560 CORAL FORMATIONS. April, 1836. 



alone build a solid reef, are never found within the 

 lagoon ; they only flourish amidst the foam of the never- 

 tiring breakers. Nevertheless, the more delicate corals, 

 though checked by several causes, such as strong tides and 

 deposits of sand, do constantly tend to fill up the lagoon ; 

 but the process must become slower and slower, as the water 

 in the shallow expanse is rendered subject to accidental 

 impurities. A curious instance of this happened at Keeling 

 Island, where a heavy tropical storm of rain killed nearly 

 all the fish. When the coral at last has filled up the lagoon 

 to the height of lowest water at spring-tides, which is the 

 extreme limit possible, — how, afterwards, is the work to be 

 completed ? There is no high land whence sediment can be 

 poured down; and the dark-blue colour of the ocean 

 bespeaks its purity. The wind, carrying calcareous dust 

 from the outer coast, is the only agent which can finally 

 convert the lagoon island into solid land, and how slow must 

 this process be ! 



Subsidence of the land must always be most difiicult to 

 detect, excepting in countries long civilized, — for the move- 

 ment itself tends to conceal all evidence of it. Neverthe- 

 less, at Keeling Island, tolerably conclusive evidence of such 

 movement could be observed. On every side of the lagoon, 

 in which the water is as tranquil as in the most sheltered 

 lake, old cocoa-nut trees were undermined and falling. 

 Captain FitzRoy likewise pointed out to me on the beach 

 the foundation-posts of a storehouse, which the inhabitants 

 said had stood, seven years before, just above high-water 

 mark, but now was daily washed by the tide. Upon asking 

 the people whether they ever experienced earthquakes, they 

 said, that lately the island had been shaken by a very bad 

 one ; and that they remembered two others during the last 

 ten years. I no longer doubted concerning the cause which 

 made the trees fall, and the storehouse to be washed by the 

 daily tide. 



At Vanikoro, the encircled island already mentioned, I 

 gathered from Captain Dillon's account, that the alluvial 



