THE CORAL-REEF PROBLEM. 55 



reef, are recognized. An encircling barrier reef differs mainly 

 from an atoll in that the assumed subsidence has not been suf- 

 ficient to completely bury the enclosed island, leaving con- 

 sequently, no internal sea, but merely a separating channel 

 formed within the coral boundary. By further subsidence, it 

 is conceived, the encircling reef would be converted into an 

 atoll. When a coral boundary extends for a great distance in 

 a more or less linear direction it is termed a linear reef, or 

 " barrier " reef proper. The great barrier reef off the island of 

 New Caledonia extends in a N. V/. and S. E. direction for a dis- 

 tance of upwards of 400 miles, and that of the northeastern 

 coast of Australia has a linear extension, with interruptions of 

 more than 1000 miles. In the case of the latter the width of 

 the intervening strait is in many places between 50 and 60 

 miles, with a depth of water reaching 350 feet. The reef 

 patches, themselves, even in their broader parts, rarely exceed 

 one or two miles in width. 



: Besides the three forms of coral structure — atolls,*encireling 

 and barrier reefs — which have been assumed to give un- 

 equivocal evidence of subsidence, there is still a fourth type, 

 that of the so-called "fringing" reef, which has generally been 

 considered to afford proof either of terrestrial stabilit}' or of 

 actual elevation. These fringing reefs hug the immediate shore 

 line, and may, indeed, be said to represent the incipient stage 

 or starting point whence the other forms of reefs were developed ; 

 by slow subsidence a fringing reef would, on the Darwinian 

 hypothesis, be converted into a barrier reef. Fringing reefs are 

 frequently continued as a series of superimposed terraces 

 above the dry land, — an unequivocal proof of elevation. They 

 rarely, if ever, descend in the water to depths much exceeding 

 120 feet, and, as might be naturally supposed from their man- 

 ner of formation, are but rarely associated with the other forms 

 of coral reefs. 



Applying the Darwinian hypothesis of subsidence to the 

 phenomena of coral structures generally, we may deduce the 

 following : A region of atolls, encircling and barrier reefs is 



