236 CORALS AND CORAL ISLANDS. 



dinary seas and those during stormy weather. We have, 

 therefore, no difficulty in comprehending how the ordinary 

 wave-action should build up and keep entire the shore plat- 

 form, while the more agitated seas may tear up parts of the 

 structure formed, and bear them on to the higher parts of the 

 island. Still more violent in action are the great earthquake- 

 waves, which move through the very depths of the ocean. 



These principles offer an explanation also of the general 

 fact that the windward reef is the highest. The ordinary 

 seas both on the leeward and windward sides, are sufficient 

 for producing coral debris and building up the reef^ and in 

 this work the two sides will go on together, though at different 

 rates of progress. We may often find no very great dif- 

 ference in the width of the leeward and windward reefs, es- 

 pecially 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 transportation of up- 

 torn 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 platform 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 Gilbert Islands (see map, on page 165.) 

 All the islands of this group are well wooded to windward — 

 the side fronting east. But the north and northeast sides of 

 Tari-tari are only a bare reef, through a distance of twenty 

 miles, although the southeast reef is a continuous line of ver- 



