differ in many respects and are by no means typical. 

 They are therefore considered separately in the next 

 chapter. 



Barrier reefs present an entirely different appearance, 

 and are both complex and extensive. The most striking 

 example is the Great Barrier Reef of Australia, which 

 has been well described by Yonge (1930) and others 

 and which was the subject of a special expedition in 

 1928. The Great Barrier Reef consists essentially of a 

 line of reefs running parallel to the mainland and sep- 

 arated from it by a lagoon channel which may be as 

 much as 100 miles wide. The depth of the channel is 

 often 15 or 100 feet and may in exceptional cases be 

 as great as 1 80 feet. 



Figure 3. Sections of a barrier reef. After Jukes. Not to scale. 



The outer edge of the reef slopes rapidly so that the 

 depth at a few miles beyond it may reach 5,000 feet 

 or more. The barrier reefs are not always continuous 

 but form more or less broken series of roughly parallel 



22 



