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SECOND BOOK. 
for some distance, dropping them on the plains and 
valleys, and thus spreading out new soil. 
6. The swift rivers carry their mud to be strewn 
at the bottom of the sea. In many parts, it may 
be plainly seen that what was once the bed of an 
ocean is now dry land, which, perhaps by some 
force inside the earth, has been thrown up above 
the level of the water. 
7 . On their way the rivers manage to sort the 
soil. First they drop the stones and coarse gravel; 
then, as their speed grows less, they drop the finer 
gravel; next the sand sinks; and, lastly, the fine 
mud is carried over the plain by the flood, or 
is dropped near the river-mouth, or in the sea. 
In this way beds of gravel, sand, and clay are 
sorted out, so as to form soils of different kinds. 
8. The waters of the sea, by their constant move- 
ments, also wear away the shores and grind them 
down. We may see that even the hard stones, 
which have been for a long time rolled about by 
the water, have had their rough edges worn off, and 
are round and smooth. 
9. There is another way in which water can help 
on the work of forming soil. It can dissolve some 
of the substances in the rocks, just as you have 
seen it dissolve salt or sugar. The water cannot 
take these substances with it when it ‘ dries up ’ 
or passes into the air. So they have to be left on 
the earth, and thus they help to form new soil. 
