellfish accumulate there, 

 and the decay of the luxuriant vegetation of the tropics 

 soon makes a swamp of a lagoon. But further out again 

 the coral polyps build a reef, and again the process is 

 repeated. In this way, by the constant increase of the 

 coral reefs and the growth of the swamps, a large part of 

 the Florida peninsula has been formed. 



As the soils increased on the surface of 

 ^^^ ^ the earth the vegetation increased, till it had 



Pre ^ared \e ^^-ken the greater part of the carbonic acid 

 Soil from the air and left the carbonaceous 



remains to form what we call soil to-day, 

 and in which we grow crops. The accumulation of 

 vegetable mold is another agency for soil-building and 

 improvement. The roots of trees themselves exert a 

 solvent action on the rocks, and at times in their growth 

 force them apart through the sweUing of their roots in the 

 rock fissures. Then other living forms invisible to the 

 naked eye find a home in the vegetable soil, and through 

 their incalculable milHons, these so-called bacteria exert 

 a great influence in changing the character and composi- 

 tion of the soil. 



UnHke green plants above ground, which throw off 

 oxygen, these bacteria evolve carbonic and other acids 



