THE CONTENTS OF THE SOIL 19 



various kinds of earthworms, one of which 

 is the common angleworm. These animals eat 

 earth, which, when excreted, is more or less 

 mixed with organic matter, and the mineral 

 particles are ground and modified. It is now 

 considered that in the tenacious soils in which 

 these animals work, the earthworms have been 

 very important agents in fitting the earth for 

 the growing of plants, and consequently for 

 agriculture. 



32.1 While the basis of most soils is dis- 

 integrated rock, there are some soils which 

 are essentially organic in origin. 1 These are 

 formed by the accumulation of vegetable mat- 

 ter, often aided by the incorporation of animal 

 remains. In the tropics, such soils are often 

 formed on shores and in lagoons by the exten- 

 sion of the trunk -like roots of mangroves 

 and other trees. In the network of roots, leaves 

 and sea-wrack are caught, and mold is formed. 

 Water plants (as marsh grasses and eel -grass) 

 are sometimes so abundant on sea margins as 

 to eventually form solid land. On the edges of 

 lakes and ponds, the accumulation of water-lily 

 rhizomes and other growths often affords a foot- 

 hold for sedges and other semi -aquatic plants ; 

 and the combined growth invades the lake and 

 often fills it. Portions of this decaying and 

 tangled mass are sometimes torn away by wind 



