CHANGES IN OCEANIC REGIONS. 385 



Chagos Isles in the Indian Ocean are coral formations of a horse-shoe shape opening to 

 the north-west, while the prevailing wind blows regularly from the south-east. 



Coraline formations are distributed into the four great classes of lagoon-islands, encirc 

 ling-reefs, f ringing-reefs, and barrier-reefs. 1. Lagoon-islands, to which their Indian name 

 of atolls is generally applied, consist of a belt of coral enclosing a lagoon, or vacant space 

 of the ocean. The belt is usually narrow, and so low, that it would not be perceptible at 

 a trifling distance, but for the palms, cocoa-nuts, and other vegetation established upon it. 

 These formations are variously circular, oval, and irregular in shape ; they occur solitary, 

 and in groups, but commonly form elongated archipelagoes, and occupy an immense area 

 of the Pacific Ocean. 2. Encircling-reefs differ only from the preceding class in having 

 one or more islands within the central expanse. Tahiti, the principal of the Society 

 Islands, is a fine example. It rises in mountains to the height of 7000 feet, and is 

 surrounded by a lagoon, like an enormous moat, from half a mile to three miles broad, 

 which is separated from the outlying ocean by a reef of coral. Both in the case of atolls 

 and encircling-reefs, the coral has openings or channels in its circuit, by which ships enter 

 the lagoons, where they find excellent harbourage. 3. Fringing -reefs are mere ribands of 

 coral, enclosing no lagoons, but immediately lining the shores. 4. Barrier-reefs extend in 

 straight lines in front of the shores of a continent or large island, frequently at a consider 

 able distance from the land. The most remarkable example is the great Australian reef. 

 Externally, it starts up with little inclination from a fathomless ocean, stretches upwards 

 of a thousand miles along the north-east coast of Australia, varies in breadth from two 

 hundred yards to a mile, and in distance from the shore from twenty to seventy miles. 

 There are transverse openings by which vessels enter the interior ocean, which is 

 everywhere safely navigable. "The long ocean swell," remarks an observer, "being 

 suddenly impeded by this barrier, lifted itself in one great continuous ridge of deep blue 

 water, which, curling over, fell on the edge of the reef in an unbroken cataract of dazzling 

 white foam. Each line of breaker runs often one or two miles in length, with not a 

 perceptible gap in its continuity. There was a simple grandeur and display of power and 

 beauty in this scene that rose even to sublimity. The unbroken roar of the surf, with its 

 regular pulsation of thunder, as each succeeding swell fell first on the outer edge of the 

 reef, was almost defeaning, yet so deep-toned as not to interfere with the slightest nearer 

 and sharper sound. Both the sound and sight were such as to impress the spectator with 

 the consciousness of standing in the presence of an overwhelming majesty and power." 

 We have here a mound constructed by animals of low organisation, in comparison with 

 which, the great wall of China, or the pyramids of Egypt, or the Plymouth Breakwater, 

 shrink into insignificance, formed, too, amid the waves of the ocean, in defiance of its storms. 





