192 ACTINOZOA. 



outer margin may be rendered sinuous by bays, 

 or the continuity of the whole reef completely 

 interrupted by one or more irregular inlets. 



Barrier-reefs differ from those just mentioned 

 in occurring at a greater distance from shore, a 

 wide channel of relatively smooth and shallow 

 water flowing between, within which terraces of 

 fringing corals sometimes find their proper en- 

 vironment. Outside the reef depths almost un- 

 fathomable have been obtained, as close as it is 

 possible to venture amid the rolling surf which 

 breaks in unceasing billows against its surface. 



Besides the larger Barrier-reefs which, in cer- 

 tain cases, attain a length of many hundred miles, 

 there are others, in all respects similar, but more 

 variable in size, which are often found surround- 

 ing the smaller islands of the Pacific. 



From such reefs to Atolls we may trace every 

 possible transition. An Atoll differs from an 

 encircling Barrier-reef in enclosing, instead of a 

 central island Avith its intervening channel, an 

 uninterrupted surface of calm water, or lagoon. 

 The low-lying strip of land separating this lagoon 

 from the line of white breakers which marks the 

 outer boundary of the Atoll is seldom more than 

 half-a-mile in breadth, and presents an imper- 

 fectly circular or prolonged crescentic form, 

 though occasionally, as in Whitsunday Island, the 

 circle is complete. 



Eeefs, in general, grow only along their outer 

 edge and, when upraised above the sea-level, 

 always appear highest to the windward side. 



Clear sea-water, well aerated, at a certain tem- 

 perature, and a depth of not more than 25 or 30 

 fathoms, are among the most important of those 



