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J. MATTHEWS ON THE HISTORY AND 



4th, Atolls, or lagoon islands. These are rings of coral enclos- 

 ing a lagoon, or portion of the ocean in their centre ; 

 usually having an opening on the leeward side, and being 

 apparently unconnected with any land. 



Structure or Coral-Reefs. — 1, Fringing-reef ; 2, Barrier-reef; 3, Atoll. 

 a, Sea level ; b, Coral-reef ; c, Primitive land ; d, Portion of sea 

 within the reef, forming a channel or lagoon. (From Nicholson's 

 " Manual of Zoology.") 



Corals exist, like all other living things, under limiting circum- 

 stances. These are light, heat, air, food, and probably electricity, 

 though we know far too little of the part played by the last to be 

 able to assign to it its due share in the general scheme. When the 

 balance of these is disturbed, the organism affected by the disturb- 

 ance endeavours within certain limits to conform to the altered con- 

 ditions, or, failing that, perishes. I will now try to explain how 

 this applies to the varying coral formations. 



1st. Coral fringes are such as have existed undisturbed in all their 

 vital and normal conditions, and along a coast line at moderate 

 depths. 



2nd. Encircling reefs are of the same formation and conditions, 

 also undisturbed in their vital relations, but such as surround an 

 island at the same moderate distance from land as the fringes. 



