26 FISHERIES OF THE NORTH SEA 



above-named minerals. In addition to 

 these there are often huge stones, remnants 

 of the glacial period, which the fisherman 

 knows to his cost, for they often destroy 

 the nets. 



First we must consider how this shallow 

 sea was formed ; it is important to under- 

 stand this, because our fisheries are de- 

 pendent on the fertility of the seabed, 

 which is the cumulative result of various 

 active forces. We all know that the 

 atmosphere acts chemically on the earth's 

 surface. Its action consists mainly in 

 oxidisation of the minerals containing 

 oxygen, and in the production of carbon- 

 ates ; this action separates the integral 

 parts of the rock. Changes of temperature, 

 by causing expansion and contraction, 

 also have a loosening and pulverizing 

 effect. This is particularly noticeable after 

 a heavy frost. Rain, by wetting and 

 rotting rock, has the same disintegrating 

 result ; it also washes the smaller frag- 

 ments from higher to lower levels. Again, 

 plants in their growth and decay convert 

 rock into soil by keeping the surface damp 

 and allowing water to penetrate and soften 



