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small. The lime carried in solution by rivers to the sea 

 goes to form the shells of these animals. The Silica carried 

 in solution to the sea is used by a minute animal called 

 Radiolaria in the formation of its body or rather the shell 

 round its body. Tripoli-earth and Barbadoes-earth used for 

 grinding purposes are old Radiolarian deposits like chalk is 

 old foraminiferous deposit. 



40. Light diatomaceous earth is of vegetable origin ; but 

 the earth is nearly pure silica. Landshells, caterpillars, moles, 

 voles, musk-shrews, and pigs may be also mentioned as 

 Nature's cultivators, though they are also to be regarded in 

 the light of pests. Locusts which are the worst of all pests 

 may be also regarded in the light of Nature's fertilizers. If 

 locusts are frightened and prevented from alighting, they 

 may not do any damage but simply leave a thick deposit of 

 droppings, rich in manurial substances culled from forests, 

 all along their track. 



41. Earth-worms have also considerable influence in the 

 formation of soils and in altering their character. They derive 

 nourishment from soil which passes through their intestinal 

 canal, some of the organic matter being digested, while the 

 whole of the earth is mixed up and triturated inside the 

 canal. Worm-casts are particularly useful to the farmer, as 

 they help to loosen and perforate the soil for penetration of 

 roots, water and air. Worms also drag down leaves, 

 pieces of straw &c. into their holes, thus incorporating 

 organic matter into the soil, and making heavy soils lighter 

 and light soils heavier. The presence of earth-worms on 

 grass-land consisting of a shallow layer of soil resting on 

 hard rocks is particularly beneficial in gradually adding to 

 the depth of the soil in an imperceptible manner. Darwin 

 computed that an acre of garden soil contains on an average 

 about 50,000 earth-worms, and in ordinary arable soils about 

 half this number. In good soils 10 tons of dry earth is 

 passed through the intestines of earth-worms annually and 



