DARWIN ON CORAL REEFS. 385 



group, as in atolls generally, that the channel is generally situated 

 on the side least exposed to the prevailing winds and the heavy 

 seas. 



It appears, with regard to the islets formed in this group, that 

 some of them have come into existence as land only within a few 

 years, while others are fast wearing away. 



The great Chagos Bank is described by Capt. Moresby, as " a 

 half-drowned atoll." Its longer axis is about 90 miles, and it is 

 70 miles in width across the broadest part. The central part 

 consists of a level muddy flat between 40 and 50 fathoms deep, 

 surrounded on all sides, with the exception of some breaches, by 

 the steep edges of a set of sand-banks, varying in breadth from 5 

 to 12 miles, and rudely arranged in a circle. These banks are about 

 16 fathoms beneath the surface, and are bordered by a rim about a 

 mile wide, consisting of smooth hard rock, and only about 5 or ] 

 fathoms beneath the surface. At the distance of half a mile from 

 one part of this rim, no bottom was found with 190 fathoms, and 

 off another point, none with 210 fathoms. The circumstances 

 and cause of this submerged condition will require subsequent 

 consideration. 



Barrier reefs resemble, in general form of structure, the atoll 

 reefs already described, but they inclose land and a lagoon channel 

 instead of a lagoon or mere sheet of water. They are, however, 

 sometimes of vast dimensions, one on the west coast of New 

 Caledonia being 400 miles in length and upwards of eight leagues 

 from the shore, and the Australian barrier extending along the 

 eastern and north-eastern coast of that vast island for nearly a 

 thousand miles, its average distance from the coast being between 

 20 and 30 miles. It is observed in many of these reefs that 

 the coral banks rise from far greater depths than the animal can 

 exist in, and the living corals form a comparatively small proportion 

 of the whole amount. 



Fringing or shore reefs differ from barrier reefs, as has been 

 already stated, by the absence of a lagoon channel and by the close 

 relation in their horizontal extension with the probable slope be- 

 neath the sea of the adjoining land. The reefs round the island 

 of Mauritius offer a good example of this class ; and here, as in 

 barrier reefs, the reef is breached by a straight passage in front of 

 every river and streamlet.* 



Many islands are fringed by reefs of this kind, but where the 

 sea deepens rapidly, the reefs are narrow, often not more than 

 50 or 100 yards wide, and they have a nearly smooth hard surface 

 scarcely uncovered at low water, and without an interior shoal 

 channel. Their dimensions and structure, in fact, depend entirely 



* This peculiarity is explained afterwards by Mr. Darwin, and an account is 

 given of other causes unfavourable to the advance of the coral animal, but it is 

 stated that one of these fringing reefs, if elevated in a perfect condition above 

 the level of the sea, ought to present the singular appearance of a broad dry 

 moat within a low mound. This, however, would be greatly modified by the 

 action of the sea during the elevation. (See p. 54.) 

 VOL. I. C C 



