632 THE WONDERS OF GEOLOGY. Lect. VI. 



have not subsided to any considerable amount, but either 

 have remained stationary, or been upheaved since the 

 growth of the coral. When a barrier reef, encircling 

 an island, gradually sinks down, the corals go on growing 

 vigorously upwards ; but, as the island sinks, the water 

 gains inch by inch on the shore, and the mountain-tops at 

 length become separate islands, within one great reef; and 

 ultimately the highest pinnacle disappears, and a perfect 

 Atoll is formed. Hence lagoon islands, having originated 

 from encircling barrier reefs, resemble them in general size 

 and form, and in the manner in which they are grouped 

 together ; and they may be regarded as outline charts or 

 models of the sunken islands they now surmount. By this 

 theory of the upward growth of polyparia during the sub- 

 sidence of the land on which they are based, Mr. Darwin 

 satisfactorily explains all the leading phenomena of those 

 marvellous structures, the barrier reefs ; and of those fairy 

 coral islands that begem the vast expanse of the Pacific, and 

 nourish in the midst of its mighty billows. The narrow 

 belt of land of these insular zones, frequently but a few 

 hundred yards wide, is surrounded by a deep and often 

 unfathomable ocean, and encloses a lake of tranquil water ; 

 it is crowned with cocoa-nut trees, clothed with a luxuriant 

 tropical vegetation, and begirt with a beach of glittering sand, 

 on which are constantly dashing the snow-white breakers of 

 the azure sea. Nowhere, as Mr. Darwin beautifully remarks, 

 can be found such wonderful proofs of the power of vita- 

 lity to repel the influence of mechanical force ; for the 

 breakers far exceed in violence those of our temperate 

 regions, and it is impossible to behold them without feeling 

 a conviction, that rocks of quartz or granite would speedily 

 be demolished by such irresistible agents ;* and yet there 

 stand, immoveable, those marvellous monuments — 



* See Mr. Darwin's delightful volume, "On the Structure and 

 Distribution of Coral Reefs." 



