CORAL EEEFS. 75 



islands being unsatisfactory, Mr. Darwin was led to re-consider the whole subject. 

 Numerous soundings taken all round the Cocos atoll showed that at ten fathoms the 

 prepared tallow in the hollow of the sounding rod came up perfectly clean, and 

 marked with the impression of living polyps. As the depth increased these impressions- 

 became less numerous, but adhering particles of sand succeeded, until it was evident that 

 the bottom consisted of smooth mud. From these observations it was obvious to him that 

 the utmost depth at which the coral polyps can construct reefs is between twenty and 

 thirty fathoms. Now, there are enormous areas in the Indian Ocean in which every 

 island is a coral formation, raised to the height to which the waves can throw up 

 fragments and the winds pile up sand ; and the only theory which seems to account for all 

 the circumstances embraced is that of the subsidence of vast regions in this ocean. " As 

 mountain after mountain and island after island slowly sank beneath the water/"' he says, 

 "fresh bases could be successively afforded for the growth of the corals. I venture to 

 defy any one to explain in any other manner how it is possible that numerous islands 

 should be distributed throughout vast areas, all the islands being low, all built of coral,, 

 absolutely requiring a foundation within a limited depth below the surface. 1 " 



Darwin's description of the island of Cocos, or Keeling, is as follows : <c The ring- 

 formed reef of the lagoon island is surmounted in the greater part of its length by linear 

 islets. On the northern or leeward side there is an opening through which vessels can 

 pass to the anchorage within. On entering, the scene was very curious and rather 

 pretty; its beauty, however, entirely depended on the brilliancy of the surrounding colours. 

 The shallow, clear, and still water of the lagoon, resting in its greater part on white 

 sand, is, when illuminated by a vertical sun, of the most vivid green. This brilliant 

 expanse, several miles in width, is on all sides divided, either by a line of snow-white 

 breakers from the dark heaving waters of the ocean, or from the blue vault of heaven 

 by the strips of land crowned by the level tops of the cocoa-nut tree. As a white cloud 

 here and there affords a pleasing contrast to the azure sky, so in the lagoon bands of 

 living coral darken the emerald-green water. 



"The next morning I went ashore on Direction Island. The strip of dry land is 

 only a few hundred yards in width ; on the lagoon side there was a white calcareous beach, 

 the radiation from which, under this sultry climate, was very oppressive. On the outer 

 coast, a solid, broad, flat coral rock served to break the violence of the open sea. 

 Excepting near the lagoon, where there is some sand, the land is entirely composed of 

 rounded fragments of coral. In such a loose, dry, stony soil, the climate of the 

 intertropical regions alone could produce so vigorous a vegetation. 



" On some of the smaller islets nothing could be more elegant than the manner in 

 which the young and full-grown cocoa-nut trees, without destroying each other's symmetry, 

 were mingled into one wood. A beach of glittering white sand formed a border to those 

 fairy spots/'' 



Mrs. Brassey writes enthusiastically of some coral fields in the South Pacific. "It is 

 really impossible to describe the beauty of the scene before us. Submarine coral forests of 

 every colour, studded with sea-flowers, anemones, and echinidse, of a brilliancy only to be 

 seen in dreamland; shoals of the brightest and swiftest fish darting and flashing in and out ; 



