FORMATION OF CORAL REEFS AND ISLANDS. 



231 



has not been rightly understood. It will be remembered that 

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

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



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. On the shores of New South Wales, 

 Australia, near Sydney, as observed by the author, the same 

 structure is exemplified along the sandstone, 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 hun- 

 dred 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 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. 



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 favor the production of a horizontal surface ; it 



THE OLD HAT. 



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 



