240 CORALS AND CORAL ISLANDS. 
increase this difference. Further, the winds work with the 
waves, and bear the lighter part of the beach-making sands 
to a higher level than can be reached by the waves, giving 
the beach a top of wind-drift deposition as already explained. 
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 débris 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- 


