78 ON CORAL REEFS AND ISLANDS. 



leeward and windward reefs, especially as the wind for some 

 parts of the year has a course opposite to its usual direction. 

 But seldom, except on the side to windward, is a sufficient force 

 brought to bear upon the edge of the platform, to detach and 

 uplift the larger coral blocks. The distance to which the waves 

 may roll on without becoming too much weakened for the trans- 

 portation of uptorn blocks, will determine the outline of the 

 forming land. With proper data as to the force of the waves, 

 the tides, and the soundings around, the extent of the shore plat- 

 form might be made a subject of calculation. 



The effect of a windward reef in diminishing the force of the 

 sea is sometimes shown 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 southeast 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 calcareous 

 grains, as explained on a following page. It has been stated that 

 coral sandrocks 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 continual progress. 

 Moreover, the structure built amid the waves will necessarily 

 have the form and condition best fitted for withstanding their ac- 

 tion. 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 devastating seas which overleap the whole land may occa- 

 sion unusual degradation from 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, show any 

 approximation to an obliteration of the lagoon. 



