2io The Realm of Nature CHAP. 



reefs. The great barrier reef of Australia, stretching for 

 1 200 miles along the east coast of Queensland, is the finest 

 example known, (c) The atoll^ which is a reef in the form 

 of a closed curve with no land in the centre. The lagoon 

 encircled by an atoll is usually shallow, and th,e bed of it 

 composed of coral which is either dead or not in vigorous 

 life. Typical examples of the true coral islands or atolls 

 are found in the Maldives, Laccadives, and Chagos groups 

 in the west of the Indian Ocean. These reefs are usually 

 very narrow compared with their length, and their surface 

 never rises higher than from 10 to 20 feet above the 

 sea. In most instances only a portion of the reef rises 

 above the surface, giving the appearance of a chain of low 

 islands separated by very shallow water. The coral polyp 

 dies when it reaches sea-level, but blocks of coral are broken 

 off by the waves and thrown on the reef, where they get 

 broken down into sand, and this becoming compacted 

 amongst the branches of living coral is raised by degrees 

 until it forms dry land. Water percolating through the 

 coral rock and sand gradually converts the whole into a 

 solid mass of coral limestone, part of the carbonate of lime 

 being dissolved and re-deposited in a crystalline form in the 

 crevices. Drifting pumice strands on the beach and 

 weathers into clay ( 311) for the formation of soil. 

 Ultimately the seeds of trees and other plants get drifted 

 to the islands and take root, birds visit them, and the coral 

 island becomes habitable. 



281. The Formation of Coral Islands. During the 

 famous voyage of H.M.S. Beagle the naturalist Darwin 

 made a detailed examination of several coral formations, 

 and he came to the conclusion that the three typical forms 

 were closely related to each other. He recognised that it 

 was possible for atolls to form if they had a submarine 

 mountain, the top of which was less than 20 fathoms below 

 the surface, as a foundation, but he did not know that such 

 peaks often occurred. He found also that the walls of 

 coral rock on the seaward face of reefs sometimes rose from 

 an enormous depth, and since coral polyps can only live and 

 build in the warm surface layer, he concluded that the 



