Tin: CONTKNTS UF THK SOIL 19 



various kinds ot" failhworiiis, oiir of wliicli 

 is tlio t'oiuiiinii aiii::l(>\vi)i-ni, Those animals eat 

 ••artli, whii-li, wlirn cxcri't^Ml, is more or loss 

 iriixoil with organic niattiT, and tlio iiiinoral 

 partiolos aro pcround and iuodiJi»'d. It is now 

 <'onsidorod that in tho tonaoious soils in wliich 

 thoso animals woi'k, tho oarth worms have Imm-m 

 vt-rv impoi-taiit aironts in litting tho oarth for 

 th«' irrnwini^ of plants, and eonsoqut-ntly for 

 airri<'uitnr<'. 



o'J. Wliih' the liasis of most soils is dis- 

 into^'at«'d rook, thoro aro some soils which 

 aro ossontially organic in origin. Thoso aro 

 formo<l l>y tho aooiimulation of vogotahlo niat- 

 t«'r, often aided l»y tho incorjtoration of animal 

 nMnains. In thi> ti-opics, sndi soils ar<' often 

 formod on shores and in lagoons by th«' exten- 

 sion of the trunk- like roots of mangroves 

 and other trees. In tht? network <tf roots, loaves 

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

 Water plants (as marsh grasses and ool-gi*ass) 

 are soniotimes so ahundant on sea margins as 

 to eventually form solid land. On the edges of 

 lakes and ponds, tho acounuilation of water- lily 

 rhizomos and other growths often atTonls a foot- 

 hold for sodgos and other semi-aquatic jilants ; 

 ami the combined gi'owth invades the lakf and 

 oft.'U fills it. Portions of this decaying and 

 tangled mass are sometimes torn awav bv wind 



