CCELENTERATA : ACTINOZOA. 183 



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 

 onlyjvery 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 consti- 

 tuted by dead coral, or " coral-rock." 



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

 lowing facts have been established : 



A. The coral-producing polypes cannot exjjaLaLlevels higher 

 than extreme low water* exposure tc T tfatt sun even for a short 

 period, provmg'Tapidly 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_.brakers which 

 constantly fall upon their outer margins. These detach Jarge 

 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 de- 

 stroyed by a continuation of this process the sea being quite 

 competent to undo what it had done unless some counter- 

 acting force were brought into play. This counteracting force 

 is found in the vital activity of the living corals, which form the 

 seaward margin of the reef, and which, by their growth, prevent 

 the sea from always destroying the masses of sediment which 

 it may have thrown up. 



B. The coral - producing polypes are essentially shallow- 

 water animals, and cannot exist at depths exceeding some 15 

 to 30 fathoms. It follows from this that no coral-reef can be 

 commenced upon a sea-bottom deeper than about 30 fathoms. 



