STRUCTURE OF CORAL ISLANDS. 



151 



accumulations going on within them are, therefore, wholly of 

 coral. The reefs within the lagoons correspond very exactly 

 in mode of growth and other characters to the inner reefs 

 under the lee of a barrier. 



IV. NOTICES OF SOME CORAL ISLANDS. 



The preceding descriptions represent the general character 

 of atolls, but are more especially drawn from the Paumotus. 

 There are some peculiarities in other seas to which we may 

 briefly allude. 



Among the scattered coral islands north of the Samoan 

 Group, the shore platform is seldom as extensive as at the 

 Paumotus. It rarely exceeds fifty yards in width, and is cut 

 up by passages often reaching almost to the beach. In some 

 places the platform is broken into islets. Enderbury's Island 

 is one of the number to which this description applies. The 

 beach is eleven or twelve feet high. For the first eight feet it 

 slopes very regularly at an angle of thirty to thirty-five degrees, 

 and consists of sand, coarse pebbles, or rounded stones of 

 coral, with some shells ; and there is the usual beach con- 

 glomerate near the water's edge. After this first slope, it is 

 horizontal for eighty to two hundred feet, and then there is a 

 gradual rise of three to four feet. Over this portion there are 

 large slabs of the beach conglomerate, along with masses from 

 the reef-rock, and some thick plates of a huge foliaceous 

 Madrepora ; and these slabs, many of which are six feet square, 

 lie inclining quite regularly against one another, as if they had 

 been taken up and laid there by hand. They incline in the 

 same direction with the slope of the beach. The large Madre- 

 pora alluded to has the. mode of growth of the Madrepora 

 palniata ; and probably the entire zoophyte extended over an 

 area twelve or fifteen feet in diameter. The fragments are 

 three to four inches thick, and thirty square feet in surface. 



As a key to the explanation of the peculiarities here observed, 

 it maybe remarked that the tides in the Paumotus are two to 

 three feet, and about Enderbury's Island five to six feet, in height. 



