142 



THE OPAL SEA 



suit is a rebound and a shooting upward of the 

 water into the apex of the wave — a dancing 

 skyward of harmless jets. Even a heavy storm 

 will not always throw waves against a cliff so 

 situated. It is the most secure of the rock 

 barriers, and centuries may elapse before such a 

 wall is finally disintegrated and cast into the sea. 

 The cliff with a shallow shore, shelving out- 

 ward, fares much worse. The waves as they 

 come in from the sea, moving in even succes- 

 sion, begin to feel a pull upon their bases as 

 soon as the shallowing commences. The drag 

 upon each wave retards the onward march 

 of the columns, with the result that the for- 

 ward waves move more slowly, and the ones 

 that come after catch up with them — close 

 up the ranks as it were — and make the col- 

 umns shorter and nearer together. As each 



The shallow 

 shore 



Friction of 

 wave bases. 



wave moves up the shelving shore the friction 

 becomes greater. The base is held back by the 

 sea bottom and pushed back by the undertow 

 running outward from the shore; while the top, 

 being less retarded than the base, is by its own 

 impetus driven ahead — pitched violently for- 

 ward. The climax is reached when the wave 

 dashes itself to pieces against the cliff and falls 

 in shattered foam among the shore bowlders. 

 Immediately its broken fragments gather them- 



