228 J. D. Dana, Esq., on Coral Beefs and Islands. 



Though apparently so peculiar, the existence of this plat- 

 form is due to the simple action of the sea, and is a necessary 

 result of this action. Passing to New Holland, from the 

 coral islands of the tropics, we there found the same struc- 

 ture exemplified along the sandstone shores of this semi-con- 

 tinent, where it is continued for scores of miles. At the basis 

 of the sandstone cliff, in most places one or more hundred 

 feet in height, there is a layer of sandstone rock, lying, like 

 the shore platform of the coral island, near low tide level, and 

 from fifty to one hundred and fifty yards in width. It is 

 continuous with the bottom layer of the cliff; the rocks which 

 once covered it have been removed by the sea ; its outer 

 edge is the surf line of the shore. At low tide it is mostly 

 a naked flat of rock, while at high tide it is wholly under 

 water, and the sea reaches the cliff. New Zealand, at the 

 Bay of Islands, afforded us the same fact again, in an argilla- 

 ceous sand rock ; and there was no stratification in this case 

 to favour the production of a horizontal surface ; it was a 

 direct result from the causes at work — the shore shelf stands 

 about five feet above low water. 



A surging wave as it comes upon a coast, gradually rears 

 itself on the shallowing shores ; finally, the waters at top, 

 through their greater velocity, plunge with violence upon the 

 barrier before it. The force of the ocean's surges is there- 

 fore mostly confined to their summit waters, which add weight 

 to superior velocity, and drive violently upon whatever ob- 

 stacle is presented. The lower waters of the surge advance 

 steadily but more slowly, owing to the retarding friction of 

 the bottom ; the motion they have is directly forward, and 

 thus they act with little mechanical advantage ; moreover, 

 they gradually swell over the shores, and receive in part the 

 force of the upper waters. The wave, after breaking, sweeps 

 up the shore till it gradually dies away. Degradation from 

 this source is consequently most active where the upper or 

 plunging portion of the breaker strikes. 



But further, we observe that at low tide the sea is com- 

 paratively quiet ; it is during the influx and efflux that the 

 surges are heaviest. The action commences after the rise, 

 is strongest from half to three-fourths tide, and then dimi- 

 nishes again near high tide. Moi*eover, the plunging part 



