FORMATION OF CORAL REEFS AND ISLANDS. 201 



rocks from the edge of the platform, and throw them on the 

 reef From the observations of Mr. Stevenson, cited on a pre- 

 ceding page (p. 191), it appears that the force of the waves 

 during the summer and winter months differs at Skerryvore 

 more than 1,200 pounds to the square foot, — in the former it 

 averaging but d^^d pounds, and in the latter 2,086 pounds, 

 while in storms it was at times equivalent to 6,083 pounds. 

 The seasons are not as unlike in the tropical part of the Pacific. 

 Still there must be a marked difference between the ordinary 

 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 platform, 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 of^er 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 h;i(les will go on together, though at different rates 

 of progress. We may often find no very great difference in 

 the width of the 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 weak- 

 ened for the transportation 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 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 133). 

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



