NORTH SEA FISHERIES 27 



them ; their roots insert themselves into 

 crevices and detach fragments. In tem- 

 perate climates, such as those of the 

 countries surrounding the North Sea where 

 there is much rain, the -disintegrating 

 effect is greatest. The rock at higher 

 levels is always being converted by nature 

 into soil and brought down to lower land. 

 Hence the most fertile lands are almost 

 always in the valleys, or are those tracts 

 nearest the sea-level. The soil is continu- 

 ally washing ' down. Watch a mountain 

 torrent after a heavy rain. The water, 

 which before was limpid, soon becomes 

 turgid and yellow with the enormous 

 quantity of sediment which the rain has 

 washed into the stream. A good pro- 

 portion of this sediment "will eventually 

 be deposited in the bed of the sea. In 

 this way the soil is always travelling 

 towards the sea bottom. The rivers bear 

 the transported matter and it is deposited 

 when the force of the current diminishes ; 

 the heavier particles fall first, the finer 

 matter is carried farther. When the river 

 reaches level country and the current 

 becomes slow these minute particles of 



