CCELENTERATA : ACTINOZOA. 149 



between twenty and thirty miles, and the depth of the inner 

 channel being from ten to sixty fathoms, whilst the sea outside 

 is " profoundly deep" (in some places over 1800 feet). 



3. Atolls (fig. 54, 3). These are oval or nearly circular reefs 

 of coral, enclosing a central expanse of water or lagoon. They 

 seldom form complete rings, the reef being usually breached 

 by one or more openings, which are always situated 'on the lee- 

 ward side, or on that side which is most completely sheltered 

 from the prevailing winds. In their structure they are iden- 

 tical with " encircling barrier-reefs," and differ from these only 

 in the fact that the lagoon which they enclose does not contain 

 an island in its centre. 



If a coral-reef be observed say a portion of an encircling 

 barrier-reef the following are the general phenomena which 

 may be noticed. The general shape of the reef is triangular, 

 presenting a steep and abrupt wall on the seaward side, and 

 having a long and gentle slope towards the land. The outer 

 margin of the reef is exposed to the beating of a tremendous 

 surf, whilst the soundings taken just outside the line of 

 breakers always indicate great depths. The longer inner slope 

 is washed by the calm waters of the inner lagoon or channel. 

 The reef is only very partially composed of living corals, which 

 are found to occupy a mere strip, or zone, along the seaward 

 margin of the reef, whilst all above this, as well as all below, 

 is constituted by dead coral, or " coral-rock." 



As to the method in which such a reef is produced, the 

 following facts have been established : 



A. The coral-producing polypes cannot exist at levels higher 

 than extreme low water, exposure to the sun, even for a short 

 period, proving rapidly fatal. It follows from this that no 

 coral-reef can be raised above the level of the sea by the efforts 

 of its builders. The agency whereby reefs are raised above the 

 surface of the sea, is the denuding power of the breakers which 

 constantly fall upon their outer margins. These detach large 

 masses of dead coral, and heap them up in particular places, 

 until an island is gradually produced. The fragments thus 

 accumulated are compacted together by the finer detritus of 

 the reef, and are cemented together by the percolation of water 

 holding carbonate of lime in solution. In this way the upper 

 surface of the reef, along a line of greater or less breadth, is 

 more or less completely raised above the level of high water. 

 It is obvious, however, that the- reef might be entirely 

 destroyed by a continuation of this process the sea being 

 quite competent to undo what it had done unless some 

 counteracting force were brought into play. This counteract- 



