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



With proper data as to the force of the waves, the tides, and 

 the soundings around, the extent of the shore platform might 

 be made a subject of calculation. 



The effect of a windward reef in diminishing the force of 

 the sea is sometimes shewn in the influence of one island on 

 another. A striking instance of this is presented by the 

 northernmost of the Tarawan islands. All the islands of 

 this group are well wooded to windward — the side fronting 

 east, between north and south. But the north side of Tari- 

 tari is nothing but a bare reef, through a distance of twenty- 

 miles, although the south-east reef is a continuous line of 

 verdure. The small island of Makin, just north of Tari-tari, 

 is the breakwater which has protected the reef referred to 

 from the heavier seas. 



Coral-island accumulations have one advantage over all 

 other shore deposits, owing to the ready agglutination of cal- 

 careous grains, as explained in a following page. It has been 

 stated, that coral sand-rocks are forming along the beaches, 

 while the reef-rock is consolidating in the water. A defence 

 of rock against encroachment is thus produced, and is in con- 

 tinual progress. Moreover, the structure built amid the 

 waves will necessarily have the form and condition best fitted 

 for withstanding their action. The little islet of an atoll is 

 therefore more enduring than hills of harder basaltic rocks. 

 Reefs of zoophytic growth but " mock the leaping billows," 

 while other lands of the same height gradually yield to the 

 assaults of the ocean. There are cases, however, of wear 

 from the sea, owing to some change of condition in the island, 

 or in the currents about it, in consequence of which, parts 

 once built up are again carried off. Moreover, those devas- 

 tating seas which overleap the whole land, may occasion un- 

 usual degradation for some parts. Yet these islets have 

 within themselves the source of their own repair, and are 

 secure from all serious injury. 



The lagoons in coral islands are constantly receiving more 

 or less debris from the reefs ; and patches of growing coral 

 within also tend to fill them up. But the effect is slow in its 

 progress, and none but islands of small size, as before stated, 

 shew any approximation to an obliteration of the lagoon. 

 {To he continued,) 



