44 SEA AXn LAXD 



swaying of the mass a considerable amount of rocky matter 

 is made into fine mud. whicii is free to drift away in the whirl- 

 in;^ waters. W'c may often see that the sea-water is per- 

 ceptibly muddied by this action for a considerable distance 

 from the shore, a wide frinq^e of the sea attesting by its tur- 

 bidity 'tlie work of this mill. If the student be an expert surf- 

 bather, he may venture beyond the shore belt to the point 

 where the waves topple over in the breakers. There he will 

 discover another mode of action of the waves, which differs 

 in many regards from that brought about by the swayings 

 against the shore. When the wave topples over, the upper 

 part of its mass falls down, it may be from the height of ten 

 feet, upon the bottom, on which it strikes with great energy. 

 If this floor of the sea be tine sand, the effect of the blow 

 is slight, and the particles are little disturbed, being trodden 

 to a Arm mass by the long-continued tramping of the surges. 

 If, however, the bottom be composed of pebbles, with their 

 faces made slippery from the water or the gelatinous ooze of 

 the sea-floor, they fly about when the falling wave strikes 

 them, giving forth a hoarse roar from their friction against 

 each other. Sometimes we may observe how these swift- 

 moving stones striking against a flrm-set bowlder skij) into 

 the air like a ball froni a bowler's hand. So we see that 

 there are two kinds of rock-grinding done on the beach — that 

 which is accomplished next the land by the swinging move- 

 ment of the waves and that which is effected by the breakers. 

 The rate at which the pebbles are reduced to sand and 

 mud by these processes of the beach varies, of course, with 

 the hardness of the materials and the energy with which the 

 waves assail them. We may judge the speed of this work 

 not only by the rapid reduction in the size of the pebbles as 



