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fastidious. They can only live in warm water, and temperatures 

 below 68deg. F. are unsuitable, thus they are consequently confined 

 to tropical seas. Moreover they do not thrive below a depth of 

 about 100 feet, though their existence is possible up to about 150 

 feet. 



With the corals live a number of other lime secreting or- 

 ganisms, including calcareous seaweeds, and sponges and the like. 

 Three types of coral reefs can be identified. 



(a) The fringing reef, where the deposits of limestone form a 

 fringe round the coastline ; occasionally a small channel separates 

 them from the mainland. Examples occur in the Sandwich and 

 Fiji Islands. 



(b) Barrier reefs run parallel to the coast and are separated 

 from it by a wide and deep channel, which is often navigable and 

 which frequently provides an excellent natural roadstead. The 

 best example is undoubtedly the Great Australian Barrier Reef, 

 which stretches roughly 1,250 miles along the coast of West 

 Australia and varies in width from 10 to 90 miles. 



(c) Atolls are circular reefs, containing apparently no land 

 centre, but enclosing a sheet of shallow water called a lagoon. 

 Usually there are one or more channels into the lagoon, which 

 are open to the scour of the tides. 



Waves and currents bring material to form soil, and fragments 

 of the reef are detached by the waves and thrown as a cement upon 

 the rest of the reef, thus assisting in forming a compact mass. 



Seeds of trees and other plants are brought by currents and by- 

 birds and the islands become covered 1 with vegetation. The palms 

 are now almost natural inhabitants of the atoll. 



In all types of reefs the constructive agency is most completely 

 manifested upon the sea-side of the reefs, evidently on account of 

 the greater abundance of food and of oxygen. 



The mode of formation of coral reefs was explained by Darwin 

 while naturalist on " H.M.S. Beagle." Taking into consideration 

 the fastidious character of the polype he concluded that the coral 

 rock must have been carried down by subsidence of the land, whilst 

 the ring of coral continued its upward growth. Ultimately the 

 island became totally submerged and the fringing reef became an 

 atoll. 



Sir John Murray, of the Challenger Expedition, has a rival 

 theory as he points out that many atolls occur where land is actu- 

 ally rising. He is of opinion that every atoll is built upon a sub- 

 marine mountain. Accumulations of muds, oozes, etc., gradually 

 raise it to the requisite height. Then coral polypes settle in 

 colonies and raise a table of coral. The question of food supply 

 and oxygen determines the production of a ring, which rises to sea 

 level and is carried above that level by the rising mountain. The 

 polypes of the interior degenerate and die out while the outer 

 circumference continues activity. 



Both theories no doubt account for some of the cases. Dar- 



