172 



COELENTERATA. 



vessels of small draft.* The width of the reef is largely dependent on the slope 

 of the sea bottom, becoming narrowed in proportion as this is steep ; and also, 

 as Semper has shown, on the set of the currents, which, when they are strong, 

 hinder the extension of the reef past which they flow. 



The edge of a fringing reef is often seen to be strewn with large masses of 

 dead coral, which have been torn from the outer slopes by the breakers and 



FIG. 144a. A barrier reef (from Darwin), seen from within, from one of the high peaks 01 

 Bolabola, one of the Society Islands. 



thrown upon the reef. Many such masses must, on the other hand, fall down 

 the slope, and here, by their gradual accumulation, together with the shells of 

 Pteropods, Foraminifera, and other pelagic organisms borne by ocean currents, 

 form a basis on which the living margin of the reef may extend outward. 



The structure and growth of a fringing reef do not appear to offer problems 

 which are very difficult to solve, but the case is different when we come to the 

 other two classes of reef, the barrier reef and atoll. 



FIG. 144b. A small atoll (from Darwin), being a sketch of Whitsunday Island in the S. Pacific, 

 taken from Captain Beechey's Voyage. The whole circle has been converted into land, which 

 is a comparatively rare occurrence. 



A barrier reef resembles a fringing reef except in one important particular, 

 namely, that it is separated from the shore by a channel, which is often of great 

 width and which may attain a depth of 50-60 fathoms. 



The great barrier reef of Australia which runs along the east coast of Queensland 

 is over 1100 miles in length, and encloses a channel which is in many places 



* Opposite the mouths of streams there are wide openings in the reef where 

 the coral does not grow, by which the fresh waters reach the sea. 



