CORAL REEFS 223 



date, or of corals found in channels that had been cleared a 

 definite number of years before, and by the measurement of 

 actual specimens on the reefs after an interval of years, we 

 are now in possession of some information which enables us 

 to judge of the rate of the growth of corals in shallow water. 

 Thus the branches of a Madrepore may grow at the rate 

 of 1-2 inches in length in a year, and a great mass of 

 Porites was found to have increased 30 inches in diameter 

 in 23 years at the rate of nearly 2 inches per annum. 

 There is probably ver}^ little uniformity in growth, the rate 

 varying a great deal according to temperature, food supply, 

 and many other natural conditions ; but it has been esti- 

 mated that under ordinary circumstances a reef might grow 

 upwards from a shallow sea-bottom at a rate of one foot 

 in ii| years, or 14I fathoms in 1000 years. ^ 



From the study of these general aspects of the recent 

 coral reefs we may now pass on to the consideration of the 

 greater geological problems of the origin of the atolls and 

 of the various theories that have been advanced in the 

 endeavour to solve them. The first serious attempt in this 

 direction was made when Darwin ^ published his famous 

 book on coral reefs, giving the results of his researches and 

 reflections on the subject during the voyage round the world 

 of H.M.S. Beagle. According to his theory all atolls and 

 barrier reefs of the world originated as fringing reefs in shallow 

 water off the coasts of tropical continental lands and islands. 

 When the land subsided by earth movements and the shores 

 became submerged the coral reefs, rising vertically as their 

 supporting rocks sank, became separated from the retreating 

 shore by ever-increasing distances. In this way the fringing 

 reefs became converted into barrier reefs and the shallow 

 sand-patched lagoons of the fringing reefs became deep-water 

 areas. In the case of islands, if the land continued to sub- 

 side until the island became entirely submerged, all that would 



^ For further information on these points see : J. Stanley Gardiner, 

 Fauna and Geog. Maldive and Laccadive Archipelagoes, vol. i. Appendix A ; 

 and A. G. Mayer, " Ecology of Murray Island," Carnegie Institute of 

 Washington Publications, vol. ix., 1918. 



- C. Darwin, Coral Reefs, ist ed., 1842 ; 3rd ed. edited by Prof. Bonncy, 

 1889. 



