FORMATION OF CORAL REEFS AND ISLANDS. 197 



been rightly understood. It will be remembered that it lies 

 but little above low-tide level, and it is often over three hun- 

 dred feet in width, with a nearly flat surface throughout. 



Though apparently so peculiar, the existence of this platform 

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

 of this action. On the shores of New South Wales, Australia, 

 near Sydney, as observed by the author, the same structure is 

 exemplified along the sandsto?ie shores of this semi-continent, 

 where it is continued for scores of miles. At the base 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 

 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 coast. 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. 



THE OLD HAT. 



New Zealand, at the Bay of Islands, affords a like fact in an 

 •argillaceous 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 small island in this bay is 

 well named the " Old Hat," the platform encircling it, as 

 shown in the above figure, forming a broad brim to a rude 

 conical crown. The water, in these cases, has worn away the 

 cliffs, leaving the basement untouched. 



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

 self on the shallowing shores ; finally, the waters at top, through 



