Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

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