n6 REEFS ON SINKING LAND. 



Persian Gulf for twenty months had a growth of coral on her copper 

 bottom two feet thick. When living on the reef, the corals nourish 

 best in the waters most exposed to the waves. 



The Barrier Reef only differs from the fringing reef in the cir- 

 cumstance that the land near which it grows has sunk or become 

 depressed to a considerable extent, so that instead of coming within 

 half a mile or a mile or two of the shore, as the rule is with fringing 

 reefs, it is usually at a distance of ten or twenty miles, or in some 

 cases even fifty miles from land. Barrier reefs occur in the middle 

 part of the Red Sea ; and like the fringing reefs on the African and 

 Arabian shores, they surround islands like the Comora Isles in the 

 Mozambique Channel, and the Pelew Isles; they extend over con- 

 siderable areas in the Pacific, notably forming the great barrier reef on 

 the north-east coast of Australia, and extend round New Caledonia, some 

 of the Fiji Islands, and the Society Islands. New Caledonia has one of 

 the most instructive of barrier reefs. It is 400 miles long, and has an 

 average distance from land of about ten miles. The island of New 

 Caledonia is 250 miles long; so that we are led to the conclusion that 

 New Caledonia has become by depression of the sea-bed 150 miles 

 shorter since coral first began to grow around it in the form of fring- 

 ing reef. 



The Australian barrier reef, which extends for uoo miles, is 

 usually about twenty miles from the shore, though sometimes the dis- 

 tance increases to fifty or ninety miles. The depth of this channel 

 varies from as little as fifty or sixty feet to as much as sixty fathoms in 

 its southern part, where it is at the greatest distance from land. 



Atoll is the name given to the reef when the land around which 

 the coral grows has become completely submerged, so that the spot 

 where it existed is only marked by the circle of coral which grew up to 

 the surface of the sea as the depression progressed. This ring is 

 occasionally perfect and frequently broken into segments. These gaps 

 are precisely similar to those observed in fringing, and barrier reefs 

 where rivers flow into the sea and bring down fresh water and mud, 

 which renders it impossible for coral to thrive in such positions ; 

 though gaps are sometimes met with where the cliffs furnish to the sea 

 a large amount of mud. Hence it has been inferred by Mr. Darwin 

 that breaks in the continuity of an atoll reef may generally be taken 

 to indicate the positions at which the streams flowed into the sea 

 which drained lands now entirely submerged in the ocean and buried 

 under a crown of coral. As a rule, the reef grows upwards as fast as 

 the sea-bed is submerged, but occasionally the depression is so com- 

 paratively rapid as to drown the coral Atolls are chiefly met with in 

 the Indian and Pacific Oceans. Among the former are the Laccadive 

 and Maldive Islands, the Chagos Bank, and the Saya de Malha, which 

 form a stretch of submerged land between India and the north of 

 Madagascar. It is impossible not to recognise that such a group of 

 islands before they were submerged may well have been higher, and 

 have formed continuous land from Mozambique to the Malabar coast. 

 Such a land would help materially to account for the African element 



